AlphaTesting

【Alpha Testing】

　　The alpha test is a last chance to reject a pixel from being written to the screen.

　　

　　After the final output color has been calculated, the color can optionally have its alpha value compared to a fixed value. If the test fails, the pixel is not written to the display.

　　在最终颜色（pixel-color）被计算后，可以通过Alpha Test来排除顶点。

[Syntax]　　

AlphaTest Off

Render all pixels (default).

AlphaTest comparison AlphaValue

Set up the alpha test to only render pixels whose alpha value is within a certain range.

Comparison

　　

AlphaValue

　　A floating-point number between 0 and 1. This can also be a variable reference to a float or range property, in which case it should be written using the standard square bracket notation ([VariableName]).

 [Alpha测试的用途]

　　对于半透明的凹形图形，用Alpha测试与Blending配合使用才能产生较好的效果。

　　

　　左图：ZWrite On，AlphaTest Equal 1.0。由于Alpha均为1，所以无Blending。在此条件下产生出一个锐利的图像。

　　中图：ZWrite On，AlphaTest Off。在此条件下，Alpha为0的部分会遮挡住Alpha非零的部分，产生奇怪的图。

　　右图：1）ZWrite On，AlphaTest Equal 1.0。此遍渲染非透明物体。

　　　　   2）ZWrite Offet，AlphaTest Less 1.0。此遍渲染透明物体。

　　　　　右图能够产生较为完美的图像。

　　右图的Shader如下：

Shader "Vegetation" {
    Properties {
        _Color ("Main Color", Color) = (.5, .5, .5, .5)
        _MainTex ("Base (RGB) Alpha (A)", 2D) = "white" {}
        _Cutoff ("Base Alpha cutoff", Range (0,.9)) = .5
    }
    SubShader {
        // Set up basic lighting
        Material {
            Diffuse [_Color]
            Ambient [_Color]
        }
        Lighting On

        // Render both front and back facing polygons.
        Cull Off

        // first pass:
        //   render any pixels that are more than [_Cutoff] opaque
        Pass {
            AlphaTest Greater [_Cutoff]
            SetTexture [_MainTex] {
                combine texture * primary, texture
            }
        }

        // Second pass:
        //   render in the semitransparent details.
        Pass {
            // Dont write to the depth buffer
            ZWrite off
            // Don't write pixels we have already written.
            ZTest Less
            // Only render pixels less or equal to the value
            AlphaTest LEqual [_Cutoff]

            // Set up alpha blending
            Blend SrcAlpha OneMinusSrcAlpha

            SetTexture [_MainTex] {
                combine texture * primary, texture
            }
        }
    }
} 

参考：file://localhost/Applications/Unity/Unity.app/Contents/Documentation/Documentation/Components/SL-AlphaTest.html