godot-angle-static/src/libANGLE/renderer/vulkan/README.md

ANGLE: Vulkan Back-end

ANGLE's Vulkan back-end implementation lives in this folder.

Vulkan is an explicit graphics API. It has a lot in common with other explicit APIs such as Microsoft's D3D12 and Apple's 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.

Back-end Design

The RendererVk class represents an EGLDisplay. RendererVk owns shared global resources like the VkDevice, VkQueue, the Vulkan format tables and internal Vulkan shaders. The ContextVk 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

A render pass has three states: unstarted, started and active (we call it active in short), started but inactive (we call it inactive in short). 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.
• onRenderPassFinished: Puts render pass into inactive state where you can not record more commands into secondary command buffer, except in some special cases where ANGLE does some optimization internally.
• flushCommandsAndEndRenderPassWithoutSubmit: Marks the end of render pass. It flushes secondary command buffer into vulkan's primary command buffer, puts secondary command buffer back to unstarted state and then goes into recycler for reuse.

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 RendererVk::finishQueueSerial, ANGLE submits the primary command buffer to a VkQueue.

See the code 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
• Shader Module Compilation
• OpenGL Line Segment Rasterization
• Format Tables and Emulation
• Deferred Clears
• Queries