Transferring line length from one axis/dimension to another in perspective (without ellipses), possible?

Hard to formulate the question, so here's what I'm not asking (unless it answers it and I am missing it...)

I do not mean mirroring a rectangle in perspective by finding the center point with crosses, and then drawing a line from one corner through the midpoint of the rectangle to extend it perfectly in scale along the axis.

I do not mean taking a distance, or height, and moving it around in 3d space to another point (like receding telephone poles)

I mean transferring the width of something in perspective, to basically make a perfect square from it, without an ellipse. I know that ellipses will form a perfect square if they tangent all lines of the square in the middle.

But due to ...some reason the software I am using, has a really crappy ellipse tool (Opentoonz) and I don't have an alternative right now...so, if there is another way, I'd be happy to hear it.

Note:I want to know this for general drawing skills, I'm just mentioning the software cause this is where not knowing it is biting me...it's for a personal thing. Trying to learn how to animate flaps and stuff.

Even a nudge in the right direction would be appreciated, or just some link to a video or text, no matter how low effort, anything than my own, at this point, paralyzed mind.

• Hi. Welcome to GDSE. Can you please share a diagram or sketch showing what you are trying to do or what has failed. As it stands, it's pretty difficult to follow your question to be honest. Sorry about that. Commented Nov 11, 2021 at 12:15
• There are ways to calculate this mathematically. But as far as I remember it is quite difficult to do – or at least I did not really get it as I was thought that in school. The most common way of doing this in drawing though is to use a perspective grid. Commented Nov 11, 2021 at 12:50
• So, you have a 2D drawing which contains only a line segment and you say it's a perspective image. That's perfectly possible, but do you know anything of the placements of the observer, imaging plane and the line segment in 3D space? If they are not restricted at all, but anything goes then there's infinitely different possibilities to draw a perfect perspective image of a square and the given line segment is one of the sides of that image. The easiest version doesn't even need any drawing, everything is already there.
– user82991
Commented Nov 12, 2021 at 13:09