# ANGLE: Vulkan Back-end ANGLE's Vulkan back-end implementation lives in this folder. [Vulkan](https://www.khronos.org/vulkan/) is an explicit graphics API. It has a lot in common with other explicit APIs such as Microsoft's [D3D12][D3D12 Guide] and Apple's [Metal](https://developer.apple.com/metal/). Compared to APIs like OpenGL or D3D11 explicit APIs can offer a number of significant benefits: * Lower API call CPU overhead. * A smaller API surface with more direct hardware control. * Better support for multi-core programming. * Vulkan in particular has open-source tooling and tests. [D3D12 Guide]: https://docs.microsoft.com/en-us/windows/desktop/direct3d12/directx-12-programming-guide ## Back-end Design The [`RendererVk`](RendererVk.cpp) class represents an `EGLDisplay`. `RendererVk` owns shared global resources like the [VkDevice][VkDevice], [VkQueue][VkQueue], the [Vulkan format tables](vk_format_utils.h) and [internal Vulkan shaders](shaders). The [ContextVk](ContextVk.cpp) class implements the back-end of a front-end OpenGL Context. ContextVk processes state changes and handles action commands like `glDrawArrays` and `glDrawElements`. ## Command recording The back-end records commands into command buffers via the following `ContextVk` APIs: * `beginNewRenderPass`: Writes out (aka flushes) prior pending commands into a primary command buffer, then starts a new render pass. Returns a secondary command buffer *inside* a render pass instance. * `getOutsideRenderPassCommandBuffer`: May flush prior command buffers and close the render pass if necessary, in addition to issuing the appropriate barriers. Returns a secondary command buffer *outside* a render pass instance. * `getStartedRenderPassCommands`: Returns a reference to the currently open render pass' commands buffer. The back-end (mostly) records Image and Buffer barriers through additional `CommandBufferAccess` APIs, the result of which is passed to `getOutsideRenderPassCommandBuffer`. Note that the barriers are not actually recorded until `getOutsideRenderPassCommandBuffer` is called: * `onBufferTransferRead` and `onBufferComputeShaderRead` accumulate `VkBuffer` read barriers. * `onBufferTransferWrite` and `onBufferComputeShaderWrite` accumulate `VkBuffer` write barriers. * `onBuffferSelfCopy` is a special case for `VkBuffer` self copies. It behaves the same as write. * `onImageTransferRead` and `onImageComputerShadeRead` accumulate `VkImage` read barriers. * `onImageTransferWrite` and `onImageComputerShadeWrite` accumulate `VkImage` write barriers. * `onImageRenderPassRead` and `onImageRenderPassWrite` accumulate `VkImage` barriers inside a started RenderPass. After the back-end records commands to the primary buffer and we flush (e.g. on swap) or when we call `ContextVk::finishToSerial`, ANGLE submits the primary command buffer to a `VkQueue`. See the [code][CommandAPIs] for more details. ### Simple command recording example In this example we'll be recording a buffer copy command: ``` // Ensure that ANGLE sets proper read and write barriers for the Buffers. vk::CommandBufferAccess access; access.onBufferTransferWrite(dstBuffer); access.onBufferTransferRead(srcBuffer); // Get a pointer to a secondary command buffer for command recording. vk::OutsideRenderPassCommandBuffer *commandBuffer; ANGLE_TRY(contextVk->getOutsideRenderPassCommandBuffer(access, &commandBuffer)); // Record the copy command into the secondary buffer. We're done! commandBuffer->copyBuffer(srcBuffer->getBuffer(), dstBuffer->getBuffer(), copyCount, copies); ``` ## Additional Reading More implementation details can be found in the `doc` directory: - [Fast OpenGL State Transitions](doc/FastOpenGLStateTransitions.md) - [Shader Module Compilation](doc/ShaderModuleCompilation.md) - [OpenGL Line Segment Rasterization](doc/OpenGLLineSegmentRasterization.md) - [Format Tables and Emulation](doc/FormatTablesAndEmulation.md) - [Deferred Clears](doc/DeferredClears.md) - [Queries](doc/Queries.md) [VkDevice]: https://www.khronos.org/registry/vulkan/specs/1.1-extensions/man/html/VkDevice.html [VkQueue]: https://www.khronos.org/registry/vulkan/specs/1.1-extensions/man/html/VkQueue.html [CommandAPIs]: https://chromium.googlesource.com/angle/angle/+/df31624eaf3df986a0bdf3f58a87b79b0cc8db5c/src/libANGLE/renderer/vulkan/ContextVk.h#620