[Therion] Background zoom-level bug

Wookey wookey at wookware.org
Mon Nov 12 04:32:12 CET 2018

There seems to be a bug in xtherion graphics editor revealed if you
manage to enter a non-power-of-two zoom level.

If you get into this state then the background image and the lines
drawn on top of it go out of registration.

Here is an easy way to reproduce the problem. You will need a mouse
with a scroll-wheel, and the latest 5.4.1 version of therion..

* Insert a jpeg into a new .th2 file in xtherion.
* Draw a line over part of it.
* Move it around and observe that the line moves with the background. This is true at 
  all 5 fixed zoom levels.
* zoom with the mouse scroll button as far out as possible. It seems to allow 12% for example. Now when you come back out you get 24%, 48%, 96% etc.
* Now if you move the image around, the image and lines stay together until you have moved a bit, but then start to move seprately. This seems to be something to do with bounding boxes. 
* Click on 25/50/100/200/400 scaling the line+ images go back into registration. 

I can't reproduce this problem right now because my therion is too
old, so the mouse wheel doesn't produce scrolling in the xtherion
window, but we had the same problem on both Windows and Linux machines
at the weekend with 5.4.1.

If others can confirm that the above is a sufficient set of
instructions, then hopefully someone with clue can work out why it
goes wrong for 'intermediate' zoom levels.

It took us quite a while to work out what on earth was going on at the weekend!

Principal hats:  Linaro, Debian, Wookware, ARM

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