This image is associated with the "Images and memory use?" thread in
p.general. The two panels in the image show the front of a box with ordinary
diffuse shading and a simple normal map. The box is lit by three lights,
red, green and blue, which are shining on to the box from the top-right,
bottom-left and bottom-right directions respectively. The box in the left
panel uses this for a normal:

normal {
  bump_map {
    png "bumptestin.png"
    interpolate 2
  }
}

bumptestin.png is just a couple of black circles drawn on a white
background. You can also do this with cells or another pattern instead of
the bump_map, it still "works".

The box on the right uses this:

#declare ImPig = pigment {
  image_map {
    png "bumptest.png"
    interpolate 2
  }
}

#declare ImFunc = function {
  pigment { ImPig }
}

normal {
  function { ImFunc(x,y,z).gray }
}

I don't understand why they look so different. I tried reducing the bump
strength using bump_size, but I couldn't get one to look like the other.
Lights, camera and geometry did not change. Also, if you use these two as
pigments instead of normals then they produce absolutely identical images.
What am I doing wrong?

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