Ex: 4 - Rotation

#version 300 es
#ifdef GL_ES
precision mediump float;
#endif

uniform vec2 u_resolution;
uniform float u_time;
out vec4 outColor;

float rectShape(vec2 position, vec2 scale) {
  scale = vec2(0.5) - scale * 0.5;
  vec2 shaper = vec2(step(scale.x, position.x), step(scale.y, position.y));
  shaper *= vec2(step(scale.x, 1.0 - position.x), step(scale.y, 1.0 - position.y));
  return shaper.x * shaper.y;
}

mat2 rotate(float angle) {
  return mat2(cos(angle), - sin(angle), sin(angle), cos(angle));
}

void main() {
  vec2 coord = gl_FragCoord.xy / u_resolution.x;
  vec3 color = vec3(0.0);

  coord -= vec2(0.5);
  coord = rotate(0.5 * sin(u_time)) * coord;
  coord += vec2(0.5);

  color += vec3(rectShape(coord, vec2(0.3, 0.3)));

  outColor = vec4(color, 1.0);
}