// Ripple: the page as a still pool a slow swell crosses. Rings travel out
// from the middle of the window at a fixed speed in pixels, so they look
// the same at any size and frame rate; the displacement is at most a couple
// of pixels, so text stays readable, and it eases in over its first half
// second instead of snapping on. Each ring catches the light a little on its
// rising side, in linear light, and the result is dithered so the shading
// never bands. It is a Shadertoy file: pardes puts its prefix before it.

const float TAU = 6.28318530718;
const float WAVELENGTH_PX = 150.0;
const float SPEED_PX = 55.0;        // outward, per second
const float AMPLITUDE_PX = 2.2;
const float SHADE = 0.035;          // light on a slope, linear
const float EASE_IN = 0.5;          // seconds

vec3 toLinear(vec3 c) {
    return mix(pow((c + 0.055) / 1.055, vec3(2.4)), c / 12.92, lessThanEqual(c, vec3(0.04045)));
}

vec3 toSrgb(vec3 c) {
    c = max(c, vec3(0.0));
    return mix(1.055 * pow(c, vec3(1.0 / 2.4)) - 0.055, c * 12.92, lessThanEqual(c, vec3(0.0031308)));
}

float ign(vec2 p) {
    return fract(52.9829189 * fract(dot(p, vec2(0.06711056, 0.00583715))));
}

void mainImage(out vec4 fragColor, in vec2 fragCoord) {
    vec2 size = iResolution.xy;
    vec2 center = 0.5 * size;
    vec2 offset = fragCoord - center;
    float radius = length(offset);
    float reach = 0.5 * length(size);
    // Strongest a third of the way out, fading to nothing at the middle
    // and at the edges, so the window's frame never wobbles.
    float envelope = smoothstep(0.0, 0.18, radius / reach) * (1.0 - smoothstep(0.35, 0.95, radius / reach));
    float ease = smoothstep(0.0, EASE_IN, iTime);
    float phase = (radius - iTime * SPEED_PX) * TAU / WAVELENGTH_PX;
    float height = sin(phase);
    float slope = cos(phase);
    vec2 dir = offset / max(radius, 1e-3);
    vec2 moved = fragCoord + dir * height * AMPLITUDE_PX * envelope * ease;

    vec4 s = texture(iChannel0, moved / size);
    vec3 col = toLinear(s.rgb) * (1.0 + SHADE * slope * envelope * ease);
    vec3 srgb = toSrgb(col) + (ign(fragCoord + float(iFrame % 64) * 5.588238) - 0.5) / 255.0;
    fragColor = vec4(clamp(srgb, 0.0, 1.0), s.a);
}