10. Diffuse lighting in vertex shader

Vertex shader

#version 130

in vec4 s_vPosition;
in vec4 s_vNormal;

uniform mat4 mM;    // model matrix
uniform mat4 mV;    // camera view matrix
uniform mat4 mP;    // perspective matrix

uniform mat4 mRotations; // all model rotations matrix

uniform vec4 vLight; // direction of light

smooth out vec4 interpolated_color;

void main () {
    
    // Rotate the normal and use as vec3:
    // No need tp normalize after rotation since it remains a unit vector:
    vec3 normal_in_cam_space = (mRotations*s_vNormal).xyz;
    // The angle between the surface normal and the light source vector
    // is called the angle of incidence of the light:
    float cos_incidence = dot( normal_in_cam_space, vLight.xyz );
    // Note that cosine of the angle of incidence can become negative
    // for angles greater than 90 degrees, hence we use the clamp() to
    // keep it in range from zero to one:
	cos_incidence = clamp( cos_incidence, 0, 1 );
    vec4 light_color = vec4( 1.0, 1.0, 0.0, 1.0 ); // R + G = yellow
    interpolated_color = vec4( light_color.rgb * cos_incidence, 1.0);

    gl_Position = mP*mV*mM*s_vPosition;
}

Fragment shader

#version 130

smooth in vec4 interpolated_color;

out vec4 fColor;

void main()
{
	fColor = interpolated_color;
}