GLSL lighting problem (normals)

Hello,

Last week I've run into a problem but can't find what's causing the problem.

The problem is as follows. I have a sphere and a light source. When the sphere isn't rotated, the lighting looks fine:


But as soon as I try to rotate it, the lighting get's all wrong. The picture below shows the sphere when it's rotated 90 degrees around the y-axis.



My first thought is that for some reason the normals aren't rotated at all or not rotated correctly.

I'm using GLSL and OpenGL 2.

I think shaders sources are useful:
vertex shader
[source]
// The diffuse value
varying float diffuse_value;

// The normalized light position
uniform vec3 NormalizedLightDirection;

void main()
{
// Calculate the actual normal
vec3 vertex_normal = normalize(gl_Normal * gl_NormalMatrix);

// Calculate the diffuse value
diffuse_value = max(dot(vertex_normal, NormalizedLightDirection), 0.0);

// Set vertex position
gl_Position = gl_ModelViewProjectionMatrix * (gl_Vertex);

gl_FrontColor = gl_Color;
}
[/source]

fragment shader:
[source]
varying float diffuse_value;

uniform vec4 DiffuseColor;

void main()
{
// Set the final color by multiplying the diffuse color with the diffuse value and then adding the ambient lighting
gl_FragColor = vec4(1.0) * diffuse_value;
}
[/source]

I appreciate any advice anybody can give me.

Correct this
vec3 vertex_normal = normalize(gl_NormalMatrix * gl_Normal );
OpenGL use column major matrices (same as your gl_Position calculation)

Correct this
vec3 vertex_normal = normalize(gl_NormalMatrix * gl_Normal );
OpenGL use column major matrices (same as your gl_Position calculation)