3.11. Code for mapping RGB triples to 4 bits   

From Glassner (1990)

/*    Mapping RGB Triples onto Four Bits by Alan Paeth

      from "Graphics Gems", Academic Press, 1990

 */


remap8(R, G, B, R2, G2, B2)

    float R, G, B, *R2, *G2, *B2;

    {

/*

 * remap8 maps floating (R,G,B) triples onto quantized

 * (R2,B2,B2) triples and returns the code (vertex)

 * value/color table entry for the quantization. The

 * points (eight) are the vertices of the cube.

 */

    int code;

        *R2 = *G2 = *B2 = 0.0;

        code = 0;

        if (R >= 0.5) { *R2 = 1.0; code |= 1; }

        if (G >= 0.5) { *G2 = 1.0; code |= 2; }

        if (B >= 0.5) { *B2 = 1.0; code |= 4; }

        return(code);

    }


/*

 * remap14 maps floating (R,G,B) triples onto quantized

 * (R2,B2,B2) triples and returns the code (vertex)

 * value/color table entry for the quantization. The

 * points (fourteen) are the vertices of the cuboctahedron.

 */


float rval[] = { 0.,.5 ,.5 , 1.,.0 , 0., 0.,.5,

                .5 , 1., 1., 1., 0.,.5 ,.5 , 1.};

float gval[] = { 0.,.5 , 0., 0.,.5 , 1., 0.,.5,

                .5 , 1., 0.,.5 , 1., 1.,.5 , 1.};

float bval[] = { 0., 0.,.5 , 0.,.5 , 0., 1.,.5,

                .5 , 0., 1.,.5 , 1.,.5 , 1., 1.};


int remap14(R, G, B,  R2, G2, B2)

    float R, G, B, *R2, *G2, *B2;

    {

    int code = 0;

        if ( R + G + B > 1.5) code |= 8;

        if (-R + G + B > 0.5) code |= 4;

        if ( R - G + B > 0.5) code |= 2;

        if ( R + G - B > 0.5) code |= 1;

        *R2 = rval[code];

        *G2 = gval[code];

        *B2 = bval[code];

        return(code);

    }