(function () {
    function FogEffect(viewer, options) {
        if (!viewer) throw new Error("viewer is undefined")
        this.viewer = viewer;
        this.fogByDistance = Cesium.defaultValue(options.fogByDistance, new Cesium.Cartesian4(10, 0.0, 1000, 0.9))
        this.maxHeight = Cesium.defaultValue(options.maxHeight, 9000)
        this.color = Cesium.defaultValue(options.color, Cesium.Color.WHITE)
        this.init();
    }

    FogEffect.prototype.init = function () {
        this.FogStage = new Cesium.PostProcessStage({
            name: 'czm_fog',
            fragmentShader: this.FragmentShader_Fog(),
            uniforms: {
                fogByDistance: () => { return this.fogByDistance; },
                fogColor: () => { return this.color; }
            }
        });
        this.FogStage.enabled = this.enabled
        this.viewer.scene.postProcessStages.add(this.FogStage)
    }

    //定义下雨场景 着色器
    FogEffect.prototype.FragmentShader_Fog = function () {
        return `float getDistance(sampler2D depthTexture, vec2 texCoords) 
        { 
            float depth = czm_unpackDepth(texture2D(depthTexture, texCoords)); 
            if (depth == 0.0) { 
                return czm_infinity; 
            } 
            vec4 eyeCoordinate = czm_windowToEyeCoordinates(gl_FragCoord.xy, depth); 
            return -eyeCoordinate.z / eyeCoordinate.w; 
        } 
        float interpolateByDistance(vec4 nearFarScalar, float distance) 
        { 
            float startDistance = nearFarScalar.x; 
            float startValue = nearFarScalar.y; 
            float endDistance = nearFarScalar.z; 
            float endValue = nearFarScalar.w; 
            float t = clamp((distance - startDistance) / (endDistance - startDistance), 0.0, 1.0); 
            return mix(startValue, endValue, t); 
        } 
        vec4 alphaBlend(vec4 sourceColor, vec4 destinationColor) 
        { 
            return sourceColor * vec4(sourceColor.aaa, 1.0) + destinationColor * (1.0 - sourceColor.a); 
        } 
        uniform sampler2D colorTexture; 
        uniform sampler2D depthTexture; 
        uniform vec4 fogByDistance; 
        uniform vec4 fogColor; 
        varying vec2 v_textureCoordinates; 
        void main(void) 
        { 
            float distance = getDistance(depthTexture, v_textureCoordinates); 
            vec4 sceneColor = texture2D(colorTexture, v_textureCoordinates); 
            float blendAmount = interpolateByDistance(fogByDistance, distance); 
            vec4 finalFogColor = vec4(fogColor.rgb, fogColor.a * blendAmount); 
            gl_FragColor = alphaBlend(finalFogColor, sceneColor); 
        }
    `
    }

    FogEffect.prototype.show = function (visible) {
        this.FogStage.enabled = visible;
    }

    FogEffect.prototype.destroy = function (visible) {
        if (!this.viewer || !this.FogStage) return;
        this.viewer.scene.postProcessStages.remove(this.FogStage);
        this.FogStage.destroy();
        delete this.alpha
    }

    Cesium.FogEffect = FogEffect
})()