Old-school snow

void main() {
    highp float grayComponent = rand(vec2(positionInput.x + u_Random, positionInput.y + u_Random));
    // Daniel's code randomises every pixel. I found that randomising only those that came
    // out higher than 0.9 made for a more old-school "television, tuned to a dead channel",
    // which I also found more hypnotic.
    if (grayComponent > 0.9) {
        gl_FragColor = vec4(grayComponent, grayComponent, grayComponent, 1.0);
    } else {
        gl_FragColor = vec4(0, 0, 0, 1.0);
    }
}