From cube, to the graphcis API

Rendering Hardware Interface

It is now time to use the RHI command. RHI stands for 'Rendering Hardware Interface'. It abstracts the real graphics API of each platform, like DirectX or Vulkan. You can treat it as a simple mapping to the graphics API. For example, RHIDrawIndexedPrimitive can be treated as DrawIndexedInstanced.

Do you remember how to render a triangle when you learned DirectX 11 or 12?

We need to set up the render pipeline state, set the shaders and parameters, and use DrawIndexedInstanced to dispatch a draw call.

If you forget, just find a hello triangle tutorial, for example:

"Direct3D 11 - Hello Triangle" - Anton's Graphics Tutorials

From Mesh Draw Command to RHI

We need to perform almost the same action for each FMeshDrawCommand. However, we should not directly call the API. Instead, we should add the command to a RHICommandList. This list will be sent to and executed by the RHI thread at a later time.

In our one cube, and the depth pass, it generates the following commands:

So our story becomes:

• The render thread takes the rendering data, including both the runtime data (render proxies from components) and the common data (render data from assets), and uses it to render the image.
  • For each FStaticMeshSceneProxy:
    • Retrieve many FMeshBatch objects.
    • For each FMeshBatch:
      • Build mesh draw commands for each pass.
  • For each pass, merge all the draw commands that belong to this pass.
  • For each pass:
    • Execute the related FMeshDrawCommand list.
      • Convert the mesh draw command into a set of RHICommand
      • Add those commands to RHI command list
      • Send the list to RHI thread
  • The RHI thread take the command list, and execute.

So let me show a summary image to conclude from FMeshBatch to FMeshDrawCommand and then to RHICommand.

A little more about RHI

RHI is not exclusive to the Unreal Engine; many engines implement their own RHI layer. Additionally, there are RHI implementations that serve as middleware, such as NvRHI.

https://github.com/NVIDIAGameWorks/nvrhi