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Sampling at the Edges of Heightmaps for Normal Mapping

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I am creating heightmaps on the GPU and then using a Sobel filter to create a normal map for that heightmap. The Sobel filter works by sampling the 8 pixels surrounding a central pixel. The obvious problem is that when the central pixel is located on the edge of a heightmap the filter will not be able to sample all the edges. To resolve this problem I decided to expand the size of the heightmap from 256x256 pixels to 258x258 pixels. Then when I generate the heightmap for a particular quad I do the following: 

vec3 p = vec3(uv.x, 0.0, uv.y);

//size is the size of the current quad
p *= (size + 2);
p -= (1.0/(size + 2.0));
//'meshoffset' offsets the scaled meshes points from the origin
p += meshOffset;

when it comes to sample the heightmap I warp the UV coordinates like so: 

//the uv coordinates range from (0.0,0.0),(0.0, 1.0/17.0),(0.0, 2.0/17.0)...-> (1.0, 1.0)
uv *= (15.0/17.0);
uv += (1.0/17.0);

yet this still does not cause the edges to match up. 


Does anyone have any experience with this?


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