// #define hue(h) clamp( abs( fract(h + vec4(3,2,1,0)/3.) * 6. - 3.) -1. , 0., 1.)
#define hue(v)  ( .6 + .6 * cos( 6.3*(v)  + vec4(0,23,21,0)  ) )

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    vec2 uv =  floor(fragCoord/iResolution.x * 12.)/12.;    
    fragColor = hue(uv.x + uv.y/3. + iTime*0.5);
}