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skia2/site/user/special/vulkan.md
Mike Klein 308b7d9933 update Vulkan docs for GN. 
BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4458
NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=4458

Change-Id: Ifb21b28440b5725775b40e79aef2a0286e1de103
Reviewed-on: https://skia-review.googlesource.com/4458
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
2016-11-07 16:11:58 +00:00

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Vulkan

Skis 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_vulkan_sdk to the path to your Vulkan SDK in args.gn. This defaults to the environment variable VULKAN_SDK.

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.

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::Create:

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

sk_sp<GrContext> context = GrContext::Create(kVulkan_GrBackend, (GrBackendContext) vkBackendContext);

When using the Vulkan backend the GrBackendObject field in GrBackendRenderTargetDesc and GrBackendTextureDesc is interpeted as a pointer to a GrVkImageInfo object. GrVkImageInfo specifies a VkImage and associated state (tiling, layout, format, etc). This allows the client to import externally created Vulkan images as destinations for Skia rendering via SkSurface factory functions or for to composite Skia rendered content using SkImage::getTextureHandle().

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.