12-03-2020, 08:15 PM
(12-03-2020, 05:42 PM)denzjos Wrote: Well, I discovered that the flange is a drawing of an ellips.The clock is a photograph or a rendered drawing that have a perspective distortion.The drawing of the flange is 3D drawing in isometric projection. The ellips (green) is not distorted but it is just a angled view. That is why it is easy to find the center-point. To find the center lines you must draw a circle (yellow - here divided in 12, not required) with this point as a center point. The 4 points where the circle is intersecting the ellips are the vertices of a rectangle (red). Lines from the center-point of the circle through the the middle of the sides of the rectangle are the center lines (red and one is discovered by a cyan line) of the ellips. I just tested the plugin to see if it is working to restore the front view of the flange with the holes and the cavity.
Ok. I am not familiar with isometric projections. But I suppose what in such projection happens to a circle is:
- the circle is mapped onto an ellipse;
- the center of the circle goes to the center of that ellipse.
But if we have a general perspective mapping (a projective transformation), then condition 1 still holds but condition 2 is no longer true! That is the case in your clock example. If you look at the distorted clock and the 1-anchor stroke you have put there, that anchor is NOT at the center of the ellipse. Not even if it is the point where the perspective mapping sends the center of the original circle.
Using that point as the would-be center, as you did, is the right thing to do. That point is precisely the point that should be sent to the center of the corrected circle. (After all, we are trying to cancel the effect of the perspective mapping.)
The plugin needs to be told what point should be sent to the center, and knowing that point may not always be so easy.