From my Medium article

pencil.glsl

#define size 25.

vec2 Hash12(float t)
{
    float x = fract(sin(t * 674.3) * 453.2);
    float y = fract(sin(t * 2674.3) * 453.2);
    
    return vec2(x,y);
}

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{

    vec2 uv = (fragCoord-.5 * iResolution.xy)/iResolution.y;
    vec3 col = vec3(0.0);
    vec3 debug = vec3(Hash12(uv.y).x);
    
    
    for(float i = 0.; i < size; i++)
    {
        vec2 dir= Hash12(i * Hash12(uv.x).x + sin(Hash12(uv.y).x)) - .5;
        
        float t = fract(iTime * .05);
        float d = length(uv-dir * (t + uv.x * t * uv.y));
        float brightness = 0.002;
        col += vec3(brightness / d);
    }
    
    fragColor = vec4(vec3(1.0, 1.0, 1.0)- col,1.0);
}