创建一个雨滴涟漪效果着色器，包含雨滴位置生成、单个涟漪、雨滴冲击和多雨滴涟漪系统，支持雨滴密度和涟漪速度调节。

vec2 generateRaindropPosition(float seed) {
    // 使用伪随机数生成雨滴位置
    float x = fract(sin(seed * 78.233) * 43758.5453);
    float y = fract(sin(seed * 12.9898) * 43758.5453);
    return vec2(x, y);
}

float singleRipple(vec2 uv, vec2 center, float time, float speed) {
    float dist = distance(uv, center);
    
    // 涟漪传播效果
    float ripple = sin(dist * 20.0 - time * speed * 5.0);
    
    // 衰减效果
    float attenuation = 1.0 / (1.0 + dist * 10.0);
    
    return ripple * attenuation * waveAmplitude;
}

float raindropImpact(vec2 uv, vec2 center, float time, float speed) {
    float dist = distance(uv, center);
    
    // 冲击波效果
    float impact = sin(dist * 30.0 - time * speed * 10.0) * 
                 exp(-dist * 15.0);
    
    return impact * 0.5;
}

float multiRippleSystem(vec2 uv, float time, float density) {
    float totalRipple = 0.0;
    int numDrops = int(density * 20.0);
    
    for (int i = 0; i < 15; i++) {
        if (i >= numDrops) break;
        
        float seed = float(i) * 123.456;
        vec2 dropPos = generateRaindropPosition(seed);
        
        // 每个雨滴的动画时间偏移
        float timeOffset = sin(seed * 45.678) * 10.0;
        
        // 单个涟漪
        float ripple = singleRipple(uv, dropPos, time + timeOffset, rippleSpeed);
        
        // 冲击效果
        float impact = raindropImpact(uv, dropPos, time + timeOffset, rippleSpeed);
        
        totalRipple += ripple + impact;
    }
    
    return totalRipple;
}

uniforms: {
    time: { value: 0 },
    resolution: { value: new THREE.Vector2(_width, _height) },
    dropDensity: { value: 0.5 },
    rippleSpeed: { value: 1.0 },
    waveAmplitude: { value: 0.3 },
    waterColor: { value: new THREE.Color(0x4a90e2) }
}