// Warm white pulse travelling along the stairs
void mainImage(out vec4 fragColor, in vec2 fragCoord) {
    float x = fragCoord.x / iResolution.x;
    float wave = 0.5 + 0.9 * sin(x * 12.2831 * 2.0 - iTime * 2);
    vec3 warm = vec3(1.0, 0.45, 0.1);
    fragColor = vec4(warm * wave, 1.0);
}