Discarding Fragments

It is possible to tell the fragment shader it shouldn't write any pixel. This can be done using the "discard" statement.

This can be used - for example - to implement a color key.

Using the following texture, all red pixels should be removed (discarded!).

Texture with with red color key

varying vec2 vTexCoord;

void main(void)
{
   vTexCoord = gl_MultiTexCoord0.xy;
   gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
}        

sampler2D myTexture;
           varying vec2 vTexCoord;

void main (void) 
{  
   vec4 color = texture2D(myTexture, vTexCoord); 
             
   if (color.rgb == vec3(1.0,0.0,0.0))
      discard; 
   
   gl_FragColor = color;
} 

If you start the shader you will get the following result:

There are still some red pixels visible. This is because the texture was filtered. This can be prevented by setting the texture parameters to:

glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_NEAREST);

This time it looks better.