[Unity]模拟雨水的折射效果

用GrabPass做的小玩具。

并不是真的计算了折射，只是简单地扰动了uv，对于雨水来说效果已经足够好了。

Shader代码：

Shader "Unlit/Rain"
{
	Properties
	{
		_MainTex("Texture", 2D) = "white" {}
		_Opaque("Opaque",float) = 0.02
	}
	SubShader
	{
		Tags{
			"RenderType" = "Opaque"
			"Queue" = "Transparent"
		}
		LOD 100

		Blend SrcAlpha OneMinusSrcAlpha
		GrabPass{ "GrabTexture" }
		Pass
		{
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag

			#include "UnityCG.cginc"

			struct appdata {
				float4 vertex : POSITION;
				float2 uv : TEXCOORD0;
			};

			struct v2f {
				float4 vertex:SV_POSITION;
				float2 uv:TEXCOORD0;
				float4 screenuv:TEXCOORD1;

			};

			sampler2D _MainTex;
			sampler2D GrabTexture;
			float4 _MainTex_ST;
			float _Opaque;

			v2f vert(appdata v) {
				v2f o;
				o.vertex = UnityObjectToClipPos(v.vertex);
				o.uv = TRANSFORM_TEX(v.uv, _MainTex);
				o.screenuv = ComputeGrabScreenPos(o.vertex);
				return o;
			}

			fixed4 frag(v2f i) : SV_Target
			{
				// sample the texture
				fixed4 col = tex2D(_MainTex, i.uv);
				float2 suv = i.screenuv / i.screenuv.w;
				//512和128是测试出来比较正常的魔数。
				//越大则扰动越剧烈。
				float2 distort = float2(sin(suv.x * 512 + _Time.w * 10),sin(suv.y * 128 + _Time.w * 10)) / 128;
				col = _Opaque + tex2D(GrabTexture, suv + distort);
				col.a = tex2D(_MainTex, i.uv).a;
				return col;
			}
			ENDCG
		}
	}
}