[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

		}

	}

}