void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    // Normalized pixel coordinates (from 0 to 1)
    vec2 uv = (fragCoord-.5*iResolution.xy)/iResolution.y;
    uv*=iTime/10.0;
    uv = vec2(atan(uv.x, uv.y)/acos(0.0), length(uv));

    uv+=fract(iTime/20.0);

    float c;
    if(fract(uv.x*20.0)>0.5)
    {
        c = 0.0;
        if(fract(uv.y*20.0)>0.5)
        {
            c = 1.0;
        }
    }
    else
    {
        c = 1.0;
        if(fract(uv.y*20.0)>0.5)
        {
            c = 0.0;
        }
    }

    // Time varying pixel color
    vec3 col = vec3(c);


    // Output to screen
    fragColor = vec4(col,1.0);
}