Shaders to render normals : NOT!

A useful debugging tool is to get a colour representation of the normals in a scene. With a bit of maths, it can also produce some nice, free colouring for an intro. Above is a correct image. It shows a whole bunch of spheres. Red is the x component of the normal, green the y and blue is z. Study it and you'll see its right. Only it took me a few hours to get this right. I made a very simple error, which maybe others do too so I thought I'd document it here.

Hmmm, this is what I actually got:

Pretty,yes. Wrong, definitely. Here is the WRONG CODE. This code has a serious bug:

const GLchar *normalsvsh="\
varying vec3 n;\
void main(){\
n = normalize(gl_Normal*gl_NormalMatrix);\
gl_Position=ftransform();\
}";

const GLchar *normalsfsh="\
varying vec3 n;\
void main(){\
gl_FragColor = vec4((n+1.0)*0.5,1);\
}";

Can you spot the error? Heres a clue, the spheres are rotating about their centres at different speeds.

The solution is...

n = normalize(gl_NormalMatrix*gl_Normal);\