# 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. 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 [`vk::Renderer`](Renderer.cpp) class represents an `EGLDisplay`. `vk::Renderer` 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

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 `CommandResources`
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
`vk::Renderer::finishQueueSerial`, 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::CommandResources resources;
    resources.onBufferTransferWrite(dstBuffer);
    resources.onBufferTransferRead(srcBuffer);

    // Get a pointer to a secondary command buffer for command recording.
    vk::OutsideRenderPassCommandBuffer *commandBuffer;
    ANGLE_TRY(contextVk->getOutsideRenderPassCommandBuffer(resources, &commandBuffer));

    // Record the copy command into the secondary buffer. We're done!
    commandBuffer->copyBuffer(srcBuffer->getBuffer(), dstBuffer->getBuffer(), copyCount, copies);
```

## Handling EGL image offsets

A `Texture` can have a number of levels and layers. An EGL image may be created out of the texture,
viewing a specific level and layer specified by `EGL_GL_TEXTURE_LEVEL_KHR` and
`EGL_GL_TEXTURE_ZOFFSET_KHR`. The EGL image can then be imported into another texture whose own
state does not include these offsets (e.g. the texture is accessed at level 0). It may similarly be
imported into a renderbuffer that doesn't even have a level and layer state.

The `TextureVk` and `RenderbufferVk` classes use `gl::LevelIndex`, `gl::LayerIndex`, and
`gl::ImageIndex` to refer to the level and layer from the point of view of the
texture/renderbuffer itself, i.e. these values will be 0 for an EGL image target. When these values
are needed as integers, e.g. to access an array, they are retrieved via `getUntranslated()`.

However, any references outside these classes to levels and layers, i.e. those that are paired with
`VkImage` and `VkImageView`, must use the source texture's original level and layer. This is done
via the `mState.toSource*()` helpers, which return a strongly typed `gl::OwnerLevel`,
`gl::OwnerLayer` and `gl::OwnerImageIndex`. The level and layer can be retrieved from these types
with a `get()`.

Ideally, _all_ the code in the front-end would use `gl::Own*` types and all the code in the backend
outside these classes would use `gl::Source*` for maximum safety, where the compiler ensures there
are no mistakes. This is not yet done, but in the meantime, any reference to level and layer in the
front-end is implicitly from the point of view of the texture/renderbuffer itself (i.e. equivalent
to `gl::Own*`). Any such reference outside `TextureVk` and `RenderbufferVk` in the backend is
implicitly from the point of view of the owner of the `VkImage` (i.e. equivalent to `gl::Source*`).

The code in `TextureVk` and `RenderbufferVk` must always use `mState.toSource*` to convert the types
before using a level or layer in conjunction with an `ImageHelper`, `ImageViewHelper`,
`RenderTargetVk`, `UtilsVk` etc calls.

## 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)
- [Format Tables and Emulation](doc/FormatTablesAndEmulation.md)
- [Deferred Clears](doc/DeferredClears.md)
- [Queries](doc/Queries.md)
- [Present Semaphores](doc/PresentSemaphores.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