highp float rand(highp float co) {
    return fract(sin(co * (91.3458)) * 47453.5453);
}

void fragment()
{
    highp vec2 uv = UV;
    highp vec2 texel = 1.0 * SCREEN_PIXEL_SIZE.xy;

    highp vec4 img = texture(TEXTURE, uv);

    // modulate offset
    highp float step_y = texel.y * (rand(uv.x) * 80) * (sin(sin(TIME * 0.5)) * 2.0 + 1.3);
    // offset without modulation
    //highp float step_y = texel.y*(rand(uv.x)*100.0);
    // shake offset and modulate it
    step_y += rand(uv.x * uv.y * TIME) * 0.025 * sin(TIME);
    // more noisy spikes
    step_y = mix(step_y, step_y * rand(uv.x * TIME) * 0.5, sin(TIME));

    if (dot(img, vec4(0.299, 0.587, 0.114, 0.0)) > 1.2*(sin(TIME) * 0.325 + 0.50)) {
        uv.y += step_y;
    } else {
        uv.y -= step_y;
    }

    COLOR = texture(TEXTURE, uv);
}