Can't get Image Effect Transparency to work

I'm trying to write a image effect to transition one camera into the other but I'm stuck on this last problem for a while now and tried googling for solutions and reading up about Blending and Colormasks but so far came up dry. I hope that someone here can help me on my way.

The problem is that instead of Camera1 transitioning to transparent to show Camera2 underneath it, it transitions to black. My guess is that I'm doing something wrong with either the stencil buffer not setting pixels to transparent but to black or overlooking something in the shader setting alpha channel. If I applied the shader to a UI/Image the transparency did work though.

The setup is as followed:

Camera1: Depth Only, Ortho, Depth: 1

Camera 2: Depth Only, Ortho, Depth: 0

This is the code on Camera1 for Blitting:

Code (CSharp):

1. void OnRenderImage (RenderTexture src, RenderTexture dst)
2.          {
3.              if (TransitionMaterial != null) {
4.                  RenderTexture rnd = RenderTexture.GetTemporary (src.width, src.height, 0, RenderTextureFormat.ARGB32);
5.                  Graphics.Blit (rnd, dst, TransitionMaterial);
6.                  //RenderTexture.ReleaseTemporary (rnd);
7.              }
8.            
9.          }

And I'm using this shader to my 1st Camera:

Code (CSharp):

1. Shader "Custom/Masking"
2. {
3.      Properties
4.      {
5.          _MainTex ("Main Texture", 2D) = "white" {}
6.          _MaskTex ("Mask Texture", 2D) = "white" {}
7.          _AmountSlider ("Mask Amount", Range (0,1)) = 0.4
8.          _Color ("Tint", Color) = (1,1,1,1)
9.          _Invert ("Invert", Float) = 0
10.  
11.          _ColorMask ("Color Mask", Float) = 15
12.      }
13.  
14.      SubShader
15.      {
16.          Tags
17.          {
18.              "Queue"="Transparent"
19.              "IgnoreProjector"="True"
20.              "RenderType"="Transparent"
21.              "PreviewType"="Plane"
22.              "CanUseSpriteAtlas"="True"
23.          }
24.      
25.          Stencil
26.          {
27.              Ref [_Stencil]
28.              Comp [_StencilComp]
29.              Pass [_StencilOp]
30.              ReadMask [_StencilReadMask]
31.              WriteMask [_StencilWriteMask]
32.          }
33.          ZWrite Off
34.          Cull Off
35.          Lighting Off
36.          ZTest Less
37.          Blend SrcAlpha OneMinusSrcAlpha
38.          ColorMask [_ColorMask]
39.          
40.          Pass
41.          {
42.          CGPROGRAM
43.              #pragma vertex vert
44.              #pragma fragment frag
45.  
46.              #include "UnityCG.cginc"
47.              #include "UnityUI.cginc"
48.          
49.              struct appdata_t
50.              {
51.                  float4 vertex   : POSITION;
52.                  float4 color    : COLOR;
53.                  float2 texcoord : TEXCOORD0;
54.              };
55.  
56.              struct v2f
57.              {
58.                  float4 vertex   : SV_POSITION;
59.                  fixed4 color    : COLOR;
60.                  half2 texcoord  : TEXCOORD0;
61.                  float4 worldPosition : TEXCOORD1;
62.                  half2 screenpos : TEXCOORD2;
63.              };
64.          
65.              fixed4 _Color;
66.              fixed4 _TextureSampleAdd;
67.  
68.              bool _UseClipRect;
69.  
70.              v2f vert(appdata_t IN)
71.              {
72.                  v2f OUT;
73.                  OUT.worldPosition = IN.vertex;
74.                  OUT.vertex = mul(UNITY_MATRIX_MVP, OUT.worldPosition);
75.  
76.                  OUT.texcoord = IN.texcoord;
77.                   OUT.screenpos = ComputeScreenPos(OUT.vertex);
78.                  #ifdef UNITY_HALF_TEXEL_OFFSET
79.                  OUT.vertex.xy += (_ScreenParams.zw-1.0)*float2(-1,1);
80.                  #endif
81.              
82.                  OUT.color = IN.color * _Color;
83.                  return OUT;
84.              }
85.  
86.              sampler2D _MainTex;
87.              sampler2D _MaskTex;
88.                float _AmountSlider;
89.                float _Invert;
90.              fixed4 frag(v2f IN) : SV_Target
91.              {
92.                  float4 color = tex2D (_MainTex, IN.texcoord );
93.                  float alpha =  (abs(_Invert-(tex2D(_MaskTex, IN.screenpos).r)) - _AmountSlider) + 1;
94.                  float4 alpha_color = float4(color.r, color.g, color.b, smoothstep(alpha, 1, 0) * color.a);
95.                  return alpha_color;
96.              }
97.          ENDCG
98.          }
99.      }
100. }

 

Should I instead use a stencil buffer or render textures to do this?

 

I think it will be easier to use a render texture

 

Not exactly like I hoped it would work, but! it does the trick.
Thanks!