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GLSL and texture coords...

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Hey, thanks for reading this but I have a question regarding GLSL and texture coords. Say I have the following shader program:
//note, 128.0 is the size of the texture width, hardcoded for now, needs to change
//to uniform variable.
void bilerp(in vec2 coords, out vec4 qtynew, in sampler2D qty)
  //figure out the texture coords
  vec4 st;
  st.xy = coords-1.0/128.0; //implied translation into texture simulation domain.
  st.zw = st.xy + 1.0/128.0; //top right corner of the "neighbor hood"
  //now access the texture
  vec4 t11 = texture2D(qty,st.xy);
  vec4 t21 = texture2D(qty,st.zy);
  vec4 t12 = texture2D(qty,st.xw);
  vec4 t22 = texture2D(qty,st.zw);
  //now lerp
  vec2 factor = (coords - st.xy)*128.0; //Factor is in simulation domain!
  //bilerp, such that, we compute lerp:
  //l1 = 11 to 21, l2 = 12 to 22, lerping long the x axis.
  //final = l1 to l2, lerping along the y axis. Please refer to any documentation on bilerp.
  qtynew = mix(mix(t11,t21,factor.x),mix(t12,t22,factor.x),factor.y);
Where is exactly is the "center" of the texture coord? In Mark Harris' GPU gem article, he uses CG. And he does this by adding a 0.5, which according to his comments is due to the fact that the center of a texel is 0.5, i.e 0.5, 1.5, ... Mark Harris had something like this:
float4 f4texRECTbilerp(samplerRECT tex, float2 s)


  float4 st;

  st.xy = floor(s - 0.5) + 0.5;

  st.zw = st.xy + 1;


  float2 t = s - st.xy; //interpolating factors 


  float4 tex11 = f4texRECT(tex, st.xy);

  float4 tex21 = f4texRECT(tex, st.zy);

  float4 tex12 = f4texRECT(tex, st.xw);

  float4 tex22 = f4texRECT(tex, st.zw);

  // bilinear interpolation

  return lerp(lerp(tex11, tex21, t.x), lerp(tex12, tex22, t.x), t.y);

Notice his offset of 0.5. So how does GLSL translate? I don't notice any difference from toggling between 0.5 and 1.0.

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The Cg instruction tex2DRECT was used for npot textures. Normal texture coordinates are "percentual", 0.0 = left, 1.0 = right, etc. TexRECT used real pixel coordinates. If you want the center coordinate of a 800x600 texture, the texcoords are not {0.5,0.5}, but {400,300}. So, in this context adding 0.5 really means adding half a pixel.

You are using "normal" tex2D functions, so this trick is not working here. If you shift with 0.5, you will pick a pixel 50% of the image width further. In your case, a half pixel =
0.5 / image.width , 0.5 / image.height


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