【OpenGL】Shader实例分析（五）- 边缘检测

转发请保持地址：http://blog.csdn.net/stalendp/article/details/23139953

这里将介绍基于法线的边缘检测方法，这里的shader是参考官方的：http://wiki.unity3d.com/index.php/Outlined_Diffuse_3；运行效果如下：

代码如下：

Shader "Outlined/Diffuse" { // see http://wiki.unity3d.com/index.php/Outlined_Diffuse_3
	Properties {
		_Color ("Main Color", Color) = (.5,.5,.5,1)
		_OutlineColor ("Outline Color", Color) = (0,0,0,1)
		_Outline ("Outline width", Range (.002, 0.03)) = .005
		_MainTex ("Base (RGB)", 2D) = "white" { }
	}
 
	CGINCLUDE
	#include "UnityCG.cginc"
	 
	struct appdata {
		float4 vertex : POSITION;
		float3 normal : NORMAL;
	};
	 
	struct v2f {
		float4 pos : POSITION;
		float4 color : COLOR;
	};
	 
	uniform float _Outline;
	uniform float4 _OutlineColor;
	 
	v2f vert(appdata v) {
		// just make a copy of incoming vertex data but scaled according to normal direction
		v2f o;
		o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
	 
		float3 norm   = mul ((float3x3)UNITY_MATRIX_IT_MV, v.normal);
		float2 offset = TransformViewToProjection(norm.xy);
		
		o.pos.xy += offset * o.pos.z * _Outline;
		// following please refer to : http://unitygems.com/noobs-guide-shaders-6-toon-shader/
		// o.pos = mul( UNITY_MATRIX_MVP, v.vertex + (float4(v.normal,0) * _Outline)); 
		o.color = _OutlineColor;
		return o;
	}
	ENDCG
 
	SubShader {
		//Tags {"Queue" = "Geometry+100" }
		CGPROGRAM
		#pragma surface surf Lambert
		 
		sampler2D _MainTex;
		fixed4 _Color;
		 
		struct Input {
			float2 uv_MainTex;
		};
		 
		void surf (Input IN, inout SurfaceOutput o) {
			fixed4 c = tex2D(_MainTex, IN.uv_MainTex) * _Color;
			o.Albedo = c.rgb;
			o.Alpha = c.a;
		}
		ENDCG
 
		// note that a vertex shader is specified here but its using the one above
		Pass {
			Name "OUTLINE"
			Tags { "LightMode" = "Always" }
			Cull Front
			ZWrite On
			ColorMask RGB
			Blend SrcAlpha OneMinusSrcAlpha
			//Offset 50,50
 
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			half4 frag(v2f i) :COLOR { return i.color; }
			ENDCG
		}
	}
 
	SubShader {
		CGPROGRAM
		#pragma surface surf Lambert
		 
		sampler2D _MainTex;
		fixed4 _Color;
		 
		struct Input {
			float2 uv_MainTex;
		};
		 
		void surf (Input IN, inout SurfaceOutput o) {
			fixed4 c = tex2D(_MainTex, IN.uv_MainTex) * _Color;
			o.Albedo = c.rgb;
			o.Alpha = c.a;
		}
		ENDCG
 
		Pass {
			Name "OUTLINE"
			Tags { "LightMode" = "Always" }
			Cull Front
			ZWrite On
			ColorMask RGB
			Blend SrcAlpha OneMinusSrcAlpha
 
			CGPROGRAM
			#pragma vertex vert
			#pragma exclude_renderers gles xbox360 ps3
			ENDCG
			SetTexture [_MainTex] { combine primary }
		}
	}
 
	Fallback "Diffuse"
}

原理介绍：

分两个pass进行渲染，第一个渲染边框，第二个渲染实物。

1）边框的渲染

在vertex Shader阶段，吧顶点按照法线的方向进行扩展， 这样物体就比原来要膨胀（关于膨胀效果，请参考Surface Shader Example中的Normal Extrusion with Vertex Modifier）。

o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
float3 norm   = mul ((float3x3)UNITY_MATRIX_IT_MV, v.normal);
float2 offset = TransformViewToProjection(norm.xy);  //计算法线的方向
o.pos.xy += offset * o.pos.z * _Outline; //按照法线的方向进行偏移

效果如下：

边框盖住里原始物体，这里只要设置只渲染背面，就可以达到效果，在渲染边框的Pass中设置“Cull Front”就可以了。

2）渲染实物

略

当然这个只是适用于3d物体，图像的边缘检测，还有canny算法等，以后再补充，这里给个Canny的地址：Canny Edge Detection Tutorial

另外一篇：http://en.wikipedia.org/wiki/Edge_detection;  还有： http://en.wikipedia.org/wiki/Canny_edge_detector

关于图像的边缘检测，可以运用在Deferred shading中的anti-aliasing中

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相关的文章：http://unitygems.com/noobs-guide-shaders-6-toon-shader/

Silhouette-Outlined Diffuse

OutlinedDiffuse

Lighted Bumped Outline

基于法线的边缘检测

Code Snippet: Edge Detector/Antialiasing Shader