Unity Shader 实现半透明效果

背景

想要实现如下效果：主角可以遮挡背景，背景遮挡主角时候半透明混合
研究了一周，shader知识了解从无到稍稍入门，终于解决了，记录一下实现方案。

实现方法

1. 使用 Blend One OneMinusSrcAlpha指令，也就是SrcColor + DstColor * (1 - SrcAlpha)，假设A和B重叠，先渲染A，然后渲染B，指令在B的shader里，那么SrcColor是B的颜色，DstColor是A的颜色，SrcAlpha是B的透明度，由此可以实现A和B混合后半透明效果
2. 使用Stencil标记重叠部分，可以实现只让重叠部分半透明
3. 所有shader基于Sprite-Default修改

步骤说明

1. 主角shader：使用Stencil标记像素索引为1，所有被主角渲染过的像素的索引均为1

				Stencil
				{
					Ref 1
					Pass replace
				}

2. 背景shader：采用两个pass

• 第一个pass：识别到不是主角渲染的像素，按照Sprite-Default正常渲染

				Stencil
				{

					Ref 1
					Comp notequal
				}

• 第二个pass：识别到主角渲染过的像素后，表明是重叠区域，混合两者颜色，实现半透明

				Stencil
				{

					Ref 1
					Comp equal
				}
				//混合半透明效果，背景 + 主角色* (1-SrcAlpha)
				Blend One OneMinusSrcAlpha
				...
				fixed4 frag(v2f IN) : SV_Target
				{
					fixed4 c = SampleSpriteTexture(IN.texcoord) * IN.color;
					c.a *= 0.5; ////c.a就是SrcAlpha
					c.rgb *= c.a;
					return c;
				}

3. 背景和主角layer均为default，但是order分别为0、1，也就是主角先渲染，最终效果为：
4. 背景和主角layer均为default，但是order分别为0、-1，也就是背景先渲染，最终效果为：
5. 最终实现：主角可以遮挡背景；背景遮挡主角时，重叠部分半透明混合效果

注意事项

在frag里通过clip可以过滤一些隐藏像素点，不会被strencil标记

shader源码

1. 主角shader

Shader "Custom/SpriteDefaltChar"
{
	Properties
	{
		[PerRendererData] _MainTex("Sprite Texture", 2D) = "white" {}
		_Color("Tint", Color) = (1,1,1,1)
		[MaterialToggle] PixelSnap("Pixel snap", Float) = 0
	}

		SubShader
		{
			Tags
			{
				"Queue" = "Transparent"
				"IgnoreProjector" = "True"
				"RenderType" = "Transparent"
				"PreviewType" = "Plane"
				"CanUseSpriteAtlas" = "True"
			}

			Cull Off
			Lighting Off
			ZWrite Off
			Blend One OneMinusSrcAlpha

			Pass
			{

				Stencil
				{
					Ref 1
					Pass replace
				}

			CGPROGRAM
				#pragma vertex vert
				#pragma fragment frag
				#pragma multi_compile _ PIXELSNAP_ON
				#include "UnityCG.cginc"

				struct appdata_t
				{
					float4 vertex   : POSITION;
					float4 color    : COLOR;
					float2 texcoord : TEXCOORD0;
				};

				struct v2f
				{
					float4 vertex   : SV_POSITION;
					fixed4 color : COLOR;
					float2 texcoord  : TEXCOORD0;
				};

				fixed4 _Color;

				v2f vert(appdata_t IN)
				{
					v2f OUT;
					OUT.vertex = UnityObjectToClipPos(IN.vertex);
					OUT.texcoord = IN.texcoord;
					OUT.color = IN.color * _Color;
					#ifdef PIXELSNAP_ON
					OUT.vertex = UnityPixelSnap(OUT.vertex);
					#endif

					return OUT;
				}

				sampler2D _MainTex;
				sampler2D _AlphaTex;
				float _AlphaSplitEnabled;

				fixed4 SampleSpriteTexture(float2 uv)
				{
					fixed4 color = tex2D(_MainTex, uv);

	#if UNITY_TEXTURE_ALPHASPLIT_ALLOWED
					if (_AlphaSplitEnabled)
						color.a = tex2D(_AlphaTex, uv).r;
	#endif //UNITY_TEXTURE_ALPHASPLIT_ALLOWED

					return color;
				}

				fixed4 frag(v2f IN) : SV_Target
				{
					fixed4 c = SampleSpriteTexture(IN.texcoord) * IN.color;
					c.rgb *= c.a;
					return c;
				}
			ENDCG
			}
		}
}

2. 背景shader

Shader "Custom/SpriteDefaltBg"
{
	Properties
	{
		[PerRendererData] _MainTex("Sprite Texture", 2D) = "white" {}
		_Color("Tint", Color) = (1,1,1,1)
		[MaterialToggle] PixelSnap("Pixel snap", Float) = 0
	}

		SubShader
		{
			Tags
			{
				"Queue" = "Transparent"
				"IgnoreProjector" = "True"
				"RenderType" = "Transparent"
				"PreviewType" = "Plane"
				"CanUseSpriteAtlas" = "True"
			}


			Pass
			{
				Stencil
				{

					Ref 1
					Comp notequal
				}

				Cull Off
				Lighting Off
				ZWrite Off
				Blend One OneMinusSrcAlpha

			CGPROGRAM
				#pragma vertex vert
				#pragma fragment frag
				#pragma multi_compile _ PIXELSNAP_ON
				#include "UnityCG.cginc"

				struct appdata_t
				{
					float4 vertex   : POSITION;
					float4 color    : COLOR;
					float2 texcoord : TEXCOORD0;
				};

				struct v2f
				{
					float4 vertex   : SV_POSITION;
					fixed4 color : COLOR;
					float2 texcoord  : TEXCOORD0;
				};

				fixed4 _Color;

				v2f vert(appdata_t IN)
				{
					v2f OUT;
					OUT.vertex = UnityObjectToClipPos(IN.vertex);
					OUT.texcoord = IN.texcoord;
					OUT.color = IN.color * _Color;
					#ifdef PIXELSNAP_ON
					OUT.vertex = UnityPixelSnap(OUT.vertex);
					#endif

					return OUT;
				}

				sampler2D _MainTex;
				sampler2D _AlphaTex;
				float _AlphaSplitEnabled;

				fixed4 SampleSpriteTexture(float2 uv)
				{
					fixed4 color = tex2D(_MainTex, uv);

	#if UNITY_TEXTURE_ALPHASPLIT_ALLOWED
					if (_AlphaSplitEnabled)
						color.a = tex2D(_AlphaTex, uv).r;
	#endif //UNITY_TEXTURE_ALPHASPLIT_ALLOWED

					return color;
				}

				fixed4 frag(v2f IN) : SV_Target
				{
					fixed4 c = SampleSpriteTexture(IN.texcoord) * IN.color;
					c.rgb *= c.a;
					return c;
				}
			ENDCG
			}

			Pass
			{
				Stencil
				{

					Ref 1
					Comp equal
				}

				Cull Off
				Lighting Off
				ZWrite Off
				Blend One OneMinusSrcAlpha

			CGPROGRAM
				#pragma vertex vert
				#pragma fragment frag
				#pragma multi_compile _ PIXELSNAP_ON
				#include "UnityCG.cginc"

				struct appdata_t
				{
					float4 vertex   : POSITION;
					float4 color    : COLOR;
					float2 texcoord : TEXCOORD0;
				};

				struct v2f
				{
					float4 vertex   : SV_POSITION;
					fixed4 color : COLOR;
					float2 texcoord  : TEXCOORD0;
				};

				fixed4 _Color;

				v2f vert(appdata_t IN)
				{
					v2f OUT;
					OUT.vertex = UnityObjectToClipPos(IN.vertex);
					OUT.texcoord = IN.texcoord;
					OUT.color = IN.color * _Color;
					#ifdef PIXELSNAP_ON
					OUT.vertex = UnityPixelSnap(OUT.vertex);
					#endif

					return OUT;
				}

				sampler2D _MainTex;
				sampler2D _AlphaTex;
				float _AlphaSplitEnabled;

				fixed4 SampleSpriteTexture(float2 uv)
				{
					fixed4 color = tex2D(_MainTex, uv);

	#if UNITY_TEXTURE_ALPHASPLIT_ALLOWED
					if (_AlphaSplitEnabled)
						color.a = tex2D(_AlphaTex, uv).r;
	#endif //UNITY_TEXTURE_ALPHASPLIT_ALLOWED

					return color;
				}

				fixed4 frag(v2f IN) : SV_Target
				{
					fixed4 c = SampleSpriteTexture(IN.texcoord) * IN.color;
					c.a *= 0.5;
					c.rgb *= c.a;
					return c;
				}
			ENDCG
			}
		}
}