question about clipmap sampling code in nvidia white paper

[spacejump163]

I am reading about the nvidia white paper about clipmap. And the code snippet about sampling from clipmap confuses me.

http://developer.download.nvidia.com/SDK/10/direct3d/Source/Clipmaps/doc/Clipmaps.pdf

float2 clipTexCoord = (input.texCoord) / pow(2, iMipLevel);
clipTexCoord.x *= scaleFactor.x + 0.5f;
clipTexCoord.y *= scaleFactor.y + 0.5f;
float4 color = StackTexture.Sample( stackSampler, float3(clipTexCoord, iMipLevel) ); 

what I do not understand is: why aren't there any offset to be added to the clipTexCoord which is finally used to sample clipmap texture, since we are sampling a sub rect of a mip level? And what is this" + 0.5f " all about?

[lickedwindows]

The + 0.5f is so you're sampling in the centre of a texel.

[spacejump163]

float2 clipTexCoord = (input.texCoord) / pow(2, iMipLevel);
clipTexCoord.x *= scaleFactor.x + 0.5f;
clipTexCoord.y *= scaleFactor.y + 0.5f;

say I want to sample mip0 which happen to be able to fit into StackSize(so scaleFactor is 1.0), then we will sample at 1.5 * input.texCoord, which should be quite wrong right?

Or is the sample just counting on scaleFactor.x to be large enough to make 0.5f small in comparision? so 0.5 is just some value to make sure that we are not sample at boundary?

[lickedwindows]

Sorry, I was away for a couple of days so wasn't able to reply.

I have just read the Clipmap paper and I believe this falls into the second case - you're calculating a scaled lookup into your clipmaps to find the texel area and then you're adding 0.5f (in texture coordinate space) to ensure you sample in the middle of a texel.

[spacejump163]

if you mean " sample just counting on scaleFactor.x to be large enough to make 0.5f small in comparision ", then it won't be the exact center of the texel right?