// Related examples and context: https://observablehq.com/@mbostock/shader

#define M_PI 3.1415926535897932384626433832795

const float size = 10.0;

mat2 rotate2d(float a) {
    return mat2(cos(a), -sin(a), sin(a), cos(a));
}

void mainImage(out vec4 fragColor, in vec2 fragCoord) {
    float angle = iTime;
    vec2 p = (fragCoord.xy - iResolution.xy / 2.0) * rotate2d(angle);
    float k = float(mod(p.x, size * 2.0) < size == mod(p.y, size * 2.0) < size);
    fragColor = vec4(vec3(k), 1.0);
}