// Paste into shadertoy.
void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    vec2 c = (2.0*iMouse.xy - iResolution.xy)/iResolution.y;
    vec2 p = (-iResolution.xy + 2.0*fragCoord)/iResolution.y;
    vec2 d = c - p;

    // angle of each pixel to the center offset by from
    float from = radians(mod(10.0*iTime,360.0));
    float a = (atan(d.x, d.y) + radians(180.0) - from)/radians(360.0);
    a = fract(a);

    vec3 col = vec3(0,0,0);
    col = vec3(a, a, a);
    fragColor = vec4( col, 1.0 );
}