Lockscreen and progress bar

[jbv at souslelogo.com]

Basically my script takes a list of images, checks the imagedata
of every image for possible white areas at the top, bottom, left
or right, crops the image if necessary, resizes the image and
groups it with fields created on the fly with text data from
an xml file.

When I simply lock the screen at the beginning of the main loop,
everything works as expected.
But when I use a group of objects on top of everything to hide
the rest, with a couple of unlock/lock screen to update a
progress bar and the content of a field in that top group, then
imagedata processing stops working. I don't get any error, but
it is as if images didn't feature any white area, when some of
them actually do.

I suspect there is a memory problem or something similar.