Fspy: Align photo with existing Mesh

Question

What I've got:

• An existing mesh of my living room (made with MeshRoom)
• A few 3D boxes added to the scene to simulate where I'll put a TV stand
• A picture of the living room. One of the pictures that got used to create the mesh of the living room with MeshRoom
• An fspy project, with axes and length reference defined

What I want:

• Once I'm in Blender and I'm seeing through the camera created by fspy, I want to align the existing mesh of the living room (and the 3D tv stand boxes I've added) to the scene

Question:

• How to align/move/scale the meshes I've got, perfectly with the view of the camera? End goal being to have the picture rendered with only the 3D boxes I've added

I know I could skip the living room mesh and re-create the boxes only from the scene. That'd probably be way easier and I'm going to do that on the side. But I find the use case super interesting, so I just want to know how one would do.

Answer 1

• In fSpy, place the world origin at some known point. Import into Blender.
  
• Snap the 3D cursor to world origin (Shift+S). Create an Empty object.
  
• Parent the fSpy camera to the new Empty (by shift-dragging it in Outliner).
  
• Move the 3D cursor to the point that you had previously chosen in fSpy.
  
• Select the Empty and Snap it to the 3D cursor position (Shift+S).
  
• If your fSpy and Blender axes don't match, rotate the Empty in 90° increments.
• If you didn't set reference distance in fSpy, roughly rescale the Empty.
  
• Enter the camera view. Select it and navigate to the camera data tab.
• Under Background Images, set Depth to "Front" and tweak Opacity to taste.
  
• Select the Empty and adjust the final transform. Hold Shift while transforming for better precision.
  

Note: in the above example, the camera is placed outside the wall, so it had to have its clipping adjusted as well. This was done off-screen, and is unnecessary for most cases.