Linear-interpolated palette cycling shader

palcycle.glsl

uniform float offset;

const int numcols = 6;
const vec4 col[numcols] = vec4[numcols](
    /* vec4(   0/255.0,   0/255.0,   0/255.0, 1), */
    /* vec4( 129/255.0,  29/255.0,  30/255.0, 1), */
    /* vec4( 253/255.0, 107/255.0,  91/255.0, 1), */
    /* vec4(  99/255.0, 191/255.0,  96/255.0, 1), */
    /* vec4( 222/255.0, 214/255.0, 162/255.0, 1)); */
    vec4(1,0,0,1),
    vec4(1,1,0,1),
    vec4(0,1,0,1),
    vec4(0,1,1,1),
    vec4(0,0,1,1),
    vec4(1,0,1,1));

vec4 effect( sampler2D texture, vec2 tc )
{
    vec4 rgba = texture2D(texture, tc) * (numcols-1);
    float key = rgba.r * 0.2126 + rgba.g * 0.7152 + rgba.b * 0.0722;
    float total = key + offset;
    int mod1 = int(mod(total, numcols));
    int mod2 = int(mod(total + 1, numcols));
    float lerp = mod(key + offset, 1);
    return
        col[mod1] * (1-lerp) +
        col[mod2] * (lerp);
}