理解 Unity3D RenderWithShader Api

主要参考：

unity, 替换shader渲染（Rendering with Replaced Shaders） ： http://www.cnblogs.com/wantnon/p/4528677.html

测试例子：

Test.cs挂到Camera上

using UnityEngine;
using System.Collections;

public class Test1 : MonoBehaviour {

    private Camera shaderCamera;
    private RenderTexture replaceRenderTexture;

    public void Start()
    {
        shaderCamera = new GameObject("Test", typeof(Camera)).GetComponent<Camera>();
        Camera origCamera = GetComponent<Camera>();
        replaceRenderTexture = new RenderTexture((int)origCamera.pixelWidth, (int)origCamera.pixelHeight, 16, RenderTextureFormat.ARGB32);
        replaceRenderTexture.wrapMode = TextureWrapMode.Clamp;
        replaceRenderTexture.useMipMap = false;
        replaceRenderTexture.filterMode = FilterMode.Bilinear;
        replaceRenderTexture.Create();
    }
    public void OnPreRender()
    {
        Camera origCamera = GetComponent<Camera>();
        shaderCamera.CopyFrom(origCamera);
        //shaderCamera.backgroundColor = Color.clear;
        shaderCamera.clearFlags = CameraClearFlags.SolidColor;
        shaderCamera.renderingPath = RenderingPath.Forward;
        shaderCamera.targetTexture = replaceRenderTexture;
        shaderCamera.depth = 10;
        
        shaderCamera.RenderWithShader(Shader.Find("Unlit/blueSphere"), "RenderType");
    }
}

redAndGreen.shader

Shader "Unlit/redAndGreenSphere"
{
	Properties
	{
		_MainTex ("Texture", 2D) = "white" {}
	}

	SubShader{
		Tags { "RenderType"="Green" }
		LOD 100
		Pass
		{
			//Tags { "RenderType"="Green" }

			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			// make fog work
			#pragma multi_compile_fog
			
			#include "UnityCG.cginc"

			struct appdata
			{
				float4 vertex : POSITION;
				float2 uv : TEXCOORD0;
			};

			struct v2f
			{
				float2 uv : TEXCOORD0;
				UNITY_FOG_COORDS(1)
				float4 vertex : SV_POSITION;
			};

			sampler2D _MainTex;
			float4 _MainTex_ST;
			
			v2f vert (appdata v)
			{
				v2f o;
				o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
				o.uv = TRANSFORM_TEX(v.uv, _MainTex);
				UNITY_TRANSFER_FOG(o,o.vertex);
				return o;
			}
			
			fixed4 frag (v2f i) : SV_Target
			{
				// sample the texture
				fixed4 col = tex2D(_MainTex, i.uv);
				// apply fog
				UNITY_APPLY_FOG(i.fogCoord, col);
				return fixed4(0, 0.5, 0, 1);
			}
			ENDCG
		}
	}

	SubShader
	{
		Tags { "RenderType"="Red" }
		LOD 100

		Pass
		{
			//Tags { "RenderType"="Red" }

			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			// make fog work
			#pragma multi_compile_fog
			
			#include "UnityCG.cginc"

			struct appdata
			{
				float4 vertex : POSITION;
				float2 uv : TEXCOORD0;
			};

			struct v2f
			{
				float2 uv : TEXCOORD0;
				UNITY_FOG_COORDS(1)
				float4 vertex : SV_POSITION;
			};

			sampler2D _MainTex;
			float4 _MainTex_ST;
			
			v2f vert (appdata v)
			{
				v2f o;
				o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
				o.uv = TRANSFORM_TEX(v.uv, _MainTex);
				UNITY_TRANSFER_FOG(o,o.vertex);
				return o;
			}
			
			fixed4 frag (v2f i) : SV_Target
			{
				// sample the texture
				fixed4 col = tex2D(_MainTex, i.uv);
				// apply fog
				UNITY_APPLY_FOG(i.fogCoord, col);
				return fixed4(0.5, 0, 0, 1);
			}
			ENDCG
		}

	}

	
}

blueSphere.shader

Shader "Unlit/blueSphere"
{
	Properties
	{
		_MainTex ("Texture", 2D) = "white" {}
	}
	SubShader
	{
		Tags { "RenderType"="Green" }
		LOD 100

		Pass
		{
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			// make fog work
			#pragma multi_compile_fog
			
			#include "UnityCG.cginc"

			struct appdata
			{
				float4 vertex : POSITION;
				float2 uv : TEXCOORD0;
			};

			struct v2f
			{
				float2 uv : TEXCOORD0;
				UNITY_FOG_COORDS(1)
				float4 vertex : SV_POSITION;
			};

			sampler2D _MainTex;
			float4 _MainTex_ST;
			
			v2f vert (appdata v)
			{
				v2f o;
				o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
				o.uv = TRANSFORM_TEX(v.uv, _MainTex);
				UNITY_TRANSFER_FOG(o,o.vertex);
				return o;
			}
			
			fixed4 frag (v2f i) : SV_Target
			{
				// sample the texture
				fixed4 col = tex2D(_MainTex, i.uv);
				// apply fog
				UNITY_APPLY_FOG(i.fogCoord, col);
				//return col;
				return fixed4(0, 0, 0.5, 1);
			}
			ENDCG
		}
	}
}

结论：

在shaderCamera 中，因为调用了RenderWithShader(Shader.Find("Unlit/blueSphere"), "RenderType");，因为blueSphere的SubShader的 "RenderType"="Green"，所以，会在场景中进行寻找 第一个SubShader的"RenderType"="Green" 的物体，用blueSphere的SubShader来代替这些物体就进行渲染。

所以，上面的挂上了redAndGreen材质的球，本来是绿色，会用blueSphere.shader来渲染，渲染为蓝色。