Smoke grenade 3: bullet holes

The effect everyone remembers from smoke grenades: shoot into the smoke and a tunnel opens up, with the bright world visible through it. Then the smoke slowly closes the hole again.

A bullet is a straight line: it starts at a point o (the gun) and flies in direction dir. The distance from any point p in space to that line is a one-liner: length(cross(p - o, dir)) (with dir normalized). A tunnel is simply every point closer to the line than some radius.

To carve it, multiply the density by a number that is 0 inside the tunnel and 1 outside: d *= smoothstep(rh * 0.4, rh, dist). The smoothstep gives the hole a soft wall.

A shader has no memory, so how does it know when each shot happened? It does not need to: shots are a function of time. Shot number i is fired at 0.3 + i * 0.5 seconds, so its age is time - that. If the age is negative the shot has not happened yet. The hole's radius shrinks with age, rh = 0.34 * exp(-age * 0.4), so the smoke fills it back in.

The four bullets start at the camera (you shoot from where you stand) and aim at four different points on the smoke ball, spread by a few sines of i.

for (int i = 0; i < 4; i++)
{
    float fi = float(i);
    float age = time - (0.3 + fi * 0.5);
    if (age > 0.0)
    {
        float2 hit = float2(0.7 * sin(fi * 2.4 + 0.5), 0.25 + 0.45 * sin(fi * 1.9));
        float3 o = float3(0.0, 0.5, -4.5) + float3(0.15 * cos(fi * 3.0), 0.08 * sin(fi * 2.0), 0.0);
        float3 dir = normalize(float3(hit, 0.0) - o);
        float dist = length(cross(p - o, dir));
        float rh = 0.34 * exp(-age * 0.4);
        d *= smoothstep(rh * 0.4, rh, dist);
    }
}