效果如下

话不多说，直接上代码

shader

Shader "Paint3D/AddTexture"
{
    Properties
	{
		_MainColor ("Main Color", Color) = (1,1,1,1) //前景图颜色
        _BackColor ("Back Color", Color) = (1,1,1,1) 
	    _MainTex ("Main Texture", 2D) = "white" {} 
        _BackTex ("Back Texture", 2D) = "white" {}
        _scaleX("scaleX",Range(-10,0)) = -5
        _scaleY("scaleY",Range(-10,0)) = -5
        ss("scaleZ",Range(-3.14,3.14)) = 0
	}
	SubShader
	{
		Tags { "Queue"="Transparent" "IgnoreProjector"="True" "RenderType"="Transparent" }
		LOD 100
 
		// ZWrite Off
    	// Blend SrcAlpha OneMinusSrcAlpha
 
		Pass
		{
			CGPROGRAM
			#pragma vertex vert
            #pragm