thebookofshaders/08/cross-rotate.frag

// Author @patriciogv - 2015
// http://patriciogonzalezvivo.com
#ifdef GL_ES
precision mediump float;
#endif

#define PI 3.14159265359

uniform vec2 u_resolution;
uniform float u_time;

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

float box(in vec2 _st, in vec2 _size){
    _size = vec2(0.5) - _size*0.5;
    vec2 uv = smoothstep(_size,
                        _size+vec2(0.001),
                        _st);
    uv *= smoothstep(_size,
                    _size+vec2(0.001),
                    vec2(1.0)-_st);
    return uv.x*uv.y;
}

float cross(in vec2 _st, float _size){
    return  box(_st, vec2(_size,_size/4.)) + 
            box(_st, vec2(_size/4.,_size));
}

void main(){
    vec2 st = gl_FragCoord.xy/u_resolution.xy;
    vec3 color = vec3(0.0);
    
    // move space from the center to the vec2(0.0)
    st -= vec2(0.5);
    // rotate the space
    st = rotate2d( sin(u_time)*PI ) * st;
    // move it back to the original place
    st += vec2(0.5);

    // Show the coordinates of the space on the background
    // color = vec3(st.x,st.y,0.0);

    // Add the shape on the foreground
    color += vec3(cross(st,0.4));

    gl_FragColor = vec4(color,1.0);
}