varying vec2 vUv;
uniform float opacity;

uniform vec3 baseColor;
uniform vec3 backColor;
uniform float occlusionDistance;
uniform float clipDistance;
uniform float maxClipFactor;


#if defined use_map
    uniform sampler2D map; 
#endif
 
#if defined(GL_EXT_frag_depth) && defined(useDepth)  
    //似乎通过gl.getExtension('EXT_frag_depth')得到的GL_EXT_frag_depth
     
    uniform sampler2D depthTexture;
    uniform float nearPlane;
    uniform float farPlane; 
    uniform vec2 resolution;
    uniform vec2 viewportOffset; //  viewportOffset 范围从0-整个画布的像素
    
    float convertToLinear(float zValue)
    {
        float z = zValue * 2.0 - 1.0;
        return (2.0 * nearPlane * farPlane) / (farPlane + nearPlane - z * (farPlane - nearPlane));
    }
#endif
  
void main() {
  
    
    vec4 color = vec4(baseColor, opacity);
    
    
    
    #if defined(GL_EXT_frag_depth) && defined(useDepth)
        // mixFactor and clipFactor define the color mixing proportion between the states of
        // full visibility and occluded visibility
        // and
        // full visibility and total invisibility
        
        float mixFactor = 0.0;
        float clipFactor = 0.0;
        
        
        // The linear depth value of the current fragment
        float fragDepth = convertToLinear(gl_FragCoord.z);

        // The coordinates of the current fragment in the depth texture
        vec2 depthTxtCoords = vec2(gl_FragCoord.x-viewportOffset.x,  gl_FragCoord.y - viewportOffset.y) / resolution;
        //gl_FragCoord大小为 viewport client大小 
        
        // The linear depth value of the pixel occupied by this fragment in the depth buffer
        float textureDepth = convertToLinear(texture2D(depthTexture, depthTxtCoords).r);

        // The difference between the two depths
        float delta =  fragDepth - textureDepth;

        if (delta > 0.0)//差距
        {
            // occlusionDistance and clipDistance define the width of the respective zones and
            // mixFactor and clipFactor express the interpolation between the two colors depending on the position
            // of the current fragment withing those zones.
            
            
            mixFactor = clamp(delta / occlusionDistance, 0.0, 1.0);
            clipFactor = clamp(delta / clipDistance, 0.0, maxClipFactor);
        }
        
        // If the fragment is totally transparent, don't bother drawing it
        if (clipFactor == 1.0)
        {
            discard;
        }else{
            
            #if defined use_map
                color = texture2D(map, vUv) * color; 
            #endif
           
            
             
            color = vec4(mix(color.rgb, backColor, mixFactor), color.a * (1.0 - clipFactor));
        }
         
    #else
        #if defined use_map
            color = texture2D(map, vUv) * color;
        #endif 
    #endif
  
    gl_FragColor = color;
  
}