Help with fSpy

Garbb · March 5, 2020, 2:03pm

Hi!
I want to make a model building from a photo. I use fSpy for this.
But I can’t do it right. I tried different settings.

Photo of the building:

Option 1:
I set the X and Y axis. Principal point: Image midpoint
The Z axis is does not correspond to the corner of the building.

Mesh in Blender:

Option 2:
Principal point: From 3rd vanishing point. Set all 3 axes.
In Fspy, everything looks right:

In the blender, the photo is shifted to the side and the mesh looks bad:

Option 3:
Set the X and Y axis. Principal point: manual.
I moved the Principal point so that the z axis is at the corner of the building:

In the blender, the photo is again shifted and the mesh looks bad.

I ask for your help.

jerzygorskiart · March 5, 2020, 2:38pm

I can think of 2 reasons why it is happening.

walls of the builidng are not perfectly paralell to each other.
the photo was made with lenses that distorts the perspective. (mosts camera do that) Lines are not perfectly straight but slightly bended.

Maybe both

fSpy most probably works properly and the fault is in the building or the photo.

colkai · March 5, 2020, 5:52pm

As you say, likely the photo, I’ve tried several photos and the results don’t line up. Even when I have the known dimensions of a room, simply because the photo has either been tweaked in Pshop, or used a correcting lens.

Bartosz_Pampuch · July 30, 2022, 9:14pm

There are a couple of reasons this picture is a litlle problematic:

1/ it’s non that apparent to figure out which lines are mutually orthogonal (eg. roof of the left building is not parallel to the main building, street is inclined, etc)

2/ I’d say this is a 2-point perspective (although the vertical lines seem to be rotated a little bit) - in such a case you should ask fSpy to find the principal point from a third vanishing point because it won’t work.

What you should do instead is set the principal point to Manual. Because vertical lines are mutually parallel it means the principal point is located on the line connecting both vanishing points

You can learn more about this and other cases in my online fSpy tutorial: https://www.youtube.com/watch?v=daiMOYR8GS8

Here’s my sketchy alignment for your photo:

(as a new user I can’t include another media to show what it looks like in Blender, but it’s pretty well aligned

best,
bartek

Bartosz_Pampuch · July 31, 2022, 1:08pm

And here’s the other picture from Blender to show the alignment