(function () {
    function SnowEffect(viewer, options) {
        if (!viewer) throw new Error("viewer is undefined")
        this.viewer = viewer;
        this.speed = Cesium.defaultValue(options.speed, 10)
        this.enabled = Cesium.defaultValue(options.enabled, true)
        this.init();
    }
    
    SnowEffect.prototype.init = function () {
        this.snowStage = new Cesium.PostProcessStage({
            name: 'czm_snow',
            fragmentShader: this.FragmentShader_Snow(),
            uniforms: {
                speed: () => { return this.speed; }
            }
        });
        this.snowStage.enabled = this.enabled
        this.viewer.scene.postProcessStages.add(this.snowStage)
    }
    
    //定义下雪场景 着色器
    SnowEffect.prototype.FragmentShader_Snow = function () {
        return `uniform sampler2D colorTexture; //输入的场景渲染照片
        varying vec2 v_textureCoordinates;
        uniform float speed;
        
        float snow(vec2 uv,float scale){
            float time = czm_frameNumber * speed / 1000.0 ;
            float w=smoothstep(1.0,0.0,-uv.y*(scale/10.0));
            if(w<0.1)return 0.0;
            uv+=time/scale;
            uv.y+=time*2.0/scale;
            uv.x+=sin(uv.y+time*0.5)/scale;
            uv*=scale;
        
            vec2 s=floor(uv),f=fract(uv),p;
            float k=3.0,d;
            p=0.5+0.35*sin(11.0*fract(sin((s+p+scale)*mat2(7,3,6,5))*5.0))-f;
            d=length(p);k=min(d,k);
            k=smoothstep(0.0,k,sin(f.x+f.y)*0.01);
            return k*w;
        }
        
        void main(void){
            vec2 resolution = czm_viewport.zw;
            vec2 uv=(gl_FragCoord.xy*2.-resolution.xy)/min(resolution.x,resolution.y);
        
            float c = 0.0;
            c+=snow(uv,15.);
            c+=snow(uv,10.);
            c+=snow(uv,9.);
            c+=snow(uv,8.);
            c+=snow(uv,7.);
            c+=snow(uv,6.);
            c+=snow(uv,5.);
            c+=snow(uv,4.);
        
            vec3 finalColor=(vec3(c)); //屏幕上雪的颜色
        
            gl_FragColor = mix(texture2D(colorTexture, v_textureCoordinates), vec4(finalColor,1.0), 0.5);  //将雪和三维场景融合
        
        }
    `
    }
    
    SnowEffect.prototype.show = function (visible) {
        this.snowStage.enabled = visible;
    }
    
    SnowEffect.prototype.destroy = function (visible) {
        if (!this.viewer || !this.snowStage) return;
        this.viewer.scene.postProcessStages.remove(this.snowStage);
        this.snowStage.destroy();
        delete this.speed
    }
    
    Cesium.SnowEffect = SnowEffect
})(Cesium)