//  Fragment ( Saturation )

#define vec1  float

const vec3 L     = vec3( 0.2125, 0.7154, 0.0721 );  //  Luminance

uniform sampler2D S;
uniform vec1      U;  //  Saturation

varying vec2      T;

void 
main( void ) 
{ 
    vec3 C       = vec3( texture2D( S, T ).rgb );

    gl_FragColor = vec4( mix( vec3( dot( C, L ) ), C, U ), 1.0 );
}

//  Fragment ( Luminance )

#define vec1  float

const vec3 L     = vec3( 0.2125, 0.7154, 0.0721 );  //  Luminance

uniform sampler2D S;

varying vec2      T;

void 
main( void ) 
{ 
    vec1 C       = vec1( dot( texture2D( S, T ).rgb, L ) );

    gl_FragColor = vec4( C, C, C, 1.0 );
}

//  Fragment ( Inversion )

uniform sampler2D S;

varying vec2      T;

void 
main( void ) 
{ 
    gl_FragColor = vec4( ( 1.0 - texture2D( S, T ).rgb ), 1.0 );
}

//  Fragment ( Default )

uniform sampler2D S;

varying vec2      T;

void 
main( void ) 
{ 
    gl_FragColor = vec4( texture2D( S, T ).rgb, 1.0 );
}

//  Fragment ( Palette )

uniform sampler2D I;  //  Index   ( i.e. RED  = R, 0, 0, 1 )
uniform sampler1D P;  //  Palette ( i.e. RGBA = R, G, B, A )

varying vec2      T;

void
main( void )
{
    gl_FragColor = vec4( texture1D( P, texture2D( I, T ).r ) );
}