thebookofshaders/09/marching_dots.frag

// Author @patriciogv - 2015 - patricio.io

#ifdef GL_ES
precision mediump float;
#endif

const float PI = 3.1415926535897932384626433832795;

uniform vec2 u_resolution;
uniform vec2 u_mouse;
uniform float u_time;

vec2 movingTiles(vec2 _st, float _zoom, float _speed){
    _st *= _zoom;
    float time = u_time*_speed;
    if( fract(time)>0.5 ){
        if (fract( _st.y * 0.5) > 0.5){
            _st.x += fract(time)*2.0;
        } else {
            _st.x -= fract(time)*2.0;
        }
    } else {
        if (fract( _st.x * 0.5) > 0.5){
            _st.y += fract(time)*2.0;
        } else {
            _st.y -= fract(time)*2.0;
        }
    }
    return fract(_st);
}

float circle(vec2 _st, float _radius){
    vec2 pos = vec2(0.5)-_st;
    return smoothstep(1.0-_radius,1.0-_radius+_radius*0.2,1.-dot(pos,pos)*3.14);
}

void main() {
    vec2 st = gl_FragCoord.xy/u_resolution.xy;
    st.x *= u_resolution.x/u_resolution.y;

    st = movingTiles(st,10.,0.5);

    vec3 color = vec3( 1.0-circle(st, 0.3 ) );

    gl_FragColor = vec4(color,1.0);
}