Implementing new render pass questions

Hi all,

I am hoping that some members of the community have some suggestions to help me get a new render pass implemented. I’m new to Unreal, but have many years of experience as a rendering engineer working on bespoke engines. I’ve been able find a number of resources online which touch on some important points, however I’m finding it very slow to understand how all the pieces holistically actually fit together.

I’m attempting to implement a new render pass which precedes the base pass that does a visibility buffer style render technique and can blend multiple materials together into the GBuffer. At this stage I’m just trying to get a prototype put together for the standard main view (I realize that I will have more work to do in time to support Lumen etc.). I’ve gotten pretty far with my technique - I implemented a new EMeshPass, a new mesh processor, and all the necessary global shaders to generate all the data I need to apply my visibility buffer to the fullscreen passes to blend the various materials together.

The trouble is in the final pass to invoke the material shaders. I really don’t want to make any changes to BasePassPixelShader.usf if I can avoid it. Instead, I’m trying to write a new .usf that is a trampoline into the base pass pixel shader which decodes my visibility buffer on entry into the shader and applies the appropriate weighting to the results of the base pass in FPixelShaderOut. On paper, it all makes sense but this is proving really difficult as the shader code and bindings feel really intertwined with the flow of FBasePassMeshProcessor. In my case, the material application is just a fullscreen pass - so I’m trying to just get something bootstrapped that is more of an “immediate” style rendering execution.

Effectively, what I’m trying to write is:
GraphBuilder.AddPass(…
{
// lambda pseudo code…
FGraphicsPipelineStateInitializer PSO; // ← set this guy up
SetGraphicsPipelineState(RHICmdList, PSO);
SetShaderBindings(); // ?
FPixelShaderUtils::DrawFullscreenTriangle();
});

It feels so straight forward that I should just be able to “extract” my shader bindings from the material and I’m good to go…

I run into problems, however, if I were to attempt to replicate the logic inside FMeshDrawCommand: where if I were to just do something like put a FMeshDrawShaderBindings on the stack and attempt to call GetShaderBindings() I immediately run into problems where there is no render proxy, because again I want to just draw a fullscreen pass.

I’ve also tried to re-implement this by making my pixel shader a FMaterialShader instead of a FMeshMaterialShader, however that doesn’t work either as again some bindings population is tightly coupled to FMeshDrawCommand.

Another attempt was to pattern match how the post process materials work, but because I am using the BasePassPixelShader I run into problems where I don’t have a vertex factory and I can’t compile my shaders.

My technique is very much like how Nanite does it’s GBuffer population so I took a look in that code, but I got a little scared away there as it is a huge system and I’m just trying to learn how to appropriately setup my shader bindings.

Again, I’m still struggling to see all the connections in the render pipeline so that I can chart an appropriate course, and I’m hoping somebody here might have a suggestion for how to proceed. Some high level questions:

Should my pixel shader be a FMeshMaterialShader? Does this mean that I need to go full Nanite and pre-build draw commands and somehow invent render proxies? Again, this felt super overkill to just draw a pass…

Should my pixel shader be a FMaterialShader? If so, how do I appropriately get shader bindings? How do I resolve the issue with the vertex factory?

Is there a simpler example than Nanite that I can pattern match against which kind of does what I want: use a fullscreen pass with the base pass pixel shader?

Bump, I am curious about this as well! :slight_smile:

I spent the whole day taking notes on how things work, but I’m no closer to a solution. I really, really wish some of this was documented.

I feel like there are only two choices here, both of which seem pretty unpleasant to me:

  1. Fully embrace the FMeshMaterialShader and have my own vertex factory and do the draws as part of a FMeshDrawCommand as opposed to just rolling my own fullscreen pass. I guess the natural evolution here is to just do exactly what Nanite is doing have my vertex factory do the decode of the visibility buffer and have a few special hooks in BasePassPixelShader just like the IS_NANITE_PASS. I kind of hate that, but feels like the least of all evils? I also don’t really know where I want to store my mesh draw commands. Nanite keeps them in the scene when a Nanite enabled proxy enters the scene so it registers the material pass. I realize that ultimately I would need something like that to cache my commands, but man… I just want a prototype of a single draw on the screen… This feels like another week of work just to see a single screen pass… Uhg.

  2. Abandon the idea of trying to re-use BasePassPixelShader.usf. I hate this idea because I don’t want to try to keep a custom shader implementation at parity with the core material shader that Unreal uses. Especially since that thing is a kitchen sink of everything, it just feels like fail.

Here are my notes, trying to understand things from the bottom up. I have a few unanswered questions in here. Maybe others in the community can tell me where I got things wrong:
Let’s figure out how the system fits together, starting from the bottom up - using FMeshDrawCommand as the guide.

How does the RHI address shader bindings?

Looks like it’s a DX11 style API where resources are bound to a specific slot, not a DX12 approach where it’s a potential root signature slot + descriptor table offset. This might exist under the hood in the DX12 RHI, I don’t think it’s important for this research to learn how the root signature is organized and constructed.

What resource types does the RHI interact with?

  • Uniform Buffer (constant buffer)
  • Sampler state
  • SRV. There is a distinction here between “FRHIShaderResourceView” and “FRHITexture,” yet both are managed in the same bucket in the FReadOnlyMeshDrawSingleShaderBindings as “SRVs.”

Where are the actual resources stored?

FShaderBindingState manages some arrays of pointers which are contextual based on what the shader needs. Wow, this thing is huge.

How do we know what slot things should be bound at?

FReadOnlyMeshDrawSingleShaderBindings manages this. I don’t know yet how to make this thing, but it understands how to distribute bindings, presumably based on what the shader dictates. It has homogeneous sections for uniform buffers, sampler states, SRVs, and “loose parameters.” Looks like a loose parameter is a constant buffer value which can overlay onto the other memory for a uniform buffer? Is the intention of this to support root constants in DX12? This seems like a really weird approach.

Why do loose parameters exist?

Looks like the DX12 implementation is a constant buffer update, not a root constant. What is a reason to actually do this? This seems terrible.

How is FReadOnlyMeshDrawSingleShaderBindings filled out?

This is just a wrapper of FMeshDrawShaderBindingsLayout to make sure you don’t modify data.

How is FMeshDrawShaderBindingsLayout filled out?

These exist in FMeshDrawShaderBindings and are initialized inside FMeshDrawShaderBindings::Initialized() given a specific FMeshProcessorShaders. Looks like the ctor is passed a FShader and it initializes FShaderParameterMapInfo based on FShader::ParameterMapInfo.

How is FShader::ParameterMapInfo filled out?

FShader::BuildParameterMapInfo() - this appears to happen as the final part of shader compilation. The map which is passed in comes from CompiledShaderInitializerType::ParameterMap.GetParameterMap().

How is CompiledShaderInitializerType’s Parameter map filled out?

The constructor gets it from FShaderCompilerOutput.ParameterMap

How is FShaderCompilerOutput.ParameterMap filled out?

These come from the backend shader compiler, so D3DShaderCompiler.inl has ExtractParameterMapFromD3DShader()

Wait, so how does that relate to FShader? How are param bindings on the C++ side actually matched against the shader compiler?

So, I guess that’s in FMeshDrawCommandStateCache? So FMeshDrawShaderBindings has a data pointer and it looks like that is casted to the appropriate thing based on the ParamMapInfo.

So, how does FMeshDrawShaderBindings::GetData() work?

FMeshDrawShaderBindings::Finalize() does validation, so it must be before that. It’s inside FMeshMaterialShader::GetShaderBindings(). The calls to ShaderBindings.Add() are what write to the data pointer.

So, what is the point of the global param structures that we pass into the graph builder?

Looks like the static uniform buffers are pulled from the params which are pulled in… Nothing else!

What defines a static uniform buffer, non-static one, or global?

Can FMeshMaterialShader::GetShaderBindings() fill in static uniform buffers?

Don’t think so… why is the shader not declarative over this?

Some of the stuff filled in FMeshMaterialShader::GetShaderBindings() need stuff like FMeshMaterialShaderElementData, where does that get filled out?

Those are just put on the stack and filled out based on what the mesh processor wants. See FEditorPrimitivesBasePassMeshProcessor::ProcessDeferredShadingPath() where it just makes a TBasePassShaderElementData ShaderElementData(nullptr); on the stack.

How is FShaderBindingState filled out?

Why is LAYOUT_FIELD() so pervasive, what does it actually do?

Makes a FFieldLayoutDesc which is a linked list of metadata in that struct.

Why is the FFieldLayoutDesc useful?

I spent more time reading how Nanite does it’s gbuffer application pass and decided to give up on that approach. My intuition is that it’s just the wrong approach. I’ve gone back to what I strongly feel should just be a super simple operation, yet I am still missing something…

My current code looks like this:

BEGIN_SHADER_PARAMETER_STRUCT(FMyShaderPassParameters, )
   SHADER_PARAMETER_STRUCT_INCLUDE(FViewShaderParameters, View)
   SHADER_PARAMETER_RDG_UNIFORM_BUFFER(FOpaqueBasePassUniformParameters, BasePass)
   ...
   RENDER_TARGET_BINDING_SLOTS()
END_SHADER_PARAMETER_STRUCT()

class FMyShaderPS : public FMeshMaterialShader
{
   DECLARE_SHADER_TYPE(FMyShaderPS, MeshMaterial);
   using FParameters = FMyShaderPassParameters;
   SHADER_USE_PARAMETER_STRUCT(FMyShaderPS, FMeshMaterialShader);
};

Then later when I get to the RDG part of things I attempt to fill out a FMyShaderPassParameters and then set that onto the command list in the lambda. Seems straight forward, right?

The problem I’m currently hitting is that I’m being told that a uniform buffer named “Material” doesn’t exist in my pass parameters. I can only guess this is introduced in FMaterial::SetupMaterialEnvironment where it does this:

	// Add the material uniform buffer definition.
	FShaderUniformBufferParameter::ModifyCompilationEnvironment(TEXT("Material"), InUniformBufferStruct, Platform, OutEnvironment);

Which naturally injects that into what the shader will compile for the material parameters. That makes sense. What I don’t see, however, on the runtime side is how I should avoid this particular failure. In FMaterial::SetParameters() I see that it’ll appropriately bind the “Material” uniform buffer, but the fact that I am failing where I am implies to me that I’m just doing something fundamentally wrong.

It feels like there are different styles of binding shader parameters, and if I take BasePassRendering as an example, it only sets up some parameters up through RDG, and some get baked as part of the mesh draw command. I also notice that the FMeshMaterial shaders used by BasePassRendering don’t actually use the SHADER_USE_PARAMETER_STRUCT() macro, how do they fill out their FShaderParameterBindings?

Again, it’d be great if any of this were documented as it’s really difficult to read this code since so much is done in macros, worse some of which stringify names together so you can’t even search the code to find what you’re looking for!

Okay, next roadblock… It sure feels like this is impossible.

As I see it, I have two options for my PS:

  • FMeshMaterialShader - this doesn’t seem possible to use because if I use DECLARE_SHADER_TYPE(…, MeshMaterial); then it won’t actually get to FShaderParameterBindings::BindForLegacyShaderParameters() and I’ll be left with a shader that has no parameter bindings. In that situation, I am pretty much broken because either:

    a) I use the SHADER_USE_PARAMETER_STRUCT() macro and the “Material” entry isn’t filled out, and I can’t get past startup

    b) I don’t use SHADER_USE_PARAMETER_STRUCT() and instead attempt to just fill that out manually by allocating it off the RDG and then bind it in the submission lambda with SetParameters(), which then fails in ValidateShaderParameters() (ShaderParameterStruct.cpp) because the shader bindings haven’t been populated yet (due to not using the appropriate shader type).

  • FMaterialShader - this won’t work because there is a deep assumption that if you have a vertex factory then you must be a FMeshMaterialShader, and I need a vertex factory in order to use BasePassPixelShader.usf.