Question How to grab real world texture ?

Hi everyone,
I am trying to replicate the "face in the wall" effect made by MattB from twitter


Basically, it's a plane distorted by face mesh.

The procedure I'm taking is :
1. place a face game object on a plane (wall);
2. for each vertex in face mesh, calculate its foot of perpendicular on the plane;
3. get the screen position of foot, divided by screen size, so I get the "UV" of each foot in screen texture
4. replace the uv of each face mesh vertex with the UV in procedure 3;
5. use screen texture (pure camera texture) as the mainTex to render the face.

The important question is: Am I in the right path ?
Is there any better ways to grab the real world texture of a plane ?

If I am in the right path, I'm encountering two problems:
1. how to get the pure camera texture (without any virtual objects) in each frame ?
I'm rendering the ARCamera (arcam) to a RenderTexture (rt) in Update() , but I still get the texture with virtual object rendered.

Code (CSharp):

1.  
2.  
3. RenderTexture.active = rt;
4. arcam.targetTexture = rt;
5. arcam.Render();
6. arcam.targetTexture = null;
7. RenderTexture.active = null;

2. I wrote following vertex shader code the get the face mesh "UV" relative to screen texture, but I get really weird result.
(I am using a half sphere instead of face for testing purpose)

Code (CSharp):

1.  
2. float4 _PlanePoint;
3. float4 _PlaneNormal;
4.  
5. v2f vert (appdata v)
6. {
7.   v2f o;
8.  
9.  
10.   float4 worldPos=mul(unity_ObjectToWorld, v.vertex);
11.   float3 toVertexDir=worldPos.xyz-_PlanePoint.xyz;
12.   float dist=dot(toVertexDir, normalize(_PlaneNormal.xyz));
13.   float3 perpendicular=dist*normalize(_PlaneNormal.xyz);
14.   float3 footWorld=worldPos.xyz-perpendicular;
15.  
16.  
17.   float4 footLocal=mul(unity_WorldToObject, float4(footWorld, 1));
18.   float4 footInClip=UnityObjectToClipPos(footLocal);
19.   float4 screenPos=ComputeScreenPos(footInClip);
20.   float2 footUV=screenPos.xy / screenPos.w;
21.  
22.   o.vertex = UnityObjectToClipPos(v.vertex);
23.  
24.   o.uv=footUV;
25.  
26.   return o;
27. }
28.  

Could any one pointing out where am I going wrong ?
Many Thx