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Hiz buffer generation in UE4

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Currently, I was trying to use hiz occlusion query to tuning my engine. I make a implementation and compare it with UE4 implementation, I found some difference, and I can't figure why. So I was here to ask for help.

 

 

Here is implementation from UE4

const float2 InvSize = 1.0f / InputTextureSize;
const float2 InUV = RenderTargetUVAndScreenPos.xy;

UV[0] = InUV + float2(-0.25f, -0.25f) * InvSize;
UV[1] = InUV + float2( 0.25f, -0.25f) * InvSize;
UV[2] = InUV + float2(-0.25f,  0.25f) * InvSize;
UV[3] = InUV + float2( 0.25f,  0.25f) * InvSize;


Depth.x = Texture.SampleLevel( TextureSampler, UV[0], 0 ).r;
Depth.y = Texture.SampleLevel( TextureSampler, UV[1], 0 ).r;
Depth.z = Texture.SampleLevel( TextureSampler, UV[2], 0 ).r;
Depth.w = Texture.SampleLevel( TextureSampler, UV[3], 0 ).r;

This one is my implementation

const float2 InvSize = 1.0f / OutputTextureSize;
const float2 InUV = ScreenPos.xy / ViewportSize;
const float2 UvWithOffset = InUV + float2(-0.25f, -0.25f) * InvSize;
float4 Depth = Texture.GatherRed(TextureSampler, UvWithOffset);

There are two major differences:

  1. UE4 use InputTextureSize as denominator, but I use OutputTextureSize to calculate the InvSize. Let's go closer to talk about this, when I have a 1024x1024 sized input texture, output texture will be 512x512. When I evaluate the depth value for position(10,10). InUV will be (10.5,10.5), in this case, I want to sample texels (20.5,20.5),(21.5,20.5),(20.5,21.5),(21.5,21.5) from input texture. So from math perspective, I should evaluate new Uv with (10.5/512-0.25/512)*1024.
  2. UE4 use SampleLevel and I use GatherRed. I don't understand why they didn't use Gather API, in my opinion, Gather* should be faster than SampleLevel, right?

 

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