// A slow lub-dub travelling up the stairs

float bump(float x, float c, float w) {
    float d = (x - c) / w;
    return exp(-d * d);
}

void mainImage(out vec4 fragColor, in vec2 fragCoord) {
    float i = floor(fragCoord.x / iResolution.x * 16.0);

    float period = 3.2;                              // slow, ~19 bpm
    float ph = fract((iTime - i * 0.15) / period) * period;
    float beat = bump(ph, 0.50, 0.20) + 0.65 * bump(ph, 1.25, 0.26);

    vec3 base = vec3(0.10, 0.0, 0.01);
    vec3 col = base + vec3(1.0, 0.04, 0.06) * beat;
    col += vec3(0.5) * bump(ph, 0.50, 0.08);         // white-ish peak
    fragColor = vec4(col, 1.0);
}