How do I convert a photo from color to black and white using OpenGL?

Question

Lets say we see an image of people watching tv in color and when I add the OpenGL code it turns into this:

http://i.dailymail.co.uk/i/pix/2008/10/18/article-1078627-02281598000005DC-210_468x382.jpg

What is the code to convert the photo from color to black and white?

Answer 1

Temptative answer : use a glsl shader

float luminance = (in.r+in.g+in.b)/3.0;
gl_FragColor = vec4(luminance, luminance, luminance, 1.0);

but this is one lame question