Water simulation 2: rain on the pond

One drop is a wave packet. Rain is a lot of drops at different places and different moments, and the best part of water is what happens when the rings meet: they pass through each other and add up, making the bright and dark crossings you see on a real pond.

In the shader that is just a sum: the height is the sum of the heights of all drops. A for loop runs over 8 drops, and each needs its own random place and its own timing.

• Place: hash21(float2(fi, 1.3)) and hash21(float2(fi, 7.7)) are two different random numbers per drop number fi, mapped to the pond with - 0.5 and a scale.
• Timing: every drop repeats every 4 seconds, but with its own offset hash21(float2(fi, 3.1)) * 4.0, so they fall out of step.

Everything else (the slope, the refraction, the sparkle) is exactly the same as the one-drop pond, because it only reads the height() function. That is the great thing about describing the surface as a function: the lighting does not care how complicated the surface is.

float h = 0.0;
for (int i = 0; i < 8; i++)
{
    float fi = float(i);
    float2 c = float2(hash21(float2(fi, 1.3)) - 0.5, hash21(float2(fi, 7.7)) - 0.5) * float2(1.5, 1.0);
    float age = fmod(time + hash21(float2(fi, 3.1)) * 4.0, 4.0);
    // ...the wave packet from last time, at distance length(p - c)...
    h += wave * 0.016 * step(0.0, x + 0.12);
}