Color saturation or gray scale implementation in OpenGL

To implement gray scaling we need to modify slightly the fragment shader. 

Conversion equation that I am using is,

Red * 0.299 + Green * 0.587 + Blue 0.144

The fragment shader used to do gray scaling is shown below,

                                                           
uniform sampler2D u_sampler;
uniform int u_compliment;

varying vec2 v_texturecoord;

void main()
{
    vec4 clr = texture2D(u_sampler, v_texturecoord);
    float l = clr.r*0.299 + clr.g*0.587 + clr.b*0.114;
    l = clamp(l, 0.0, 1.0);
    if (u_compliment == 1) {
        l = 1.0 - l;
    }                   
    gl_FragColor = vec4(l, l, l, clr.a);
}

For complete source code visit https://github.com/trsquarelab/glexamples