Vulkan

Skia has a Vulkan implementation of its GPU backend. The Vulkan backend can be built alongside the OpenGL backend. The client can select between the OpenGL and Vulkan implementation at runtime. The Vulkan backend has reached feature parity with the OpenGL backend. At this time we find that many Vulkan drivers have bugs that Skia triggers for which we have no workaround. We are reporting bugs to vendors as we find them.

Windows and Linux

To build the Vulkan backend, set skia_use_vulkan=true in args.gn.

Android

The Vulkan backend can run on any device with Vulkan drivers, including all Android N+ devices. To build the Vulkan backend, set ndk_api = 24 in args.gn to target Android N.

Mac

The Vulkan backend can be run in software emulation using SwiftShader. This will allow for testing and debugging via dm. (Vulkan is not supported in interactive apps like viewer.)

Skia already includes the SwiftShader library as an external dependency. To build it, you will first need to install CMake. Set up CMake for command line use by opening the app and following the instructions in Tools > How to Install For Command Line Use. Once CMake has been prepared, SwiftShader needs to be compiled. Follow these steps, substituting your actual Skia directory instead of $(SKIA_DIR) below:

$ cd $(SKIA_DIR)/third_party/externals/swiftshader/build
$ cmake ..
$ cmake --build . --parallel

Once its build completes, SwiftShader’s build directory should include a Darwin subdirectory containing libvk_swiftshader.dylib. To allow Skia to see this library, we need to reference it in args.gn like so:

skia_use_vulkan = true
extra_cflags = [ "-D", "SK_GPU_TOOLS_VK_LIBRARY_NAME=$(SKIA_DIR)/third_party/externals/swiftshader/build/Darwin/libvk_swiftshader.dylib" ]

Using the Vulkan Backend

To create a GrContext that is backed by Vulkan the client creates a Vulkan device and queue, initializes a GrVkBackendContext to describe the context, and then calls GrContext::MakeVulkan:

sk_sp<GrVkBackendContext> vkContext = new GrVkBackendContext;
vkBackendContext.fInstance = vkInstance;
vkBackendContext.fPhysicalDevice = vkPhysDevice;
...
vkBackendContext.fInterface.reset(GrVkCreateInterface(instance, vkPhysDevice, extensionFlags);
...

sk_sp<GrContext> context = GrContext::MakeVulkan(vkBackendContext);

When using the Vulkan backend, GrVkImageInfo is used to construct GrBackendTexture and GrBackendRenderTarget objects that in turn are used to create SkSurface and SkImage objects that refer to VkImages created by the Skia client.

The GrBackendObject returned by SkImage::getTextureHandle(), SkSurface::getTextureHandle(), and SkSurface::getRenderTargetHandle() should be interpreted as a GrVkImageInfo*. This allows a client to get the backing VkImage of a SkImage or SkSurface.

GrVkImageInfo specifies a VkImage and associated state (tiling, layout, format, etc). After getting a GrVkImageInfo* via getTextureHandle() or getRenderTargetHandle(), the client should check the fImageLayout field to know what layout Skia left the VkImage in before using the VkImage. If the client changes the layout of the VkImage, GrVkImageInfo::updateImageLayout(VkImageLayout layout) should be called before resuming Skia rendering.

The client is responsible for any synchronization or barriers needed before Skia performs I/O on a VkImage imported into Skia via GrVkImageInfo. Skia will assume it can start issuing commands referencing the VkImage without the need for additional synchronization.