// Liquid trails: the previous output is dragged along the motion, swirled a little,
// and slowly fades back to the live camera.
uniform float persistence;  // 0.0 .. 0.99 = 0.92
uniform float drag;         // 0.0 .. 6.0 = 2.5
uniform float swirl;        // 0.0 .. 0.01 = 0.002

void main() {
    vec2 centred = uv - 0.5;
    vec2 from = uv - texture(flow, uv).xy * drag + vec2(-centred.y, centred.x) * swirl;
    vec3 trail = texture(prev, from).rgb;
    vec3 live = texture(camera, uv).rgb;
    color = vec4(mix(live, trail, persistence), 1.0);
}