Fall back to D3D12 if Vulkan is not supported and vice versa

doc/classes/ProjectSettings.xml

@@ -2735,6 +2735,10 @@
 		<member name="rendering/rendering_device/d3d12/agility_sdk_version" type="int" setter="" getter="" default="613">
 			Version code of the [url=https://devblogs.microsoft.com/directx/directx12agility/]Direct3D 12 Agility SDK[/url] to use ([code]D3D12SDKVersion[/code]). This must match the [i]minor[/i] version that is installed next to the editor binary and in the export templates directory for the current editor version. For example, if you have [code]1.613.3[/code] installed, you need to input [code]613[/code] here.
 		</member>
+		<member name="rendering/rendering_device/d3d12/fallback_to_d3d12" type="bool" setter="" getter="" default="true">
+			If [code]true[/code], the forward renderer will fall back to Direct3D 12 if Vulkan is not supported.
+			[b]Note:[/b] This setting is implemented only on Windows.
+		</member>
 		<member name="rendering/rendering_device/d3d12/max_misc_descriptors_per_frame" type="int" setter="" getter="" default="512">
 			The number of entries in the miscellaneous descriptors heap the Direct3D 12 rendering driver uses each frame, used for various operations like clearing a texture.
 			Depending on the complexity of scenes, this value may be lowered or may need to be raised.
@@ -2792,6 +2796,10 @@
 			[b]Note:[/b] This property is only read when the project starts. There is currently no way to change this value at run-time.
 			[b]Note:[/b] Some platforms may restrict the actual value.
 		</member>
+		<member name="rendering/rendering_device/vulkan/fallback_to_vulkan" type="bool" setter="" getter="" default="true">
+			If [code]true[/code], the forward renderer will fall back to Vulkan if Direct3D 12 is not supported.
+			[b]Note:[/b] This setting is implemented only on Windows.
+		</member>
 		<member name="rendering/rendering_device/vulkan/max_descriptors_per_pool" type="int" setter="" getter="" default="64">
 		</member>
 		<member name="rendering/scaling_3d/fsr_sharpness" type="float" setter="" getter="" default="0.2">

main/main.cpp

@@ -1956,6 +1956,9 @@ Error Main::setup(const char *execpath, int argc, char *argv[], bool p_second_ph
 		GLOBAL_DEF_RST_NOVAL(PropertyInfo(Variant::STRING, "rendering/rendering_device/driver.android", PROPERTY_HINT_ENUM, driver_hints), default_driver);
 		GLOBAL_DEF_RST_NOVAL(PropertyInfo(Variant::STRING, "rendering/rendering_device/driver.ios", PROPERTY_HINT_ENUM, driver_hints), default_driver);
 		GLOBAL_DEF_RST_NOVAL(PropertyInfo(Variant::STRING, "rendering/rendering_device/driver.macos", PROPERTY_HINT_ENUM, driver_hints), default_driver);
+
+		GLOBAL_DEF_RST("rendering/rendering_device/vulkan/fallback_to_vulkan", true);
+		GLOBAL_DEF_RST("rendering/rendering_device/d3d12/fallback_to_d3d12", true);
 	}
 
 	{

platform/windows/display_server_windows.cpp

@@ -5929,10 +5929,37 @@ DisplayServerWindows::DisplayServerWindows(const String &p_rendering_driver, Win
 
 	if (rendering_context) {
 		if (rendering_context->initialize() != OK) {
-			memdelete(rendering_context);
-			rendering_context = nullptr;
-			r_error = ERR_UNAVAILABLE;
-			return;
+			bool failed = true;
+#if defined(VULKAN_ENABLED)
+			bool fallback_to_vulkan = GLOBAL_GET("rendering/rendering_device/vulkan/fallback_to_vulkan");
+			if (failed && fallback_to_vulkan && rendering_driver != "vulkan") {
+				memdelete(rendering_context);
+				rendering_context = memnew(RenderingContextDriverVulkanWindows);
+				if (rendering_context->initialize() == OK) {
+					WARN_PRINT("Your video card drivers seem not to support Direct3D 12, switching to Vulkan.");
+					rendering_driver = "vulkan";
+					failed = false;
+				}
+			}
+#endif
+#if defined(D3D12_ENABLED)
+			bool fallback_to_d3d12 = GLOBAL_GET("rendering/rendering_device/d3d12/fallback_to_d3d12");
+			if (failed && fallback_to_d3d12 && rendering_driver != "d3d12") {
+				memdelete(rendering_context);
+				rendering_context = memnew(RenderingContextDriverD3D12);
+				if (rendering_context->initialize() == OK) {
+					WARN_PRINT("Your video card drivers seem not to support Vulkan, switching to Direct3D 12.");
+					rendering_driver = "d3d12";
+					failed = false;
+				}
+			}
+#endif
+			if (failed) {
+				memdelete(rendering_context);
+				rendering_context = nullptr;
+				r_error = ERR_UNAVAILABLE;
+				return;
+			}
 		}
 	}
 #endif