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Using gatheralpha for alpha test - problem with BC3 (DXT5) textures

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Hi - I've implemented a pretty effective alpha test using Texture2D.GatherAlpha, works even better with alpha-to-coverage enabled.  If it gets past the clip(), i multiply the scalar alpha return value with the final color.

cbuffer PAlphaTest : register (cb2)
{
  float4 alphaTestThresh;
};

float MainSampleAlphaTest (Texture2D tex, SamplerState samp, float2 texCoord)
{
  float4 alphaSamp = tex.GatherAlpha (samp, texCoord, int2(0,0));
  float alpha = dot (alphaSamp, 0.25);
  clip (alpha - alphaTestThresh.x);
  return (alpha);
}

WIth a DXGI_FORMAT_R8G8B8A8_UNORM_SRGB texture I get this:

 

[attachment=28982:a0.jpg]

 

WIth a DXGI_FORMAT_BC3_UNORM_SRGB texture I get this:

 

[attachment=28983:a1.jpg]

 

Strange bunch of lines with the BC3 texture - anyone seen this effect before?

Thanks.

 

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I've seen driver bugs in the past when gather is involved. Did you test that you actually get the proper results if you issue four regular samples?


I agree, it sounds like a driver bug. You can also try using the reference device to see if that produces the correct results. If it does, then it's probably a driver bug.

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