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。