This section explains how the generated mesh draw commands are converted into the final RHI commands.

Culling

First, the mesh draw commands are culled. However, this step is postponed so as not to interrupt the chain of FMeshBatch → FMeshDrawCommand → RHICommand. Hence, we will assume that the mesh draw commands are already culled.

We will talk about culling in this chapter.

Add Pass and Dispatch Draw

<aside> ℹ️ FDeferredShadingSceneRenderer::RenderPrePass

</aside>

Next, the core part of the whole rendering system can be found in the FDeferredShadingSceneRenderer::Render function in DeferredShadingRenderer.cpp. This function consists of about 2000 lines of code, so we will not explain each line. Instead, let's take another look at the depth prepass.

We will revisit the rendering pipeline later. For now, let's focus on the FDeferredShadingSceneRenderer::RenderPrePass function, which is called by FDeferredShadingSceneRenderer::Render. Finally, it calls DispatchDraw.

graph TD
  FDeferredShadingSceneRenderer::Render --> RenderPrepass
	subgraph RenderPrepass
			GetDepthPassParameters --> BuildRenderingCommands --> AddPass
	end
	subgraph AddPass
			DispatchDraw["View.ParallelMeshDrawCommandPasses[EMeshPass::DepthPass].DispatchDraw"]
	end
	

From Mesh Draw Command to RHI Command

As mentioned earlier, the Mesh Draw Command contains the necessary information to initiate one Draw Call.

However, this does not mean that one Mesh Draw Command corresponds to one RHI API call.

This is because the data required for the Draw Call, such as Vertex, Pixel Shaders, and Pipeline State, needs to be set through API calls.

In summary, when executing a Mesh Draw Command, multiple RHI API Commands are filled in the RHI Command List. However, there is usually only one Draw Call API Command for drawing, which is located at the end.

<aside> ℹ️ FParallelMeshDrawCommandPass::DispatchDraw

</aside>

Let’s take a close look of the mapping between FMeshDrawCommand and RHICommand , which is the content of View.ParallelMeshDrawCommandPasses[EMeshPass::DepthPass].DispatchDraw .

graph LR
  subgraph FMeshDrawCommand
		direction TB
		CachedPipelineId			
		VertexStreams
	end
	CachedPipelineId	--> MeshPipelineState 
	MeshPipelineState --> SetGraphicsPipelineState
	
	VertexStreams --> SetStreamSource
	MeshPipelineState --> BoundShaderState --> SetShaderBindings
	BoundShaderState --> SetShaderBindings1
	subgraph RHICommands
		direction TB
		SetGraphicsPipelineState
		SetStreamSource
		SetShaderBindings
		SetShaderBindings1["SetShaderBindings(PixelShader)"]
		DrawIndexedPrimitive
	end
	subgraph SetShaderBindings["SetShaderBindings(VertexShader)"]
		direction LR
		SetSamplerParameter --> AllShaderParameters
		SetSrvParameter-->AllShaderParameters
		SetTextureParameter-->AllShaderParameters
		SetLooseParameters --> AllShaderParameters
		AllShaderParameters--> SetShaderParameters
		AllShaderParameters(["AllShaderParameters"])
	end

<aside> ℹ️ FMeshDrawCommand::SubmitDraw

</aside>

This is a simplified version, if you don’t remember the mapping to the FMeshDrawCommand, check this part. And if you want to check the details, check this function: FMeshDrawCommand::SubmitDraw