这个教程涵盖了各种各样的关于透明纹理贴图的共用，i.e.RGBA 纹理贴图包含了一个A（alpha）部分 是纹理的不透明的值。

它联合了shader的代码关于“Textured sphere”部分的概念，介绍了 “cutaways”和”Transparency”。

如果你没有读过这篇教程，这将会是一个很好的机会来阅读。

丢弃透明片段（Discarding Transparent Fragments）

让我们开始说明丢弃片段的”cutaways”部分。按照这些步骤描述的在”Texture Spheres”部分，并把图像分配给材质球的左边根据以下shader ：

Shader "Cg texturing with alpha discard" {
   Properties {
      _MainTex ("RGBA Texture Image", 2D) = "white" {} 
      _Cutoff ("Alpha Cutoff", Float) = 0.5
   }
   SubShader {
      Pass {    
         Cull Off // since the front is partially transparent, 
            // we shouldn't cull the back
 
         CGPROGRAM
 
         #pragma vertex vert  
         #pragma fragment frag 
 
         uniform sampler2D _MainTex;    
         uniform float _Cutoff;
 
         struct vertexInput {
            float4 vertex : POSITION;
            float4 texcoord : TEXCOORD0;
         };
         struct vertexOutput {
            float4 pos : SV_POSITION;
            float4 tex : TEXCOORD0;
         };
 
         vertexOutput vert(vertexInput input) 
         {
            vertexOutput output;
 
            output.tex = input.texcoord;
            output.pos = mul(UNITY_MATRIX_MVP, input.vertex);
            return output;
         }
 
         float4 frag(vertexOutput input) : COLOR
         {
            float4 textureColor = tex2D(_MainTex, input.tex.xy);  
            if (textureColor.a < _Cutoff)
               // alpha value less than user-specified threshold?
            {
               discard; // yes: discard this fragment
            }
            return textureColor;
         }
 
         ENDCG
      }
   }
 
   // The definition of a fallback shader should be commented out 
   // during development:
   // Fallback "Unlit/Transparent Cutout"
}

片段着色器读取RGBA贴图并与用户指定的范围值大小比较alpha值。如果 alpha值比范围值小，就丢弃着色片段使之透明。注意： 这项指令在某些平台上是非常缓慢的，特别是在移动设备上。因此，混合（blending）通常更有效。

混合（blending）

“Transparency”部分描述了怎样用alpha混合去渲染半透明的物体。在这段代码中用RGBA贴图：

Shader "Cg texturing with alpha blending" {
   Properties {
      _MainTex ("RGBA Texture Image", 2D) = "white" {} 
   }
   SubShader {
      Tags {"Queue" = "Transparent"} 
 
      Pass {	
         Cull Front // first render the back faces
         ZWrite Off // don't write to depth buffer 
            // in order not to occlude other objects
         Blend SrcAlpha OneMinusSrcAlpha 
            // blend based on the fragment's alpha value
 
         CGPROGRAM
 
         #pragma vertex vert  
         #pragma fragment frag 
 
         uniform sampler2D _MainTex;    
         uniform float _Cutoff;
 
         struct vertexInput {
            float4 vertex : POSITION;
            float4 texcoord : TEXCOORD0;
         };
         struct vertexOutput {
            float4 pos : SV_POSITION;
            float4 tex : TEXCOORD0;
         };
 
         vertexOutput vert(vertexInput input) 
         {
            vertexOutput output;
 
            output.tex = input.texcoord;
            output.pos = mul(UNITY_MATRIX_MVP, input.vertex);
            return output;
         }
 
         float4 frag(vertexOutput input) : COLOR
         {
            return tex2D(_MainTex, input.tex.xy);  
         }
 
         ENDCG
      }
 
      Pass {	
         Cull Back // now render the front faces
         ZWrite Off // don't write to depth buffer 
            // in order not to occlude other objects
         Blend SrcAlpha OneMinusSrcAlpha 
            // blend based on the fragment's alpha value
 
         CGPROGRAM
 
         #pragma vertex vert  
         #pragma fragment frag 
 
         uniform sampler2D _MainTex;    
         uniform float _Cutoff;
 
         struct vertexInput {
            float4 vertex : POSITION;
            float4 texcoord : TEXCOORD0;
         };
         struct vertexOutput {
            float4 pos : SV_POSITION;
            float4 tex : TEXCOORD0;
         };
 
         vertexOutput vert(vertexInput input) 
         {
            vertexOutput output;
 
            output.tex = input.texcoord;
            output.pos = mul(UNITY_MATRIX_MVP, input.vertex);
            return output;
         }
 
         float4 frag(vertexOutput input) : COLOR
         {
            return tex2D(_MainTex, input.tex.xy);  
         }
 
         ENDCG
      }
   }
   // The definition of a fallback shader should be commented out 
   // during development:
   // Fallback "Unlit/Transparent"
}

未完待续。。。。

by-----wolf96

unity3d 内部Transparent （透明）shader 代码