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
}

1. 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;
}

1. 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：
2. 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：
3. 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

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
}
}
}

1. 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
}
}
}