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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.

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    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.
    – Billy Kerr
    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.
    – Julian Steinmann
    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
    Nov 12, 2021 at 13:09

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After asking Scott Robertson, which I didn not expect an answer from, doing it with ellipses simply is the way to go. Since ellipses are perfect circles in perspective, and perfect circles tangent the lines of a perfect square in the middle of each line. And thus, with an ellipse, one can arrive at equally long lines in perspective, too.

Wish there had been something else that wasn't super complicated, but, anything but the ellipse way is going into some crazy territory, it seems.

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  • Well, you can do it in other ways too. Its not super complicated its just building a matrix, inverting it dividing and there you go, but its definitely higher order math. Building a better ellipse tool is building a matrix and inverting it. But on that note why dont you implement (or design and get someone else to implement) a better ellipse tool? I mean what is the purpose of you using open source if you cant do so?
    – joojaa
    Nov 12, 2021 at 6:41

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