Plotting a perfect square in 2p perspective?

kritskiy

Hey everyone,
Does anyone know if it's even possible to plot a perfect square in 2-point perspective? I'm struggling with this for some time now. What I'm trying to do is to take a render of a plane from 3d application, draw a quadrangle in given perspective and supposedly this new quadrangle should concur with a plane in the 3d-app. I've tried 4 or 5 different algorithms but none of them gives the the quadrangle similar to the one I'm getting later in 3d.
Maybe my initial assumption is wrong and 2 vanishing points is not enough to get the proportions right?

This algorithm gives me the best result, but the result is still a bit off
https://www.youtube.com/watch?v=OB3UEpxFlj8

Here's my process:
1) render with template perspective


2) calculations


3) The resulting quadrangle as background (green) + new plane in 3d (blue). The green quadrangle I got from calculations is wider than the blue 3d-plane


This drives me crazy!
If you have any ideas I'll be glad to listen to them.

Thanks.
Sergey.

Eric Chadwick

Why not just draw the vanishing point lines outwards from the edges of the 3d plane?

RN

Eric Chadwick said:

Why not just draw the vanishing point lines outwards from the edges of the 3d plane?

If you draw the vanishing lines from the 3d plane or from the grid they should reach the same point, since they're parallel (in theory).

I tried to do the same and failed as well. Then I tried to do it directly in the 3D software (using edge extrusions as if drawing lines on the canvas), the same methodology of transfering a distance by going through the middle etc. and it worked.



Since those could've been done in 2D easily rather than 3D, I think the problem is precision. When you work with the raster program every pixel may be slightly off and accumulate towards the end for a noticeable error. That, or that the method of using diagonals is more reliable than the measuring bar.

kritskiy

Thanks guys! I'll try diagonals and I'll try not to draw the lines by hand, but lay them via code to be as precise as possible