Question: Using fSpy - How can you tell exactly where to align an object with the picture?

All 3 of these planes match the top of the table in the picture, but when you rotate the scene, they are all in different locations.

I'm guessing there could be an infinite number of possibilities along the vector, but how do you choose the correct one?

[![enter image description here](https://i.stack.imgur.com/8tjHJ.jpg)](https://i.stack.imgur.com/8tjHJ.jpg)
[![enter image description here](https://i.stack.imgur.com/Tutcq.jpg)](https://i.stack.imgur.com/Tutcq.jpg)

Answer: 
I am struggling with this a bit myself. My work flow has been to try to get the outside structures in first to give me some solid reference points.

In your screen shot, I didn’t see a wall on the right side of your model.

I think if you place that wall, you will have a good reference to the right side of your counter top.

And then the front and back of the counter can be approximated. from the sink position on the left side of the scene.
This is the problem of the perspective projection.
It’s impossible to find all three coordinates from a flat image. You need to know at least one to determine the other two. Therefore you need to set origin in fSpy and then start modeling from that point (knowing which planes have one of there coordinates set to 0). The more aligned objects you have the more reference points you get. You’ll get a perfect match then.

Keep in mind that the proper focal length is also important. Otherwise you’ll get wrong proportions.

Take a look at my tutorial on fSpy. It covers all its parameters and also presents the case you’re having problem with.

<https://youtu.be/daiMOYR8GS8>

Good luck