A simple but useful tool ...
For Linux, Ubuntu:
A ScreenRuler will display a ruler on screen which allows you to measure the other objects that you've there, in scale 1:1
In French:
Afficher une règle à l'écran échelle 1:1:
calibrer le dpi écran, (screen ruler),
exemple, dans mon cas si dpi image = 72x72 dpi, et, ecran = 87x87dpi , alors j'ai une échelle de~ 1:1 , donc = zoom 100%
https://apps.ubuntu.com/cat/application ... reenruler/
For Windows, in French (not tested):
http://www.logitheque.com/logiciels/win ... e_8814.htm
see here too: http://newsgroup.xnview.com/viewtopic.p ... 42#p130442