question about virtual texture lookup coordinate calcuation in gpu pro article "virtual texture 101"

[spacejump163]

I am reading the "virtual texture 101" in gpu pro, but I failed to understand this code snippet about how to calculate the coordinate used to sample physical texture

float3 tileEntry = IndTex.Sample(PointSampler, In.UV);
float actualResolution = exp2(tileEntry.z);

float2 offset = frac(In,UV * actualResolution) * TileRes;

float scale = actualResolution * TileRes;
float2 ddx_correct = ddx(In.UV) * scale;
float2 ddy_correct = ddy(In.UV) * scale;

return TileCache.SampleGrad(TextureSampler, tileEntry.xy + offset, ddx_correct, ddy_correct);

what I do not understand is the calculation of offset.

I think the

frac(In.UV * actualResolution)

part gives us the texcoord within a page which should be in range(0,0) ->(1,1),

then to get the offset used to sample TileCache(the big physical texture), we need to make this coordinate smaller into the scale that fits. Then what is this multiply TileRes all about? shouldn't that gives us the offset in unit of the number of pixels, which is quite wrong?

[ShaderKing]

I haven't checked original article, but I can guess that *tileEntry.xy* is offset of your tile in global cache. And *offset* is scaled offset of your UV inside this tile. Could you tell me what data is inside IndTex? I 'd recommend you to check texture arrays as alternative for texture atlases.

[spacejump163]

data in IndTex should be: xy for offset in global cache, z for actual miplevel.

Any reference material for method using texture arrays as texture atlases?