Cesium下雪效果

上一篇写了一个下雨的效果，本文是一个下雪的效果

1. 下雪效果的类代码：

class SnowEffect {
    constructor(viewer, options) {
        if (!viewer) throw new Error('no viewer object!');
        options = options || {};
        this.snowSize = Cesium.defaultValue(options.snowSize, 0.02); //最好小于0.02
        this.snowSpeed = Cesium.defaultValue(options.snowSpeed, 60.0);
        this.viewer = viewer;
        this.init();
    }

    init() {
        this.snowStage = new Cesium.PostProcessStage({
            name: 'czm_snow',
            fragmentShader: this.snow(),
            uniforms: {
                snowSize: () => {
                    return this.snowSize;
                },
                snowSpeed: () => {
                    return this.snowSpeed;
                }
            }
        });
        this.viewer.scene.postProcessStages.add(this.snowStage);
    }

    destroy() {
        if (!this.viewer || !this.snowStage) return;
        this.viewer.scene.postProcessStages.remove(this.snowStage);
        this.snowStage.destroy();
        delete this.snowSize;
        delete this.snowSpeed;
    }

    show(visible) {
        this.snowStage.enabled = visible;
    }

    snow() {
        return "uniform sampler2D colorTexture;\n\
            varying vec2 v_textureCoordinates;\n\
            uniform float snowSpeed;\n\
                    uniform float snowSize;\n\
            float snow(vec2 uv,float scale)\n\
            {\n\
                float time=czm_frameNumber/snowSpeed;\n\
                float w=smoothstep(1.,0.,-uv.y*(scale/10.));if(w<.1)return 0.;\n\
                uv+=time/scale;uv.y+=time*2./scale;uv.x+=sin(uv.y+time*.5)/scale;\n\
                uv*=scale;vec2 s=floor(uv),f=fract(uv),p;float k=3.,d;\n\
                p=.5+.35*sin(11.*fract(sin((s+p+scale)*mat2(7,3,6,5))*5.))-f;d=length(p);k=min(d,k);\n\
                k=smoothstep(0.,k,sin(f.x+f.y)*snowSize);\n\
                return k*w;\n\
            }\n\
            void main(void){\n\
                vec2 resolution=czm_viewport.zw;\n\
                vec2 uv=(gl_FragCoord.xy*2.-resolution.xy)/min(resolution.x,resolution.y);\n\
                vec3 finalColor=vec3(0);\n\
                //float c=smoothstep(1.,0.3,clamp(uv.y*.3+.8,0.,.75));\n\
                float c=0.;\n\
                c+=snow(uv,30.)*.0;\n\
                c+=snow(uv,20.)*.0;\n\
                c+=snow(uv,15.)*.0;\n\
                c+=snow(uv,10.);\n\
                c+=snow(uv,8.);\n\
                c+=snow(uv,6.);\n\
                c+=snow(uv,5.);\n\
                finalColor=(vec3(c));\n\
                gl_FragColor=mix(texture2D(colorTexture,v_textureCoordinates),vec4(finalColor,1),.5);\n\
                }\n\
                ";
    }
}

Cesium.SnowEffect = SnowEffect;

2. 页面使用方式
   引入上面的js即可

new Cesium.SnowEffect(viewer, {
            snowSize: 0.02, //雪大小 ，默认可不写
            snowSpeed: 60.0 //雪速，默认可不写
        });

3. 效果图

   
   cesium_snow.gif