Custom Render Passes for Unreal Engine on iOS

Once I get the custom render pass for Unreal working fine on Windows, I started the painful journey to make it working on mobile (iOS to be specific).

To simplify, the custom render pass has two parts, the first one is computing shader to generate the instance data in the pre render base pass and then draw these instances in the base pass. For destop platform, it’s very straight forward to add the two custom passes into FDeferredShadingSceneRenderer::Render.

For the mobile platform, the compute pass added into FMobileSceneRenderer::Render and draw pass added into FMobileSceneRenderer::RenderDeferredMultiPass, which works as expected in Unreal editor preview on both Windows and Mac.

But while testing on iOS, it’s not working. Going into the GPU debug in XCode shows that the compute pass is working fine but the draw pass is never called. And then I learnt that for iOS, it’s using FMobileSceneRenderer::RenderDeferredSinglePass.

But adding the draw pass there after the SceneColorRendering pass is still not working on device and GPU debug shows that the draw pass is correctly run and the SceneColor/GBuffer result is as expected. The problem is that it’s too late, because SceneColor/GBuffer is already consumed for back buffer in the SceneColorRendering pass previously (yeah, it’s called for RenderDeferredSinglePass for a reason : ). So the correct way is to make the draw pass a subpass inside the SceneColorRendering pass. Or a better solution is to use unreal vertex factory for drawing our instances, more powerful and flexible, and also it will be handled by FMobileBasePassMeshPassProcessor, which then will be part of the subpass there.

A quick test with a simple vertex factory shows promising result, so I start to fully implement all our features into the vertex factory and then I start getting a crash error while starting the game.

[UE] Ensure condition failed: Texture [File:./Runtime/Renderer/Private/MeshPassProcessor.cpp] [Line: 828] Shader TBasePassVSFNoLightMapPolicy with vertex factory FMyCustomVertexFactory never set texture at BaseIndex 0. This can cause GPU hangs, depending on how the shader uses it.

Which is very wired because I don’t have any texture parameters in my shader. But surprisingly, after checking the compiled shader file for METAL, it turns out somehow my Buffer<uint> ReserveBuffer is translated to texture_buffer ReservedBuffer [[texture(0)]]! Some searching saying this is expected behavior, so for now my fix is to use StructuredBuffer<uint> in my shader.

Now, no crash or errors, but one feature is not working correctly on iOS. And I notice the data structure of this feature is not aligned with 16-bytes.

 struct FMyDataStructure 
{
    FVector3f Pos;
    FVector3f UpVector;
    float Height;
    float Radius;
}

So change it into

struct FMyDataStructure 
{
    FVector4f PosAndHeight;    // .xyz = Pos, .w = Height
    FVector4f UpVectorAndRadius; // .xyz = UpVector, .w = Radius
};

And now everything works on iOS : )