Unity3D Shader官方教程翻译(十)----Shader语法：Pass的Alpha测试

我就是我 2022-05-14 03:51 238阅读 0赞

# ShaderLab syntax: Alpha testing #

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

alpha测试是最后一个机会，拒绝将一个像素写到屏幕上。（可以理解为在将画面显示到屏幕上的最后1次修改，这次修改只能让该像素显示或不显示）

[![点击浏览下一页][SL-AlphaTest-0.jpg]][SL-AlphaTest-0.jpg]

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.

在最后的输出颜色被计算之后，这个颜色可以选择是否让自己的alpha值和一个固定值做比较。如果测试失败，这个像素将不在屏幕上显示。

指导：当每个像素即将绘制时，如果启动了Alpha测试，OpenGL会检查像素的Alpha值，只有Alpha值满足条件的像素才会进行绘制（严格的说，满足条件的像素会通过本项测试，进行下一种测试，只有所有测试都通过，才能进行绘制），不满足条件的则不进行绘制。这个“条件”可以是：始终通过（默认情况）、始终不通过、大于设定值则通过、小于设定值则通过、等于设定值则通过、大于等于设定值则通过、小于等于设定值则通过、不等于设定值则通过。

如果我们需要绘制一幅图片，而这幅图片的某些部分又是透明的（想象一下，你先绘制一幅相片，然后绘制一个相框，则相框这幅图片有很多地方都是透明的，这样就可以透过相框看到下面的照片），这时可以使用Alpha测试。将图片中所有需要透明的地方的Alpha值设置为0.0，不需要透明的地方Alpha值设置为1.0，然后设置Alpha测试的通过条件为：“大于0.5则通过”，这样便能达到目的。当然也可以设置需要透明的地方Alpha值为1.0，不需要透明的地方Alpha值设置为0.0，然后设置条件为“小于0.5则通过”。Alpha测试的设置方式往往不只一种，可以根据个人喜好和实际情况需要进行选择。

## Syntax 语法 ##

**AlphaTest** Off

Render all pixels (default).

关闭alpha测试，渲染所有像素

**AlphaTest**  *comparison AlphaValue*

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

设置alpha测试：仅当像素的alpha值处在一定范围之内才渲染它

### Comparison 比较命令 ###

Comparison is one of the following words:

比较命令式下面中的1个

<table style="padding-bottom:0px; list-style-type:none; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> 
 <tbody> 
 <tr style="padding-bottom:0px; list-style-type:none; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> 
 <td align="left" style="padding-bottom:0px; list-style-type:none; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> Greater</td> 
 <td align="left" style="padding-bottom:0px; list-style-type:none; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> Only render pixels whose alpha is greater than&nbsp;AlphaValue.&nbsp; 只渲染alpha值比AlphaValue大的像素</td> 
 </tr> 
 <tr style="padding-bottom:0px; list-style-type:none; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> 
 <td align="left" style="padding-bottom:0px; list-style-type:none; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> GEqual</td> 
 <td align="left" style="padding-bottom:0px; list-style-type:none; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> Only render pixels whose alpha is greater than or equal to&nbsp;AlphaValue. 只渲染alpha值&gt;=AlphaValue的像素</td> 
 </tr> 
 <tr style="padding-bottom:0px; list-style-type:none; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> 
 <td align="left" style="padding-bottom:0px; list-style-type:none; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> Less</td> 
 <td align="left" style="padding-bottom:0px; list-style-type:none; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> Only render pixels whose alpha value is less than&nbsp;AlphaValue. 只渲染alpha值&lt;AlphaValue的像素</td> 
 </tr> 
 <tr style="padding-bottom:0px; list-style-type:none; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> 
 <td align="left" style="padding-bottom:0px; list-style-type:none; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> LEqual</td> 
 <td align="left" style="padding-bottom:0px; list-style-type:none; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> Only render pixels whose alpha value is less than or equal to from&nbsp;AlphaValue. 只渲染alpha值&lt;=&nbsp;AlphaValue的像素</td> 
 </tr> 
 <tr style="padding-bottom:0px; list-style-type:none; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> 
 <td align="left" style="padding-bottom:0px; list-style-type:none; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> Equal</td> 
 <td align="left" style="padding-bottom:0px; list-style-type:none; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> Only render pixels whose alpha value equals&nbsp;AlphaValue. 只渲染alpha值=AlphaValue的像素</td> 
 </tr> 
 <tr style="padding-bottom:0px; list-style-type:none; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> 
 <td align="left" style="padding-bottom:0px; list-style-type:none; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> NotEqual</td> 
 <td align="left" style="padding-bottom:0px; list-style-type:none; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> Only render pixels whose alpha value differs from&nbsp;AlphaValue. 只渲染alpha值不等于AlphaValue的像素</td> 
 </tr> 
 <tr style="padding-bottom:0px; list-style-type:none; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> 
 <td align="left" style="padding-bottom:0px; list-style-type:none; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> Always</td> 
 <td align="left" style="padding-bottom:0px; list-style-type:none; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> Render all pixels. This is functionally equivalent to&nbsp;AlphaTest Off. 渲染所有像素相当于关闭alpha测试</td> 
 </tr> 
 <tr style="padding-bottom:0px; list-style-type:none; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> 
 <td align="left" style="padding-bottom:0px; list-style-type:none; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> Never</td> 
 <td align="left" style="padding-bottom:0px; list-style-type:none; margin:0px; padding-left:0px; padding-right:0px; padding-top:0px"> Don't render any pixels. 不渲染任何像素</td> 
 </tr> 
 </tbody> 
</table>

### AlphaValue  alpha值 ###

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值是0~1之间的浮点数。这也可以是一个可以被引用的float变量或范围属性，在这种情况下，它应该使用标准的方括号来表示（\[变量名\]）。

## Details 详情 ##

The alpha test is important when rendering concave objects with transparent parts. The graphics card maintains a record of the depth of every pixel written to the screen. If a new pixel is further away than one already rendered, the new pixel is not written to the display. This means that even with [Blending][], objects will not show through.

当渲染1个凹面物体的半透明部分的时候，alpha测试是很重要的。显卡会持续记录写入到屏幕上的每1个像素的深度值。如果1个新像素比1个已经渲染的像素在同一个屏幕上的位置更远时候，这个新的像素将不会被渲染。这意味着就算使用混合效果，这个物体仍然不会被显示。

[![点击浏览下一页][SL-AlphaTest-1.jpg]][SL-AlphaTest-1.jpg]

In this figure, the tree on the left is rendered using **AlphaTest**. Note how the pixels in it are either completely transparent or opaque. The center tree is rendered using only **Alpha Blending** - notice how transparent parts of nearby branches cover the distant leaves because of the depth buffer. The tree on the right is rendered using the last example shader - which implements a combination of blending and alpha testing to hide any artifacts.

在此图中，左边的树使用了alpha测试，请注意它的像素或者是全透明或者是不透明。中间的树仅仅使用了alpha混合，请注意临近树枝的透明部分由于深度缓冲的原因覆盖了远处的树叶。右边的树使用了最后1个例子的着色器，这个着色器实现了混合效果和alpha测试，隐藏了假象。

## Examples 例子 ##

The simplest possible example, assign a texture with an alpha channel to it. The object will only be visible where alpha is greater than 0.5

这是1个尽可能简单的例子，分配给它一个带有alpha通道的纹理。这个物体中仅有alpha值大于0.5的部分可见。

Shader "Simple Alpha Test" {
    	Properties {
    		_MainTex ("Base (RGB) Transparency (A)", 2D) = "" {}
    	}
    	SubShader {
    		Pass {
    			// Only render pixels with an alpha larger than 50%
    			AlphaTest Greater 0.5
    			SetTexture [_MainTex] { combine texture }
    		}
    	}
    }

This is not much good by itself. Let us add some lighting and make the cutoff value tweakable:

这个效果不是很好，我们添加一些光照和一个变量给它，使它可以调整AlphaTest的值

Shader "Cutoff Alpha" {
    	Properties {
    		_MainTex ("Base (RGB) Transparency (A)", 2D) = "" {}
    		_Cutoff ("Alpha cutoff", Range (0,1)) = 0.5
    	}
    	SubShader {
    		Pass {
    			// Use the Cutoff parameter defined above to determine
    			// what to render.
    			AlphaTest Greater [_Cutoff]
    			Material {
    				Diffuse (1,1,1,1)
    				Ambient (1,1,1,1)
    			}
    			Lighting On
    			SetTexture [_MainTex] { combine texture * primary }
    		}
    	}
    }

When rendering plants and trees, many games have the hard edges typical of alpha testing. A way around that is to render the object twice. In the first pass, we use alpha testing to only render pixels that are more than 50% opaque. In the second pass, we alpha-blend the graphic in the parts that were cut away, without recording the depth of the pixel. We might get a bit of confusion as further away branches overwrite the nearby ones, but in practice, that is hard to see as leaves have a lot of visual detail in them.

我们渲染了植物和树木，许多游戏具有典型alpha测试的硬边界。解决这个问题的办法是渲染这个物体2次。在第1个pass中，我们使用alpha测试仅仅去渲染那些半透明值大于50%的像素。在第2个pass中，我们使用alpha混合那些我们切掉的图形，不再记录这些像素的深度。远处的叶子覆盖掉了近处的叶子，在这里我们也许有些混乱，但在现实中，我们很难看到叶子的许多细节。

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
    			}
    		}
    	}
    }

Note that we have some setup inside the SubShader, rather than in the individual passes. Any state set in the SubShader is inherited as defaults in passes inside it.

注意我们在SubShader中做了一些设置，而不是在独立的pass中。所有在SubShader中做的设置将被当做其中pass的默认值。

由[www.J2meGame.com][]原创，转载请说明。

[SL-AlphaTest-0.jpg]: http://unity3d.com/support/documentation/Images/manual/SL-AlphaTest-0.jpg
[Blending]: http://unity3d.com/support/documentation/Components/SL-Blend.html
[SL-AlphaTest-1.jpg]: http://unity3d.com/support/documentation/Images/manual/SL-AlphaTest-1.jpg
[www.J2meGame.com]: http://www.j2megame.com/