OpenXR: Change timing of xrWaitFrame and add thread safety features to OpenXR

doc/classes/RenderingServer.xml

@@ -1979,6 +1979,12 @@
 				[b]Warning:[/b] This function is primarily intended for editor usage. For in-game use cases, prefer physics collision.
 			</description>
 		</method>
+		<method name="is_on_render_thread">
+			<return type="bool" />
+			<description>
+				Returns [code]true[/code] if our code is currently executing on the rendering thread.
+			</description>
+		</method>
 		<method name="light_directional_set_blend_splits">
 			<return type="void" />
 			<param index="0" name="light" type="RID" />

doc/classes/XRServer.xml

@@ -37,8 +37,8 @@
 				You should call this method after a few seconds have passed. For example, when the user requests a realignment of the display holding a designated button on a controller for a short period of time, or when implementing a teleport mechanism.
 			</description>
 		</method>
-		<method name="clear_reference_frame" qualifiers="const">
+		<method name="clear_reference_frame">
-			<return type="Transform3D" />
+			<return type="void" />
 			<description>
 				Clears the reference frame that was set by previous calls to [method center_on_hmd].
 			</description>

modules/openxr/doc_classes/OpenXRAPIExtension.xml

@@ -54,7 +54,7 @@
 		<method name="get_next_frame_time">
 			<return type="int" />
 			<description>
-				Returns the timing for the next frame.
+				Returns the predicted display timing for the next frame.
 			</description>
 		</method>
 		<method name="get_play_space">
@@ -63,6 +63,12 @@
 				Returns the play space, which is an [url=https://registry.khronos.org/OpenXR/specs/1.0/man/html/XrSpace.html]XrSpace[/url] cast to an integer.
 			</description>
 		</method>
+		<method name="get_predicted_display_time">
+			<return type="int" />
+			<description>
+				Returns the predicted display timing for the current frame.
+			</description>
+		</method>
 		<method name="get_session">
 			<return type="int" />
 			<description>

modules/openxr/doc_classes/OpenXRInterface.xml

@@ -147,6 +147,11 @@
 		</member>
 	</members>
 	<signals>
+		<signal name="instance_exiting">
+			<description>
+				Informs our OpenXR instance is exiting.
+			</description>
+		</signal>
 		<signal name="pose_recentered">
 			<description>
 				Informs the user queued a recenter of the player position.
@@ -169,6 +174,11 @@
 				Informs our OpenXR session now has focus.
 			</description>
 		</signal>
+		<signal name="session_loss_pending">
+			<description>
+				Informs our OpenXR session is in the process of being lost.
+			</description>
+		</signal>
 		<signal name="session_stopping">
 			<description>
 				Informs our OpenXR session is stopping.

modules/openxr/extensions/openxr_composition_layer_extension.cpp

@@ -274,7 +274,7 @@ bool OpenXRViewportCompositionLayerProvider::update_and_acquire_swapchain(bool p
 		if (swapchain_size == viewport_size && !p_static_image && !static_image) {
 			// We're all good! Just acquire it.
 			// We can ignore should_render here, return will be false.
-			XrBool32 should_render = true;
+			bool should_render = true;
 			return swapchain_info.acquire(should_render);
 		}
 
@@ -296,7 +296,7 @@ bool OpenXRViewportCompositionLayerProvider::update_and_acquire_swapchain(bool p
 
 	// Acquire our image so we can start rendering into it,
 	// we can ignore should_render here, ret will be false.
-	XrBool32 should_render = true;
+	bool should_render = true;
 	bool ret = swapchain_info.acquire(should_render);
 
 	swapchain_size = viewport_size;

modules/openxr/extensions/openxr_hand_tracking_extension.cpp

@@ -128,7 +128,7 @@ void OpenXRHandTrackingExtension::on_process() {
 	}
 
 	// process our hands
-	const XrTime time = OpenXRAPI::get_singleton()->get_next_frame_time(); // This data will be used for the next frame we render
+	const XrTime time = OpenXRAPI::get_singleton()->get_predicted_display_time();
 	if (time == 0) {
 		// we don't have timing info yet, or we're skipping a frame...
 		return;

modules/openxr/openxr_api.cpp

@@ -160,7 +160,7 @@ void OpenXRAPI::OpenXRSwapChainInfo::free() {
 	}
 }
 
-bool OpenXRAPI::OpenXRSwapChainInfo::acquire(XrBool32 &p_should_render) {
+bool OpenXRAPI::OpenXRSwapChainInfo::acquire(bool &p_should_render) {
 	ERR_FAIL_COND_V(image_acquired, true); // This was not released when it should be, error out and reuse...
 
 	OpenXRAPI *openxr_api = OpenXRAPI::get_singleton();
@@ -193,10 +193,18 @@ bool OpenXRAPI::OpenXRSwapChainInfo::acquire(XrBool32 &p_should_render) {
 	XrSwapchainImageWaitInfo swapchain_image_wait_info = {
 		XR_TYPE_SWAPCHAIN_IMAGE_WAIT_INFO, // type
 		nullptr, // next
-		17000000 // timeout in nanoseconds
+		1000000000 // 1s timeout in nanoseconds
 	};
 
-	result = openxr_api->xrWaitSwapchainImage(swapchain, &swapchain_image_wait_info);
+	// Wait for a maximum of 10 seconds before calling it a critical failure...
+	for (int retry = 0; retry < 10; retry++) {
+		result = openxr_api->xrWaitSwapchainImage(swapchain, &swapchain_image_wait_info);
+		if (result != XR_TIMEOUT_EXPIRED) {
+			break;
+		}
+		WARN_PRINT("OpenXR: timed out waiting for swapchain image.");
+	}
+
 	if (!XR_UNQUALIFIED_SUCCESS(result)) {
 		// Make sure end_frame knows we need to submit an empty frame
 		p_should_render = false;
@@ -206,6 +214,8 @@ bool OpenXRAPI::OpenXRSwapChainInfo::acquire(XrBool32 &p_should_render) {
 			print_line("OpenXR: failed to wait for swapchain image [", openxr_api->get_error_string(result), "]");
 			return false;
 		} else {
+			WARN_PRINT("OpenXR: couldn't to wait for swapchain but not a complete error [" + openxr_api->get_error_string(result) + "]");
+
 			// Make sure to skip trying to acquire the swapchain image in the next frame
 			skip_acquire_swapchain = true;
 			return false;
@@ -760,21 +770,6 @@ bool OpenXRAPI::load_supported_view_configuration_views(XrViewConfigurationType
 		print_verbose(String(" - recommended render sample count: ") + itos(view_configuration_views[i].recommendedSwapchainSampleCount));
 	}
 
-	// Allocate buffers we'll be populating with view information.
-	views = (XrView *)memalloc(sizeof(XrView) * view_count);
-	ERR_FAIL_NULL_V_MSG(views, false, "OpenXR Couldn't allocate memory for views");
-	memset(views, 0, sizeof(XrView) * view_count);
-
-	projection_views = (XrCompositionLayerProjectionView *)memalloc(sizeof(XrCompositionLayerProjectionView) * view_count);
-	ERR_FAIL_NULL_V_MSG(projection_views, false, "OpenXR Couldn't allocate memory for projection views");
-	memset(projection_views, 0, sizeof(XrCompositionLayerProjectionView) * view_count);
-
-	if (submit_depth_buffer && OpenXRCompositionLayerDepthExtension::get_singleton()->is_available()) {
-		depth_views = (XrCompositionLayerDepthInfoKHR *)memalloc(sizeof(XrCompositionLayerDepthInfoKHR) * view_count);
-		ERR_FAIL_NULL_V_MSG(depth_views, false, "OpenXR Couldn't allocate memory for depth views");
-		memset(depth_views, 0, sizeof(XrCompositionLayerDepthInfoKHR) * view_count);
-	}
-
 	return true;
 }
 
@@ -927,6 +922,9 @@ bool OpenXRAPI::setup_play_space() {
 
 	// If we've previously created a play space, clean it up first.
 	if (play_space != XR_NULL_HANDLE) {
+		// TODO Investigate if destroying our play space here is safe,
+		// it may still be used in the rendering thread.
+
 		xrDestroySpace(play_space);
 	}
 	play_space = new_play_space;
@@ -936,7 +934,11 @@ bool OpenXRAPI::setup_play_space() {
 	if (emulating_local_floor) {
 		// We'll use the STAGE space to get the floor height, but we can't do that until
 		// after xrWaitFrame(), so just set this flag for now.
+		// Render state will be updated then.
 		should_reset_emulated_floor_height = true;
+	} else {
+		// Update render state so this play space is used rendering the upcoming frame.
+		set_render_play_space(play_space);
 	}
 
 	return true;
@@ -1016,7 +1018,7 @@ bool OpenXRAPI::reset_emulated_floor_height() {
 		identityPose, // pose
 	};
 
-	result = xrLocateSpace(stage_space, local_space, get_next_frame_time(), &stage_location);
+	result = xrLocateSpace(stage_space, local_space, get_predicted_display_time(), &stage_location);
 
 	xrDestroySpace(local_space);
 	xrDestroySpace(stage_space);
@@ -1042,6 +1044,9 @@ bool OpenXRAPI::reset_emulated_floor_height() {
 	// report that as the reference space to the outside world.
 	reference_space = XR_REFERENCE_SPACE_TYPE_LOCAL_FLOOR_EXT;
 
+	// Update render state so this play space is used rendering the upcoming frame.
+	set_render_play_space(play_space);
+
 	return true;
 }
 
@@ -1136,6 +1141,7 @@ bool OpenXRAPI::obtain_swapchain_formats() {
 }
 
 bool OpenXRAPI::create_main_swapchains(Size2i p_size) {
+	ERR_NOT_ON_RENDER_THREAD_V(false);
 	ERR_FAIL_NULL_V(graphics_extension, false);
 	ERR_FAIL_COND_V(session == XR_NULL_HANDLE, false);
 
@@ -1154,12 +1160,12 @@ bool OpenXRAPI::create_main_swapchains(Size2i p_size) {
 		as we render 3D content into internal buffers that are copied into the swapchain, we do now have (basic) VRS support
 	*/
 
-	main_swapchain_size = p_size;
+	render_state.main_swapchain_size = p_size;
 	uint32_t sample_count = 1;
 
 	// We start with our color swapchain...
 	if (color_swapchain_format != 0) {
-		if (!main_swapchains[OPENXR_SWAPCHAIN_COLOR].create(0, XR_SWAPCHAIN_USAGE_SAMPLED_BIT | XR_SWAPCHAIN_USAGE_COLOR_ATTACHMENT_BIT | XR_SWAPCHAIN_USAGE_MUTABLE_FORMAT_BIT, color_swapchain_format, main_swapchain_size.width, main_swapchain_size.height, sample_count, view_count)) {
+		if (!render_state.main_swapchains[OPENXR_SWAPCHAIN_COLOR].create(0, XR_SWAPCHAIN_USAGE_SAMPLED_BIT | XR_SWAPCHAIN_USAGE_COLOR_ATTACHMENT_BIT | XR_SWAPCHAIN_USAGE_MUTABLE_FORMAT_BIT, color_swapchain_format, render_state.main_swapchain_size.width, render_state.main_swapchain_size.height, sample_count, view_count)) {
 			return false;
 		}
 	}
@@ -1169,7 +1175,7 @@ bool OpenXRAPI::create_main_swapchains(Size2i p_size) {
 	// - we support our depth layer extension
 	// - we have our spacewarp extension (not yet implemented)
 	if (depth_swapchain_format != 0 && submit_depth_buffer && OpenXRCompositionLayerDepthExtension::get_singleton()->is_available()) {
-		if (!main_swapchains[OPENXR_SWAPCHAIN_DEPTH].create(0, XR_SWAPCHAIN_USAGE_SAMPLED_BIT | XR_SWAPCHAIN_USAGE_DEPTH_STENCIL_ATTACHMENT_BIT, depth_swapchain_format, main_swapchain_size.width, main_swapchain_size.height, sample_count, view_count)) {
+		if (!render_state.main_swapchains[OPENXR_SWAPCHAIN_DEPTH].create(0, XR_SWAPCHAIN_USAGE_SAMPLED_BIT | XR_SWAPCHAIN_USAGE_DEPTH_STENCIL_ATTACHMENT_BIT, depth_swapchain_format, render_state.main_swapchain_size.width, render_state.main_swapchain_size.height, sample_count, view_count)) {
 			return false;
 		}
 	}
@@ -1180,36 +1186,36 @@ bool OpenXRAPI::create_main_swapchains(Size2i p_size) {
 		// TBD
 	}
 
-	for (uint32_t i = 0; i < view_count; i++) {
+	for (uint32_t i = 0; i < render_state.view_count; i++) {
-		views[i].type = XR_TYPE_VIEW;
+		render_state.views[i].type = XR_TYPE_VIEW;
-		views[i].next = nullptr;
+		render_state.views[i].next = nullptr;
 
-		projection_views[i].type = XR_TYPE_COMPOSITION_LAYER_PROJECTION_VIEW;
+		render_state.projection_views[i].type = XR_TYPE_COMPOSITION_LAYER_PROJECTION_VIEW;
-		projection_views[i].next = nullptr;
+		render_state.projection_views[i].next = nullptr;
-		projection_views[i].subImage.swapchain = main_swapchains[OPENXR_SWAPCHAIN_COLOR].get_swapchain();
+		render_state.projection_views[i].subImage.swapchain = render_state.main_swapchains[OPENXR_SWAPCHAIN_COLOR].get_swapchain();
-		projection_views[i].subImage.imageArrayIndex = i;
+		render_state.projection_views[i].subImage.imageArrayIndex = i;
-		projection_views[i].subImage.imageRect.offset.x = 0;
+		render_state.projection_views[i].subImage.imageRect.offset.x = 0;
-		projection_views[i].subImage.imageRect.offset.y = 0;
+		render_state.projection_views[i].subImage.imageRect.offset.y = 0;
-		projection_views[i].subImage.imageRect.extent.width = main_swapchain_size.width;
+		render_state.projection_views[i].subImage.imageRect.extent.width = render_state.main_swapchain_size.width;
-		projection_views[i].subImage.imageRect.extent.height = main_swapchain_size.height;
+		render_state.projection_views[i].subImage.imageRect.extent.height = render_state.main_swapchain_size.height;
 
-		if (submit_depth_buffer && OpenXRCompositionLayerDepthExtension::get_singleton()->is_available() && depth_views) {
+		if (render_state.submit_depth_buffer && OpenXRCompositionLayerDepthExtension::get_singleton()->is_available() && render_state.depth_views) {
-			projection_views[i].next = &depth_views[i];
+			render_state.projection_views[i].next = &render_state.depth_views[i];
 
-			depth_views[i].type = XR_TYPE_COMPOSITION_LAYER_DEPTH_INFO_KHR;
+			render_state.depth_views[i].type = XR_TYPE_COMPOSITION_LAYER_DEPTH_INFO_KHR;
-			depth_views[i].next = nullptr;
+			render_state.depth_views[i].next = nullptr;
-			depth_views[i].subImage.swapchain = main_swapchains[OPENXR_SWAPCHAIN_DEPTH].get_swapchain();
+			render_state.depth_views[i].subImage.swapchain = render_state.main_swapchains[OPENXR_SWAPCHAIN_DEPTH].get_swapchain();
-			depth_views[i].subImage.imageArrayIndex = i;
+			render_state.depth_views[i].subImage.imageArrayIndex = i;
-			depth_views[i].subImage.imageRect.offset.x = 0;
+			render_state.depth_views[i].subImage.imageRect.offset.x = 0;
-			depth_views[i].subImage.imageRect.offset.y = 0;
+			render_state.depth_views[i].subImage.imageRect.offset.y = 0;
-			depth_views[i].subImage.imageRect.extent.width = main_swapchain_size.width;
+			render_state.depth_views[i].subImage.imageRect.extent.width = render_state.main_swapchain_size.width;
-			depth_views[i].subImage.imageRect.extent.height = main_swapchain_size.height;
+			render_state.depth_views[i].subImage.imageRect.extent.height = render_state.main_swapchain_size.height;
 			// OpenXR spec says that: minDepth < maxDepth.
-			depth_views[i].minDepth = 0.0;
+			render_state.depth_views[i].minDepth = 0.0;
-			depth_views[i].maxDepth = 1.0;
+			render_state.depth_views[i].maxDepth = 1.0;
 			// But we can reverse near and far for reverse-Z.
-			depth_views[i].nearZ = 100.0; // Near and far Z will be set to the correct values in fill_projection_matrix
+			render_state.depth_views[i].nearZ = 100.0; // Near and far Z will be set to the correct values in fill_projection_matrix
-			depth_views[i].farZ = 0.01;
+			render_state.depth_views[i].farZ = 0.01;
 		}
 	};
 
@@ -1217,23 +1223,33 @@ bool OpenXRAPI::create_main_swapchains(Size2i p_size) {
 };
 
 void OpenXRAPI::destroy_session() {
-	if (running && session != XR_NULL_HANDLE) {
+	// TODO need to figure out if we're still rendering our current frame
-		xrEndSession(session);
+	// in a separate rendering thread and if so,
+	// if we need to wait for completion.
+	// We could be pulling the rug from underneath rendering...
+
+	if (running) {
+		if (session != XR_NULL_HANDLE) {
+			xrEndSession(session);
+		}
+
+		running = false;
+		render_state.running = false;
 	}
 
-	if (views != nullptr) {
+	if (render_state.views != nullptr) {
-		memfree(views);
+		memfree(render_state.views);
-		views = nullptr;
+		render_state.views = nullptr;
 	}
 
-	if (projection_views != nullptr) {
+	if (render_state.projection_views != nullptr) {
-		memfree(projection_views);
+		memfree(render_state.projection_views);
-		projection_views = nullptr;
+		render_state.projection_views = nullptr;
 	}
 
-	if (depth_views != nullptr) {
+	if (render_state.depth_views != nullptr) {
-		memfree(depth_views);
+		memfree(render_state.depth_views);
-		depth_views = nullptr;
+		render_state.depth_views = nullptr;
 	}
 
 	free_main_swapchains();
@@ -1248,6 +1264,7 @@ void OpenXRAPI::destroy_session() {
 	if (play_space != XR_NULL_HANDLE) {
 		xrDestroySpace(play_space);
 		play_space = XR_NULL_HANDLE;
+		render_state.play_space = XR_NULL_HANDLE;
 	}
 	if (view_space != XR_NULL_HANDLE) {
 		xrDestroySpace(view_space);
@@ -1298,6 +1315,7 @@ bool OpenXRAPI::on_state_ready() {
 
 	// we're running
 	running = true;
+	set_render_session_running(true);
 
 	for (OpenXRExtensionWrapper *wrapper : registered_extension_wrappers) {
 		wrapper->on_state_ready();
@@ -1374,34 +1392,37 @@ bool OpenXRAPI::on_state_stopping() {
 		}
 
 		running = false;
+		set_render_session_running(false);
 	}
 
-	// TODO further cleanup
-
 	return true;
 }
 
 bool OpenXRAPI::on_state_loss_pending() {
 	print_verbose("On state loss pending");
 
+	if (xr_interface) {
+		xr_interface->on_state_loss_pending();
+	}
+
 	for (OpenXRExtensionWrapper *wrapper : registered_extension_wrappers) {
 		wrapper->on_state_loss_pending();
 	}
 
-	// TODO need to look into the correct action here, read up on the spec but we may need to signal Godot to exit (if it's not already exiting)
-
 	return true;
 }
 
 bool OpenXRAPI::on_state_exiting() {
 	print_verbose("On state existing");
 
+	if (xr_interface) {
+		xr_interface->on_state_exiting();
+	}
+
 	for (OpenXRExtensionWrapper *wrapper : registered_extension_wrappers) {
 		wrapper->on_state_exiting();
 	}
 
-	// TODO need to look into the correct action here, read up on the spec but we may need to signal Godot to exit (if it's not already exiting)
-
 	return true;
 }
 
@@ -1419,10 +1440,7 @@ void OpenXRAPI::set_view_configuration(XrViewConfigurationType p_view_configurat
 
 bool OpenXRAPI::set_requested_reference_space(XrReferenceSpaceType p_requested_reference_space) {
 	requested_reference_space = p_requested_reference_space;
-
+	play_space_is_dirty = true;
-	if (is_initialized()) {
-		return setup_play_space();
-	}
 
 	return true;
 }
@@ -1625,11 +1643,6 @@ bool OpenXRAPI::initialize_session() {
 		return false;
 	}
 
-	if (!setup_play_space()) {
-		destroy_session();
-		return false;
-	}
-
 	if (!setup_view_space()) {
 		destroy_session();
 		return false;
@@ -1645,6 +1658,8 @@ bool OpenXRAPI::initialize_session() {
 		return false;
 	}
 
+	allocate_view_buffers(view_count, submit_depth_buffer);
+
 	return true;
 }
 
@@ -1696,12 +1711,18 @@ XrHandTrackerEXT OpenXRAPI::get_hand_tracker(int p_hand_index) {
 }
 
 Size2 OpenXRAPI::get_recommended_target_size() {
+	RenderingServer *rendering_server = RenderingServer::get_singleton();
 	ERR_FAIL_NULL_V(view_configuration_views, Size2());
 
 	Size2 target_size;
 
-	target_size.width = view_configuration_views[0].recommendedImageRectWidth * render_target_size_multiplier;
+	if (rendering_server && rendering_server->is_on_render_thread()) {
-	target_size.height = view_configuration_views[0].recommendedImageRectHeight * render_target_size_multiplier;
+		target_size.width = view_configuration_views[0].recommendedImageRectWidth * render_state.render_target_size_multiplier;
+		target_size.height = view_configuration_views[0].recommendedImageRectHeight * render_state.render_target_size_multiplier;
+	} else {
+		target_size.width = view_configuration_views[0].recommendedImageRectWidth * render_target_size_multiplier;
+		target_size.height = view_configuration_views[0].recommendedImageRectHeight * render_target_size_multiplier;
+	}
 
 	return target_size;
 }
@@ -1713,14 +1734,12 @@ XRPose::TrackingConfidence OpenXRAPI::get_head_center(Transform3D &r_transform,
 		return XRPose::XR_TRACKING_CONFIDENCE_NONE;
 	}
 
-	// xrWaitFrame not run yet
+	// Get display time
-	if (frame_state.predictedDisplayTime == 0) {
+	XrTime display_time = get_predicted_display_time();
+	if (display_time == 0) {
 		return XRPose::XR_TRACKING_CONFIDENCE_NONE;
 	}
 
-	// Get timing for the next frame, as that is the current frame we're processing
-	XrTime display_time = get_next_frame_time();
-
 	XrSpaceVelocity velocity = {
 		XR_TYPE_SPACE_VELOCITY, // type
 		nullptr, // next
@@ -1764,54 +1783,47 @@ XRPose::TrackingConfidence OpenXRAPI::get_head_center(Transform3D &r_transform,
 }
 
 bool OpenXRAPI::get_view_transform(uint32_t p_view, Transform3D &r_transform) {
-	if (!running) {
+	ERR_NOT_ON_RENDER_THREAD_V(false);
-		return false;
-	}
 
-	// xrWaitFrame not run yet
+	if (!render_state.running) {
-	if (frame_state.predictedDisplayTime == 0) {
 		return false;
 	}
 
 	// we don't have valid view info
-	if (views == nullptr || !view_pose_valid) {
+	if (render_state.views == nullptr || !render_state.view_pose_valid) {
 		return false;
 	}
 
 	// Note, the timing of this is set right before rendering, which is what we need here.
-	r_transform = transform_from_pose(views[p_view].pose);
+	r_transform = transform_from_pose(render_state.views[p_view].pose);
 
 	return true;
 }
 
 bool OpenXRAPI::get_view_projection(uint32_t p_view, double p_z_near, double p_z_far, Projection &p_camera_matrix) {
+	ERR_NOT_ON_RENDER_THREAD_V(false);
 	ERR_FAIL_NULL_V(graphics_extension, false);
 
-	if (!running) {
+	if (!render_state.running) {
-		return false;
-	}
-
-	// xrWaitFrame not run yet
-	if (frame_state.predictedDisplayTime == 0) {
 		return false;
 	}
 
 	// we don't have valid view info
-	if (views == nullptr || !view_pose_valid) {
+	if (render_state.views == nullptr || !render_state.view_pose_valid) {
 		return false;
 	}
 
 	// if we're using depth views, make sure we update our near and far there...
-	if (depth_views != nullptr) {
+	if (render_state.depth_views != nullptr) {
-		for (uint32_t i = 0; i < view_count; i++) {
+		for (uint32_t i = 0; i < render_state.view_count; i++) {
 			// As we are using reverse-Z these need to be flipped.
-			depth_views[i].nearZ = p_z_far;
+			render_state.depth_views[i].nearZ = p_z_far;
-			depth_views[i].farZ = p_z_near;
+			render_state.depth_views[i].farZ = p_z_near;
 		}
 	}
 
 	// now update our projection
-	return graphics_extension->create_projection_fov(views[p_view].fov, p_z_near, p_z_far, p_camera_matrix);
+	return graphics_extension->create_projection_fov(render_state.views[p_view].fov, p_z_near, p_z_far, p_camera_matrix);
 }
 
 bool OpenXRAPI::poll_events() {
@@ -1934,6 +1946,69 @@ bool OpenXRAPI::poll_events() {
 	}
 }
 
+void OpenXRAPI::_allocate_view_buffers(uint32_t p_view_count, bool p_submit_depth_buffer) {
+	// Must be called from rendering thread!
+	ERR_NOT_ON_RENDER_THREAD;
+
+	OpenXRAPI *openxr_api = OpenXRAPI::get_singleton();
+	ERR_FAIL_NULL(openxr_api);
+
+	openxr_api->render_state.view_count = p_view_count;
+	openxr_api->render_state.submit_depth_buffer = p_submit_depth_buffer;
+
+	// Allocate buffers we'll be populating with view information.
+	openxr_api->render_state.views = (XrView *)memalloc(sizeof(XrView) * p_view_count);
+	ERR_FAIL_NULL_MSG(openxr_api->render_state.views, "OpenXR Couldn't allocate memory for views");
+	memset(openxr_api->render_state.views, 0, sizeof(XrView) * p_view_count);
+
+	openxr_api->render_state.projection_views = (XrCompositionLayerProjectionView *)memalloc(sizeof(XrCompositionLayerProjectionView) * p_view_count);
+	ERR_FAIL_NULL_MSG(openxr_api->render_state.projection_views, "OpenXR Couldn't allocate memory for projection views");
+	memset(openxr_api->render_state.projection_views, 0, sizeof(XrCompositionLayerProjectionView) * p_view_count);
+
+	if (p_submit_depth_buffer && OpenXRCompositionLayerDepthExtension::get_singleton()->is_available()) {
+		openxr_api->render_state.depth_views = (XrCompositionLayerDepthInfoKHR *)memalloc(sizeof(XrCompositionLayerDepthInfoKHR) * p_view_count);
+		ERR_FAIL_NULL_MSG(openxr_api->render_state.depth_views, "OpenXR Couldn't allocate memory for depth views");
+		memset(openxr_api->render_state.depth_views, 0, sizeof(XrCompositionLayerDepthInfoKHR) * p_view_count);
+	}
+}
+
+void OpenXRAPI::_set_render_session_running(bool p_is_running) {
+	// Must be called from rendering thread!
+	ERR_NOT_ON_RENDER_THREAD;
+
+	OpenXRAPI *openxr_api = OpenXRAPI::get_singleton();
+	ERR_FAIL_NULL(openxr_api);
+	openxr_api->render_state.running = p_is_running;
+}
+
+void OpenXRAPI::_set_render_display_info(XrTime p_predicted_display_time, bool p_should_render) {
+	// Must be called from rendering thread!
+	ERR_NOT_ON_RENDER_THREAD;
+
+	OpenXRAPI *openxr_api = OpenXRAPI::get_singleton();
+	ERR_FAIL_NULL(openxr_api);
+	openxr_api->render_state.predicted_display_time = p_predicted_display_time;
+	openxr_api->render_state.should_render = p_should_render;
+}
+
+void OpenXRAPI::_set_render_play_space(uint64_t p_play_space) {
+	// Must be called from rendering thread!
+	ERR_NOT_ON_RENDER_THREAD;
+
+	OpenXRAPI *openxr_api = OpenXRAPI::get_singleton();
+	ERR_FAIL_NULL(openxr_api);
+	openxr_api->render_state.play_space = XrSpace(p_play_space);
+}
+
+void OpenXRAPI::_set_render_state_multiplier(double p_render_target_size_multiplier) {
+	// Must be called from rendering thread!
+	ERR_NOT_ON_RENDER_THREAD;
+
+	OpenXRAPI *openxr_api = OpenXRAPI::get_singleton();
+	ERR_FAIL_NULL(openxr_api);
+	openxr_api->render_state.render_target_size_multiplier = p_render_target_size_multiplier;
+}
+
 bool OpenXRAPI::process() {
 	ERR_FAIL_COND_V(instance == XR_NULL_HANDLE, false);
 
@@ -1945,42 +2020,11 @@ bool OpenXRAPI::process() {
 		return false;
 	}
 
-	for (OpenXRExtensionWrapper *wrapper : registered_extension_wrappers) {
+	// We call xrWaitFrame as early as possible, this will allow OpenXR to get
-		wrapper->on_process();
+	// proper timing info between this point, and when we're ready to start rendering.
-	}
+	// As the name suggests, OpenXR can pause the thread to minimize the time between
-
+	// retrieving tracking data and using that tracking data to render.
-	return true;
+	// OpenXR thus works best if rendering is performed on a separate thread.
-}
-
-void OpenXRAPI::free_main_swapchains() {
-	for (int i = 0; i < OPENXR_SWAPCHAIN_MAX; i++) {
-		main_swapchains[i].queue_free();
-	}
-}
-
-void OpenXRAPI::pre_render() {
-	ERR_FAIL_COND(instance == XR_NULL_HANDLE);
-
-	if (!running) {
-		return;
-	}
-
-	// Process any swapchains that were queued to be freed
-	OpenXRSwapChainInfo::free_queued();
-
-	Size2i swapchain_size = get_recommended_target_size();
-	if (swapchain_size != main_swapchain_size) {
-		// Out with the old.
-		free_main_swapchains();
-
-		// In with the new.
-		create_main_swapchains(swapchain_size);
-	}
-
-	// Waitframe does 2 important things in our process:
-	// 1) It provides us with predictive timing, telling us when OpenXR expects to display the frame we're about to commit
-	// 2) It will use the previous timing to pause our thread so that rendering starts as close to displaying as possible
-	// This must thus be called as close to when we start rendering as possible
 	XrFrameWaitInfo frame_wait_info = { XR_TYPE_FRAME_WAIT_INFO, nullptr };
 	frame_state.predictedDisplayTime = 0;
 	frame_state.predictedDisplayPeriod = 0;
@@ -1995,7 +2039,9 @@ void OpenXRAPI::pre_render() {
 		frame_state.predictedDisplayPeriod = 0;
 		frame_state.shouldRender = false;
 
-		return;
+		set_render_display_info(0, false);
+
+		return false;
 	}
 
 	if (frame_state.predictedDisplayPeriod > 500000000) {
@@ -2004,11 +2050,53 @@ void OpenXRAPI::pre_render() {
 		frame_state.predictedDisplayPeriod = 0;
 	}
 
+	set_render_display_info(frame_state.predictedDisplayTime, frame_state.shouldRender);
+
+	if (unlikely(play_space_is_dirty)) {
+		setup_play_space();
+		play_space_is_dirty = false;
+	}
+
 	if (unlikely(should_reset_emulated_floor_height)) {
 		reset_emulated_floor_height();
 		should_reset_emulated_floor_height = false;
 	}
 
+	for (OpenXRExtensionWrapper *wrapper : registered_extension_wrappers) {
+		wrapper->on_process();
+	}
+
+	return true;
+}
+
+void OpenXRAPI::free_main_swapchains() {
+	for (int i = 0; i < OPENXR_SWAPCHAIN_MAX; i++) {
+		render_state.main_swapchains[i].queue_free();
+	}
+}
+
+void OpenXRAPI::pre_render() {
+	ERR_FAIL_COND(session == XR_NULL_HANDLE);
+
+	// Must be called from rendering thread!
+	ERR_NOT_ON_RENDER_THREAD;
+
+	if (!render_state.running) {
+		return;
+	}
+
+	// Process any swapchains that were queued to be freed
+	OpenXRSwapChainInfo::free_queued();
+
+	Size2i swapchain_size = get_recommended_target_size();
+	if (swapchain_size != render_state.main_swapchain_size) {
+		// Out with the old.
+		free_main_swapchains();
+
+		// In with the new.
+		create_main_swapchains(swapchain_size);
+	}
+
 	for (OpenXRExtensionWrapper *wrapper : registered_extension_wrappers) {
 		wrapper->on_pre_render();
 	}
@@ -2028,8 +2116,8 @@ void OpenXRAPI::pre_render() {
 		XR_TYPE_VIEW_LOCATE_INFO, // type
 		nullptr, // next
 		view_configuration, // viewConfigurationType
-		frame_state.predictedDisplayTime, // displayTime
+		render_state.predicted_display_time, // displayTime
-		play_space // space
+		render_state.play_space // space
 	};
 	XrViewState view_state = {
 		XR_TYPE_VIEW_STATE, // type
@@ -2037,7 +2125,7 @@ void OpenXRAPI::pre_render() {
 		0 // viewStateFlags
 	};
 	uint32_t view_count_output;
-	result = xrLocateViews(session, &view_locate_info, &view_state, view_count, &view_count_output, views);
+	XrResult result = xrLocateViews(session, &view_locate_info, &view_state, render_state.view_count, &view_count_output, render_state.views);
 	if (XR_FAILED(result)) {
 		print_line("OpenXR: Couldn't locate views [", get_error_string(result), "]");
 		return;
@@ -2050,9 +2138,9 @@ void OpenXRAPI::pre_render() {
 			pose_valid = false;
 		}
 	}
-	if (view_pose_valid != pose_valid) {
+	if (render_state.view_pose_valid != pose_valid) {
-		view_pose_valid = pose_valid;
+		render_state.view_pose_valid = pose_valid;
-		if (!view_pose_valid) {
+		if (!render_state.view_pose_valid) {
 			print_verbose("OpenXR View pose became invalid");
 		} else {
 			print_verbose("OpenXR View pose became valid");
@@ -2071,23 +2159,24 @@ void OpenXRAPI::pre_render() {
 	}
 
 	// Reset this, we haven't found a viewport for output yet
-	has_xr_viewport = false;
+	render_state.has_xr_viewport = false;
 }
 
 bool OpenXRAPI::pre_draw_viewport(RID p_render_target) {
-	// We found an XR viewport!
+	// Must be called from rendering thread!
-	has_xr_viewport = true;
+	ERR_NOT_ON_RENDER_THREAD_V(false);
 
-	if (!can_render()) {
+	// We found an XR viewport!
+	render_state.has_xr_viewport = true;
+
+	if (instance == XR_NULL_HANDLE || session == XR_NULL_HANDLE || !render_state.running || !render_state.view_pose_valid || !render_state.should_render) {
 		return false;
 	}
 
-	// TODO: at some point in time we may support multiple viewports in which case we need to handle that...
-
 	// Acquire our images
 	for (int i = 0; i < OPENXR_SWAPCHAIN_MAX; i++) {
-		if (!main_swapchains[i].is_image_acquired() && main_swapchains[i].get_swapchain() != XR_NULL_HANDLE) {
+		if (!render_state.main_swapchains[i].is_image_acquired() && render_state.main_swapchains[i].get_swapchain() != XR_NULL_HANDLE) {
-			if (!main_swapchains[i].acquire(frame_state.shouldRender)) {
+			if (!render_state.main_swapchains[i].acquire(render_state.should_render)) {
 				return false;
 			}
 		}
@@ -2101,24 +2190,33 @@ bool OpenXRAPI::pre_draw_viewport(RID p_render_target) {
 }
 
 XrSwapchain OpenXRAPI::get_color_swapchain() {
-	return main_swapchains[OPENXR_SWAPCHAIN_COLOR].get_swapchain();
+	ERR_NOT_ON_RENDER_THREAD_V(XR_NULL_HANDLE);
+
+	return render_state.main_swapchains[OPENXR_SWAPCHAIN_COLOR].get_swapchain();
 }
 
 RID OpenXRAPI::get_color_texture() {
-	return main_swapchains[OPENXR_SWAPCHAIN_COLOR].get_image();
+	ERR_NOT_ON_RENDER_THREAD_V(RID());
+
+	return render_state.main_swapchains[OPENXR_SWAPCHAIN_COLOR].get_image();
 }
 
 RID OpenXRAPI::get_depth_texture() {
+	ERR_NOT_ON_RENDER_THREAD_V(RID());
+
 	// Note, image will not be acquired if we didn't have a suitable swap chain format.
-	if (submit_depth_buffer) {
+	if (render_state.submit_depth_buffer && render_state.main_swapchains[OPENXR_SWAPCHAIN_DEPTH].is_image_acquired()) {
-		return main_swapchains[OPENXR_SWAPCHAIN_DEPTH].get_image();
+		return render_state.main_swapchains[OPENXR_SWAPCHAIN_DEPTH].get_image();
 	} else {
 		return RID();
 	}
 }
 
 void OpenXRAPI::post_draw_viewport(RID p_render_target) {
-	if (!can_render()) {
+	// Must be called from rendering thread!
+	ERR_NOT_ON_RENDER_THREAD;
+
+	if (instance == XR_NULL_HANDLE || session == XR_NULL_HANDLE || !render_state.running || !render_state.view_pose_valid || !render_state.should_render) {
 		return;
 	}
 
@@ -2130,30 +2228,33 @@ void OpenXRAPI::post_draw_viewport(RID p_render_target) {
 void OpenXRAPI::end_frame() {
 	XrResult result;
 
-	ERR_FAIL_COND(instance == XR_NULL_HANDLE);
+	ERR_FAIL_COND(session == XR_NULL_HANDLE);
 
-	if (!running) {
+	// Must be called from rendering thread!
+	ERR_NOT_ON_RENDER_THREAD;
+
+	if (!render_state.running) {
 		return;
 	}
 
-	if (frame_state.shouldRender && view_pose_valid) {
+	if (render_state.should_render && render_state.view_pose_valid) {
-		if (!has_xr_viewport) {
+		if (!render_state.has_xr_viewport) {
 			print_line("OpenXR: No viewport was marked with use_xr, there is no rendered output!");
-		} else if (!main_swapchains[OPENXR_SWAPCHAIN_COLOR].is_image_acquired()) {
+		} else if (!render_state.main_swapchains[OPENXR_SWAPCHAIN_COLOR].is_image_acquired()) {
 			print_line("OpenXR: No swapchain could be acquired to render to!");
 		}
 	}
 
 	// must have:
-	// - shouldRender set to true
+	// - should_render set to true
 	// - a valid view pose for projection_views[eye].pose to submit layer
 	// - an image to render
-	if (!frame_state.shouldRender || !view_pose_valid || !main_swapchains[OPENXR_SWAPCHAIN_COLOR].is_image_acquired()) {
+	if (!render_state.should_render || !render_state.view_pose_valid || !render_state.main_swapchains[OPENXR_SWAPCHAIN_COLOR].is_image_acquired()) {
 		// submit 0 layers when we shouldn't render
 		XrFrameEndInfo frame_end_info = {
 			XR_TYPE_FRAME_END_INFO, // type
 			nullptr, // next
-			frame_state.predictedDisplayTime, // displayTime
+			render_state.predicted_display_time, // displayTime
 			environment_blend_mode, // environmentBlendMode
 			0, // layerCount
 			nullptr // layers
@@ -2170,14 +2271,14 @@ void OpenXRAPI::end_frame() {
 
 	// release our swapchain image if we acquired it
 	for (int i = 0; i < OPENXR_SWAPCHAIN_MAX; i++) {
-		if (main_swapchains[i].is_image_acquired()) {
+		if (render_state.main_swapchains[i].is_image_acquired()) {
-			main_swapchains[i].release();
+			render_state.main_swapchains[i].release();
 		}
 	}
 
-	for (uint32_t eye = 0; eye < view_count; eye++) {
+	for (uint32_t eye = 0; eye < render_state.view_count; eye++) {
-		projection_views[eye].fov = views[eye].fov;
+		render_state.projection_views[eye].fov = render_state.views[eye].fov;
-		projection_views[eye].pose = views[eye].pose;
+		render_state.projection_views[eye].pose = render_state.views[eye].pose;
 	}
 
 	Vector<OrderedCompositionLayer> ordered_layers_list;
@@ -2210,9 +2311,9 @@ void OpenXRAPI::end_frame() {
 		XR_TYPE_COMPOSITION_LAYER_PROJECTION, // type
 		nullptr, // next
 		layer_flags, // layerFlags
-		play_space, // space
+		render_state.play_space, // space
-		view_count, // viewCount
+		render_state.view_count, // viewCount
-		projection_views, // views
+		render_state.projection_views, // views
 	};
 	ordered_layers_list.push_back({ (const XrCompositionLayerBaseHeader *)&projection_layer, 0 });
 
@@ -2228,7 +2329,7 @@ void OpenXRAPI::end_frame() {
 	XrFrameEndInfo frame_end_info = {
 		XR_TYPE_FRAME_END_INFO, // type
 		nullptr, // next
-		frame_state.predictedDisplayTime, // displayTime
+		render_state.predicted_display_time, // displayTime
 		environment_blend_mode, // environmentBlendMode
 		static_cast<uint32_t>(layers_list.size()), // layerCount
 		layers_list.ptr() // layers
@@ -2271,6 +2372,7 @@ double OpenXRAPI::get_render_target_size_multiplier() const {
 
 void OpenXRAPI::set_render_target_size_multiplier(double multiplier) {
 	render_target_size_multiplier = multiplier;
+	set_render_state_multiplier(multiplier);
 }
 
 bool OpenXRAPI::is_foveation_supported() const {
@@ -2414,10 +2516,6 @@ OpenXRAPI::OpenXRAPI() {
 
 		submit_depth_buffer = GLOBAL_GET("xr/openxr/submit_depth_buffer");
 	}
-
-	// Reset a few things that can't be done in our class definition.
-	frame_state.predictedDisplayTime = 0;
-	frame_state.predictedDisplayPeriod = 0;
 }
 
 OpenXRAPI::~OpenXRAPI() {
@@ -3132,7 +3230,7 @@ XRPose::TrackingConfidence OpenXRAPI::get_action_pose(RID p_action, RID p_tracke
 		return XRPose::XR_TRACKING_CONFIDENCE_NONE;
 	}
 
-	XrTime display_time = get_next_frame_time();
+	XrTime display_time = get_predicted_display_time();
 	if (display_time == 0) {
 		return XRPose::XR_TRACKING_CONFIDENCE_NONE;
 	}

modules/openxr/openxr_api.h

@@ -46,13 +46,11 @@
 #include "core/templates/rb_map.h"
 #include "core/templates/rid_owner.h"
 #include "core/templates/vector.h"
+#include "servers/rendering_server.h"
 #include "servers/xr/xr_pose.h"
 
 #include <openxr/openxr.h>
 
-// Note, OpenXR code that we wrote for our plugin makes use of C++20 notation for initializing structs which ensures zeroing out unspecified members.
-// Godot is currently restricted to C++17 which doesn't allow this notation. Make sure critical fields are set.
-
 // forward declarations, we don't want to include these fully
 class OpenXRInterface;
 
@@ -77,7 +75,7 @@ public:
 		static void free_queued();
 		void free();
 
-		bool acquire(XrBool32 &p_should_render);
+		bool acquire(bool &p_should_render);
 		bool release();
 		RID get_image();
 	};
@@ -151,9 +149,6 @@ private:
 
 	uint32_t view_count = 0;
 	XrViewConfigurationView *view_configuration_views = nullptr;
-	XrView *views = nullptr;
-	XrCompositionLayerProjectionView *projection_views = nullptr;
-	XrCompositionLayerDepthInfoKHR *depth_views = nullptr; // Only used by Composition Layer Depth Extension if available
 
 	enum OpenXRSwapChainTypes {
 		OPENXR_SWAPCHAIN_COLOR,
@@ -164,14 +159,11 @@ private:
 
 	int64_t color_swapchain_format = 0;
 	int64_t depth_swapchain_format = 0;
-	Size2i main_swapchain_size = { 0, 0 };
-	OpenXRSwapChainInfo main_swapchains[OPENXR_SWAPCHAIN_MAX];
 
+	bool play_space_is_dirty = true;
 	XrSpace play_space = XR_NULL_HANDLE;
 	XrSpace view_space = XR_NULL_HANDLE;
-	bool view_pose_valid = false;
 	XRPose::TrackingConfidence head_pose_confidence = XRPose::XR_TRACKING_CONFIDENCE_NONE;
-	bool has_xr_viewport = false;
 
 	bool emulating_local_floor = false;
 	bool should_reset_emulated_floor_height = false;
@@ -328,6 +320,72 @@ private:
 	// convenience
 	void copy_string_to_char_buffer(const String p_string, char *p_buffer, int p_buffer_len);
 
+	// Render state, Only accessible in rendering thread
+	struct RenderState {
+		bool running = false;
+		bool should_render = false;
+		bool has_xr_viewport = false;
+		XrTime predicted_display_time = 0;
+		XrSpace play_space = XR_NULL_HANDLE;
+		double render_target_size_multiplier = 1.0;
+
+		uint32_t view_count = 0;
+		XrView *views = nullptr;
+		XrCompositionLayerProjectionView *projection_views = nullptr;
+		XrCompositionLayerDepthInfoKHR *depth_views = nullptr; // Only used by Composition Layer Depth Extension if available
+		bool submit_depth_buffer = false; // if set to true we submit depth buffers to OpenXR if a suitable extension is enabled.
+		bool view_pose_valid = false;
+
+		Size2i main_swapchain_size;
+		OpenXRSwapChainInfo main_swapchains[OPENXR_SWAPCHAIN_MAX];
+	} render_state;
+
+	static void _allocate_view_buffers(uint32_t p_view_count, bool p_submit_depth_buffer);
+	static void _set_render_session_running(bool p_is_running);
+	static void _set_render_display_info(XrTime p_predicted_display_time, bool p_should_render);
+	static void _set_render_play_space(uint64_t p_play_space);
+	static void _set_render_state_multiplier(double p_render_target_size_multiplier);
+
+	_FORCE_INLINE_ void allocate_view_buffers(uint32_t p_view_count, bool p_submit_depth_buffer) {
+		// If we're rendering on a separate thread, we may still be processing the last frame, don't communicate this till we're ready...
+		RenderingServer *rendering_server = RenderingServer::get_singleton();
+		ERR_FAIL_NULL(rendering_server);
+
+		rendering_server->call_on_render_thread(callable_mp_static(&OpenXRAPI::_allocate_view_buffers).bind(p_view_count, p_submit_depth_buffer));
+	}
+
+	_FORCE_INLINE_ void set_render_session_running(bool p_is_running) {
+		// If we're rendering on a separate thread, we may still be processing the last frame, don't communicate this till we're ready...
+		RenderingServer *rendering_server = RenderingServer::get_singleton();
+		ERR_FAIL_NULL(rendering_server);
+
+		rendering_server->call_on_render_thread(callable_mp_static(&OpenXRAPI::_set_render_session_running).bind(p_is_running));
+	}
+
+	_FORCE_INLINE_ void set_render_display_info(XrTime p_predicted_display_time, bool p_should_render) {
+		// If we're rendering on a separate thread, we may still be processing the last frame, don't communicate this till we're ready...
+		RenderingServer *rendering_server = RenderingServer::get_singleton();
+		ERR_FAIL_NULL(rendering_server);
+
+		rendering_server->call_on_render_thread(callable_mp_static(&OpenXRAPI::_set_render_display_info).bind(p_predicted_display_time, p_should_render));
+	}
+
+	_FORCE_INLINE_ void set_render_play_space(XrSpace p_play_space) {
+		// If we're rendering on a separate thread, we may still be processing the last frame, don't communicate this till we're ready...
+		RenderingServer *rendering_server = RenderingServer::get_singleton();
+		ERR_FAIL_NULL(rendering_server);
+
+		rendering_server->call_on_render_thread(callable_mp_static(&OpenXRAPI::_set_render_play_space).bind(uint64_t(p_play_space)));
+	}
+
+	_FORCE_INLINE_ void set_render_state_multiplier(double p_render_target_size_multiplier) {
+		// If we're rendering on a separate thread, we may still be processing the last frame, don't communicate this till we're ready...
+		RenderingServer *rendering_server = RenderingServer::get_singleton();
+		ERR_FAIL_NULL(rendering_server);
+
+		rendering_server->call_on_render_thread(callable_mp_static(&OpenXRAPI::_set_render_state_multiplier).bind(p_render_target_size_multiplier));
+	}
+
 public:
 	XrInstance get_instance() const { return instance; };
 	XrSystemId get_system_id() const { return system_id; };
@@ -384,9 +442,13 @@ public:
 	bool initialize_session();
 	void finish();
 
-	XrSpace get_play_space() const { return play_space; }
+	_FORCE_INLINE_ XrSpace get_play_space() const { return play_space; }
-	XrTime get_next_frame_time() { return frame_state.predictedDisplayTime + frame_state.predictedDisplayPeriod; }
+	_FORCE_INLINE_ XrTime get_predicted_display_time() { return frame_state.predictedDisplayTime; }
-	bool can_render() { return instance != XR_NULL_HANDLE && session != XR_NULL_HANDLE && running && view_pose_valid && frame_state.shouldRender; }
+	_FORCE_INLINE_ XrTime get_next_frame_time() { return frame_state.predictedDisplayTime + frame_state.predictedDisplayPeriod; }
+	_FORCE_INLINE_ bool can_render() {
+		ERR_ON_RENDER_THREAD_V(false);
+		return instance != XR_NULL_HANDLE && session != XR_NULL_HANDLE && running && frame_state.shouldRender;
+	}
 
 	XrHandTrackerEXT get_hand_tracker(int p_hand_index);
 

modules/openxr/openxr_api_extension.cpp

@@ -48,6 +48,7 @@ void OpenXRAPIExtension::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("is_running"), &OpenXRAPIExtension::is_running);
 
 	ClassDB::bind_method(D_METHOD("get_play_space"), &OpenXRAPIExtension::get_play_space);
+	ClassDB::bind_method(D_METHOD("get_predicted_display_time"), &OpenXRAPIExtension::get_predicted_display_time);
 	ClassDB::bind_method(D_METHOD("get_next_frame_time"), &OpenXRAPIExtension::get_next_frame_time);
 	ClassDB::bind_method(D_METHOD("can_render"), &OpenXRAPIExtension::can_render);
 
@@ -130,8 +131,17 @@ uint64_t OpenXRAPIExtension::get_play_space() {
 	return (uint64_t)OpenXRAPI::get_singleton()->get_play_space();
 }
 
+int64_t OpenXRAPIExtension::get_predicted_display_time() {
+	ERR_FAIL_NULL_V(OpenXRAPI::get_singleton(), 0);
+	return (XrTime)OpenXRAPI::get_singleton()->get_predicted_display_time();
+}
+
 int64_t OpenXRAPIExtension::get_next_frame_time() {
 	ERR_FAIL_NULL_V(OpenXRAPI::get_singleton(), 0);
+
+	// In the past we needed to look a frame ahead, may be calling this unintentionally so lets warn the dev.
+	WARN_PRINT_ONCE("OpenXR: Next frame timing called, verify this is intended.");
+
 	return (XrTime)OpenXRAPI::get_singleton()->get_next_frame_time();
 }
 

modules/openxr/openxr_api_extension.h

@@ -69,6 +69,7 @@ public:
 	bool is_running();
 
 	uint64_t get_play_space();
+	int64_t get_predicted_display_time();
 	int64_t get_next_frame_time();
 	bool can_render();
 

modules/openxr/openxr_interface.cpp

@@ -43,6 +43,8 @@ void OpenXRInterface::_bind_methods() {
 	ADD_SIGNAL(MethodInfo("session_stopping"));
 	ADD_SIGNAL(MethodInfo("session_focussed"));
 	ADD_SIGNAL(MethodInfo("session_visible"));
+	ADD_SIGNAL(MethodInfo("session_loss_pending"));
+	ADD_SIGNAL(MethodInfo("instance_exiting"));
 	ADD_SIGNAL(MethodInfo("pose_recentered"));
 	ADD_SIGNAL(MethodInfo("refresh_rate_changed", PropertyInfo(Variant::FLOAT, "refresh_rate")));
 
@@ -1258,6 +1260,14 @@ void OpenXRInterface::on_state_stopping() {
 	emit_signal(SNAME("session_stopping"));
 }
 
+void OpenXRInterface::on_state_loss_pending() {
+	emit_signal(SNAME("session_loss_pending"));
+}
+
+void OpenXRInterface::on_state_exiting() {
+	emit_signal(SNAME("instance_exiting"));
+}
+
 void OpenXRInterface::on_pose_recentered() {
 	emit_signal(SNAME("pose_recentered"));
 }

modules/openxr/openxr_interface.h

@@ -31,6 +31,29 @@
 #ifndef OPENXR_INTERFACE_H
 #define OPENXR_INTERFACE_H
 
+// A note on multithreading and thread safety in OpenXR.
+//
+// Most entry points will be called from the main thread in Godot
+// however a number of entry points will be called from the
+// rendering thread, potentially while we're already processing
+// the next frame on the main thread.
+//
+// OpenXR itself has been designed with threading in mind including
+// a high likelihood that the XR runtime runs in separate threads
+// as well.
+// Hence all the frame timing information, use of swapchains and
+// sync functions.
+// Do note that repeated calls to tracking APIs will provide
+// increasingly more accurate data for the same timestamp as
+// tracking data is continuously updated.
+//
+// For our code we mostly implement this in our OpenXRAPI class.
+// We store data accessed from the rendering thread in a separate
+// struct, setting values through our renderer command queue.
+//
+// As some data is setup before we start rendering, and cleaned up
+// after we've stopped, that is accessed directly from both threads.
+
 #include "action_map/openxr_action_map.h"
 #include "extensions/openxr_hand_tracking_extension.h"
 #include "openxr_api.h"
@@ -173,6 +196,8 @@ public:
 	void on_state_visible();
 	void on_state_focused();
 	void on_state_stopping();
+	void on_state_loss_pending();
+	void on_state_exiting();
 	void on_pose_recentered();
 	void on_refresh_rate_changes(float p_new_rate);
 	void tracker_profile_changed(RID p_tracker, RID p_interaction_profile);

scene/main/viewport.cpp

@@ -688,6 +688,18 @@ void Viewport::_process_picking() {
 		physics_picking_events.clear();
 		return;
 	}
+#ifndef _3D_DISABLED
+	if (use_xr) {
+		if (XRServer::get_singleton() != nullptr) {
+			Ref<XRInterface> xr_interface = XRServer::get_singleton()->get_primary_interface();
+			if (xr_interface.is_valid() && xr_interface->is_initialized() && xr_interface->get_view_count() > 1) {
+				WARN_PRINT_ONCE("Object picking can't be used when stereo rendering, this will be turned off!");
+				physics_object_picking = false; // don't try again.
+				return;
+			}
+		}
+	}
+#endif
 
 	_drop_physics_mouseover(true);
 

servers/rendering/rendering_server_default.h

@@ -1048,6 +1048,10 @@ public:
 	virtual void init() override;
 	virtual void finish() override;
 
+	virtual bool is_on_render_thread() override {
+		return Thread::get_caller_id() == server_thread;
+	}
+
 	virtual void call_on_render_thread(const Callable &p_callable) override {
 		if (Thread::get_caller_id() == server_thread) {
 			command_queue.flush_if_pending();

servers/rendering_server.cpp

@@ -3426,6 +3426,7 @@ void RenderingServer::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("get_rendering_device"), &RenderingServer::get_rendering_device);
 	ClassDB::bind_method(D_METHOD("create_local_rendering_device"), &RenderingServer::create_local_rendering_device);
 
+	ClassDB::bind_method(D_METHOD("is_on_render_thread"), &RenderingServer::is_on_render_thread);
 	ClassDB::bind_method(D_METHOD("call_on_render_thread", "callable"), &RenderingServer::call_on_render_thread);
 
 #ifndef DISABLE_DEPRECATED

servers/rendering_server.h

@@ -41,6 +41,32 @@
 #include "servers/display_server.h"
 #include "servers/rendering/rendering_device.h"
 
+// Helper macros for code outside of the rendering server, but that is
+// called by the rendering server.
+#ifdef DEBUG_ENABLED
+#define ERR_ON_RENDER_THREAD                                              \
+	RenderingServer *rendering_server = RenderingServer::get_singleton(); \
+	ERR_FAIL_NULL(rendering_server);                                      \
+	ERR_FAIL_COND(rendering_server->is_on_render_thread());
+#define ERR_ON_RENDER_THREAD_V(m_ret)                                     \
+	RenderingServer *rendering_server = RenderingServer::get_singleton(); \
+	ERR_FAIL_NULL_V(rendering_server, m_ret);                             \
+	ERR_FAIL_COND_V(rendering_server->is_on_render_thread(), m_ret);
+#define ERR_NOT_ON_RENDER_THREAD                                          \
+	RenderingServer *rendering_server = RenderingServer::get_singleton(); \
+	ERR_FAIL_NULL(rendering_server);                                      \
+	ERR_FAIL_COND(!rendering_server->is_on_render_thread());
+#define ERR_NOT_ON_RENDER_THREAD_V(m_ret)                                 \
+	RenderingServer *rendering_server = RenderingServer::get_singleton(); \
+	ERR_FAIL_NULL_V(rendering_server, m_ret);                             \
+	ERR_FAIL_COND_V(!rendering_server->is_on_render_thread(), m_ret);
+#else
+#define ERR_ON_RENDER_THREAD
+#define ERR_ON_RENDER_THREAD_V(m_ret)
+#define ERR_NOT_ON_RENDER_THREAD
+#define ERR_NOT_ON_RENDER_THREAD_V(m_ret)
+#endif
+
 template <typename T>
 class TypedArray;
 
@@ -1684,7 +1710,7 @@ public:
 
 #ifndef DISABLE_DEPRECATED
 	// Never actually used, should be removed when we can break compatibility.
-	enum Features {
+	enum Features{
 		FEATURE_SHADERS,
 		FEATURE_MULTITHREADED,
 	};
@@ -1708,6 +1734,7 @@ public:
 	bool is_render_loop_enabled() const;
 	void set_render_loop_enabled(bool p_enabled);
 
+	virtual bool is_on_render_thread() = 0;
 	virtual void call_on_render_thread(const Callable &p_callable) = 0;
 
 #ifdef TOOLS_ENABLED

servers/xr/xr_interface.h

@@ -122,17 +122,21 @@ public:
 
 	/** rendering and internal **/
 
+	// These methods are called from the main thread.
+	virtual Transform3D get_camera_transform() = 0; /* returns the position of our camera, only used for updating reference frame. For monoscopic this is equal to the views transform, for stereoscopic this should be an average */
+	virtual void process() = 0;
+
+	// These methods can be called from both main and render thread.
 	virtual Size2 get_render_target_size() = 0; /* returns the recommended render target size per eye for this device */
 	virtual uint32_t get_view_count() = 0; /* returns the view count we need (1 is monoscopic, 2 is stereoscopic but can be more) */
-	virtual Transform3D get_camera_transform() = 0; /* returns the position of our camera for updating our camera node. For monoscopic this is equal to the views transform, for stereoscopic this should be an average */
+
+	// These methods are called from the rendering thread.
 	virtual Transform3D get_transform_for_view(uint32_t p_view, const Transform3D &p_cam_transform) = 0; /* get each views transform */
 	virtual Projection get_projection_for_view(uint32_t p_view, double p_aspect, double p_z_near, double p_z_far) = 0; /* get each view projection matrix */
 	virtual RID get_vrs_texture(); /* obtain VRS texture */
 	virtual RID get_color_texture(); /* obtain color output texture (if applicable) */
 	virtual RID get_depth_texture(); /* obtain depth output texture (if applicable, used for reprojection) */
 	virtual RID get_velocity_texture(); /* obtain velocity output texture (if applicable, used for spacewarp) */
-
-	virtual void process() = 0;
 	virtual void pre_render(){};
 	virtual bool pre_draw_viewport(RID p_render_target) { return true; }; /* inform XR interface we are about to start our viewport draw process */
 	virtual Vector<BlitToScreen> post_draw_viewport(RID p_render_target, const Rect2 &p_screen_rect) = 0; /* inform XR interface we finished our viewport draw process */

servers/xr_server.cpp

@@ -51,7 +51,7 @@ XRServer *XRServer::singleton = nullptr;
 
 XRServer *XRServer::get_singleton() {
 	return singleton;
-};
+}
 
 void XRServer::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("get_world_scale"), &XRServer::get_world_scale);
@@ -59,7 +59,7 @@ void XRServer::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("get_world_origin"), &XRServer::get_world_origin);
 	ClassDB::bind_method(D_METHOD("set_world_origin", "world_origin"), &XRServer::set_world_origin);
 	ClassDB::bind_method(D_METHOD("get_reference_frame"), &XRServer::get_reference_frame);
-	ClassDB::bind_method(D_METHOD("clear_reference_frame"), &XRServer::get_reference_frame);
+	ClassDB::bind_method(D_METHOD("clear_reference_frame"), &XRServer::clear_reference_frame);
 	ClassDB::bind_method(D_METHOD("center_on_hmd", "rotation_mode", "keep_height"), &XRServer::center_on_hmd);
 	ClassDB::bind_method(D_METHOD("get_hmd_transform"), &XRServer::get_hmd_transform);
 
@@ -104,11 +104,20 @@ void XRServer::_bind_methods() {
 	ADD_SIGNAL(MethodInfo("tracker_added", PropertyInfo(Variant::STRING_NAME, "tracker_name"), PropertyInfo(Variant::INT, "type")));
 	ADD_SIGNAL(MethodInfo("tracker_updated", PropertyInfo(Variant::STRING_NAME, "tracker_name"), PropertyInfo(Variant::INT, "type")));
 	ADD_SIGNAL(MethodInfo("tracker_removed", PropertyInfo(Variant::STRING_NAME, "tracker_name"), PropertyInfo(Variant::INT, "type")));
-};
+}
 
 double XRServer::get_world_scale() const {
-	return world_scale;
+	RenderingServer *rendering_server = RenderingServer::get_singleton();
-};
+
+	if (rendering_server && rendering_server->is_on_render_thread()) {
+		// Return the value with which we're currently rendering,
+		// if we're on the render thread
+		return render_state.world_scale;
+	} else {
+		// Return our current value
+		return world_scale;
+	}
+}
 
 void XRServer::set_world_scale(double p_world_scale) {
 	if (p_world_scale < 0.01) {
@@ -118,19 +127,58 @@ void XRServer::set_world_scale(double p_world_scale) {
 	}
 
 	world_scale = p_world_scale;
-};
+	set_render_world_scale(world_scale);
+}
+
+void XRServer::_set_render_world_scale(double p_world_scale) {
+	// Must be called from rendering thread!
+	ERR_NOT_ON_RENDER_THREAD;
+
+	XRServer *xr_server = XRServer::get_singleton();
+	ERR_FAIL_NULL(xr_server);
+	xr_server->render_state.world_scale = p_world_scale;
+}
 
 Transform3D XRServer::get_world_origin() const {
-	return world_origin;
+	RenderingServer *rendering_server = RenderingServer::get_singleton();
-};
+
+	if (rendering_server && rendering_server->is_on_render_thread()) {
+		// Return the value with which we're currently rendering,
+		// if we're on the render thread
+		return render_state.world_origin;
+	} else {
+		// Return our current value
+		return world_origin;
+	}
+}
 
 void XRServer::set_world_origin(const Transform3D &p_world_origin) {
 	world_origin = p_world_origin;
-};
+	set_render_world_origin(world_origin);
+}
+
+void XRServer::_set_render_world_origin(const Transform3D &p_world_origin) {
+	// Must be called from rendering thread!
+	ERR_NOT_ON_RENDER_THREAD;
+
+	XRServer *xr_server = XRServer::get_singleton();
+	ERR_FAIL_NULL(xr_server);
+	xr_server->render_state.world_origin = p_world_origin;
+}
 
 Transform3D XRServer::get_reference_frame() const {
-	return reference_frame;
+	RenderingServer *rendering_server = RenderingServer::get_singleton();
-};
+	ERR_FAIL_NULL_V(rendering_server, reference_frame);
+
+	if (rendering_server->is_on_render_thread()) {
+		// Return the value with which we're currently rendering,
+		// if we're on the render thread
+		return render_state.reference_frame;
+	} else {
+		// Return our current value
+		return reference_frame;
+	}
+}
 
 void XRServer::center_on_hmd(RotationMode p_rotation_mode, bool p_keep_height) {
 	if (primary_interface == nullptr) {
@@ -156,27 +204,38 @@ void XRServer::center_on_hmd(RotationMode p_rotation_mode, bool p_keep_height) {
 	} else if (p_rotation_mode == 2) {
 		// remove our rotation, we're only interesting in centering on position
 		new_reference_frame.basis = Basis();
-	};
+	}
 
 	// don't negate our height
 	if (p_keep_height) {
 		new_reference_frame.origin.y = 0.0;
-	};
+	}
 
 	reference_frame = new_reference_frame.inverse();
-};
+	set_render_reference_frame(reference_frame);
+}
 
 void XRServer::clear_reference_frame() {
 	reference_frame = Transform3D();
+	set_render_reference_frame(reference_frame);
+}
+
+void XRServer::_set_render_reference_frame(const Transform3D &p_reference_frame) {
+	// Must be called from rendering thread!
+	ERR_NOT_ON_RENDER_THREAD;
+
+	XRServer *xr_server = XRServer::get_singleton();
+	ERR_FAIL_NULL(xr_server);
+	xr_server->render_state.reference_frame = p_reference_frame;
 }
 
 Transform3D XRServer::get_hmd_transform() {
 	Transform3D hmd_transform;
 	if (primary_interface != nullptr) {
 		hmd_transform = primary_interface->get_camera_transform();
-	};
+	}
 	return hmd_transform;
-};
+}
 
 void XRServer::add_interface(const Ref<XRInterface> &p_interface) {
 	ERR_FAIL_COND(p_interface.is_null());
@@ -185,12 +244,12 @@ void XRServer::add_interface(const Ref<XRInterface> &p_interface) {
 		if (interfaces[i] == p_interface) {
 			ERR_PRINT("Interface was already added");
 			return;
-		};
+		}
-	};
+	}
 
 	interfaces.push_back(p_interface);
 	emit_signal(SNAME("interface_added"), p_interface->get_name());
-};
+}
 
 void XRServer::remove_interface(const Ref<XRInterface> &p_interface) {
 	ERR_FAIL_COND(p_interface.is_null());
@@ -200,33 +259,33 @@ void XRServer::remove_interface(const Ref<XRInterface> &p_interface) {
 		if (interfaces[i] == p_interface) {
 			idx = i;
 			break;
-		};
+		}
-	};
+	}
 
 	ERR_FAIL_COND_MSG(idx == -1, "Interface not found.");
 	print_verbose("XR: Removed interface \"" + p_interface->get_name() + "\"");
 	emit_signal(SNAME("interface_removed"), p_interface->get_name());
 	interfaces.remove_at(idx);
-};
+}
 
 int XRServer::get_interface_count() const {
 	return interfaces.size();
-};
+}
 
 Ref<XRInterface> XRServer::get_interface(int p_index) const {
 	ERR_FAIL_INDEX_V(p_index, interfaces.size(), nullptr);
 
 	return interfaces[p_index];
-};
+}
 
 Ref<XRInterface> XRServer::find_interface(const String &p_name) const {
 	for (int i = 0; i < interfaces.size(); i++) {
 		if (interfaces[i]->get_name() == p_name) {
 			return interfaces[i];
-		};
+		}
-	};
+	}
 	return Ref<XRInterface>();
-};
+}
 
 TypedArray<Dictionary> XRServer::get_interfaces() const {
 	Array ret;
@@ -238,14 +297,14 @@ TypedArray<Dictionary> XRServer::get_interfaces() const {
 		iface_info["name"] = interfaces[i]->get_name();
 
 		ret.push_back(iface_info);
-	};
+	}
 
 	return ret;
-};
+}
 
 Ref<XRInterface> XRServer::get_primary_interface() const {
 	return primary_interface;
-};
+}
 
 void XRServer::set_primary_interface(const Ref<XRInterface> &p_primary_interface) {
 	if (p_primary_interface.is_null()) {
@@ -256,7 +315,7 @@ void XRServer::set_primary_interface(const Ref<XRInterface> &p_primary_interface
 
 		print_verbose("XR: Primary interface set to: " + primary_interface->get_name());
 	}
-};
+}
 
 void XRServer::add_tracker(const Ref<XRTracker> &p_tracker) {
 	ERR_FAIL_COND(p_tracker.is_null());
@@ -272,7 +331,7 @@ void XRServer::add_tracker(const Ref<XRTracker> &p_tracker) {
 		trackers[tracker_name] = p_tracker;
 		emit_signal(SNAME("tracker_added"), tracker_name, p_tracker->get_tracker_type());
 	}
-};
+}
 
 void XRServer::remove_tracker(const Ref<XRTracker> &p_tracker) {
 	ERR_FAIL_COND(p_tracker.is_null());
@@ -285,7 +344,7 @@ void XRServer::remove_tracker(const Ref<XRTracker> &p_tracker) {
 		// and remove it
 		trackers.erase(tracker_name);
 	}
-};
+}
 
 Dictionary XRServer::get_trackers(int p_tracker_types) {
 	Dictionary res;
@@ -307,7 +366,7 @@ Ref<XRTracker> XRServer::get_tracker(const StringName &p_name) const {
 		// tracker hasn't been registered yet, which is fine, no need to spam the error log...
 		return Ref<XRTracker>();
 	}
-};
+}
 
 PackedStringArray XRServer::get_suggested_tracker_names() const {
 	PackedStringArray arr;
@@ -369,9 +428,9 @@ void XRServer::_process() {
 			// ignore, not a valid reference
 		} else if (interfaces[i]->is_initialized()) {
 			interfaces.write[i]->process();
-		};
+		}
-	};
+	}
-};
+}
 
 void XRServer::pre_render() {
 	// called from RendererViewport.draw_viewports right before we start drawing our viewports
@@ -383,8 +442,8 @@ void XRServer::pre_render() {
 			// ignore, not a valid reference
 		} else if (interfaces[i]->is_initialized()) {
 			interfaces.write[i]->pre_render();
-		};
+		}
-	};
+	}
 }
 
 void XRServer::end_frame() {
@@ -396,14 +455,13 @@ void XRServer::end_frame() {
 			// ignore, not a valid reference
 		} else if (interfaces[i]->is_initialized()) {
 			interfaces.write[i]->end_frame();
-		};
+		}
-	};
+	}
 }
 
 XRServer::XRServer() {
 	singleton = this;
-	world_scale = 1.0;
+}
-};
 
 XRServer::~XRServer() {
 	primary_interface.unref();
@@ -412,4 +470,4 @@ XRServer::~XRServer() {
 	trackers.clear();
 
 	singleton = nullptr;
-};
+}

servers/xr_server.h

@@ -36,6 +36,7 @@
 #include "core/os/thread_safe.h"
 #include "core/templates/rid.h"
 #include "core/variant/variant.h"
+#include "rendering_server.h"
 
 class XRInterface;
 class XRTracker;
@@ -92,10 +93,46 @@ private:
 
 	Ref<XRInterface> primary_interface; /* we'll identify one interface as primary, this will be used by our viewports */
 
-	double world_scale; /* scale by which we multiply our tracker positions */
+	double world_scale = 1.0; /* scale by which we multiply our tracker positions */
 	Transform3D world_origin; /* our world origin point, maps a location in our virtual world to the origin point in our real world tracking volume */
 	Transform3D reference_frame; /* our reference frame */
 
+	// As we may be updating our main state for our next frame while we're still rendering our previous frame,
+	// we need to keep copies around.
+	struct RenderState {
+		double world_scale = 1.0; /* scale by which we multiply our tracker positions */
+		Transform3D world_origin; /* our world origin point, maps a location in our virtual world to the origin point in our real world tracking volume */
+		Transform3D reference_frame; /* our reference frame */
+	} render_state;
+
+	static void _set_render_world_scale(double p_world_scale);
+	static void _set_render_world_origin(const Transform3D &p_world_origin);
+	static void _set_render_reference_frame(const Transform3D &p_reference_frame);
+
+	_FORCE_INLINE_ void set_render_world_scale(double p_world_scale) {
+		// If we're rendering on a separate thread, we may still be processing the last frame, don't communicate this till we're ready...
+		RenderingServer *rendering_server = RenderingServer::get_singleton();
+		ERR_FAIL_NULL(rendering_server);
+
+		rendering_server->call_on_render_thread(callable_mp_static(&XRServer::_set_render_world_scale).bind(p_world_scale));
+	}
+
+	_FORCE_INLINE_ void set_render_world_origin(const Transform3D &p_world_origin) {
+		// If we're rendering on a separate thread, we may still be processing the last frame, don't communicate this till we're ready...
+		RenderingServer *rendering_server = RenderingServer::get_singleton();
+		ERR_FAIL_NULL(rendering_server);
+
+		rendering_server->call_on_render_thread(callable_mp_static(&XRServer::_set_render_world_origin).bind(p_world_origin));
+	}
+
+	_FORCE_INLINE_ void set_render_reference_frame(const Transform3D &p_reference_frame) {
+		// If we're rendering on a separate thread, we may still be processing the last frame, don't communicate this till we're ready...
+		RenderingServer *rendering_server = RenderingServer::get_singleton();
+		ERR_FAIL_NULL(rendering_server);
+
+		rendering_server->call_on_render_thread(callable_mp_static(&XRServer::_set_render_reference_frame).bind(p_reference_frame));
+	}
+
 protected:
 	static XRServer *singleton;