Issue when trying to convert position by matrix

Hello,

Here is an image of what I am trying to do :
C is like a camera with a frustum projected onto the wall.

My goal is to find the matrix which transform the world space point A in C local space by applying the projection.

My code looks like this :

Code (CSharp):

1.  
2. Matrix4x4 l_Proj = Matrix4x4.Perspective(40f, 1f, m_Distance, m_MaxDistance);
3. Matrix4x4 l_GLProj = GL.GetGPUProjectionMatrix(l_Proj,false);
4. Matrix4x4 l_WorldToFrustum = l_GLProj * C.worldToLocalMatrix;
5.  
6. Vector3 B = l_WorldToFrustum.MultiplyPoint(A);
7. B.z = 0;
8.  

I must find the matrix because I use it in a shader later.

I was expecting to retreive local position roughly the size of my violet square but my computed local points, such as B, are way too far from the point C compared to where A is located.

My question is : where do I mess up ? I'm not sure of the matrix order when multiplying them, I tried invert the order without success...

 

Why don't you just use transfrom.worldToLocalMatrix?

 

Because it does not take into account the projection.
If I use only this matrix on my four world space points, my local points will form a trapeze. I want that my four world space points form a local square.

As you see, my four blue dot related by dotted lines form a square. But not of the good size.

 

Here is an exemple when I only use C.worldToLocalMatrix

Code (CSharp):

1. Matrix4x4 l_WorldToFrustum =  C.worldToLocalMatrix;
2. Vector3 B = l_WorldToFrustum.MultiplyPoint(A);
3. B.z = 0;

I have a trapeze but i want a square...

 

Well... turns out this was in fact correct since my coord for each corner are ~ (1,1), (-1,-1), (-1,1), (1,-1).
Ahah... silly me...