【智商捉急系列】我该怎么改这个shader 实现我需要的效果。

Shader "UnityCoder/CanvasWithMask"
{
Properties {
_MainTex ("Canvas (RGBA)", 2D) = "white" {}
_MaskTex ("Mask (RGBA)", 2D) = "white" {}
}
SubShader {
Tags { "RenderType"="Opaque" "Queue" = "Transparent" }
Blend SrcAlpha OneMinusSrcAlpha
//Blend One One
LOD 100
Pass {
Tags { "LightMode" = "Vertex" }
Lighting Off
SetTexture [_MainTex]
{
combine texture
}
SetTexture [_MaskTex]
{
//combine previous*texture
combine texture lerp (texture) previous, previous+texture
}
} // pass
} // subshader
}