Weirdest bug ever

Yesterday I was debugging some weird bugs. Actually spent several hours on it.

With the file transfer previews, Windows Live Messenger chops of some parts. This can be solved by sending the image as 96x96, with transparent parts added. QImage to the resque! Well.. sort of. I ran into a pretty annoying bug.

This is the code to generate a transparent image:

int size = QMAX( image.width(), image.height() );
QImage fixedImage( size, size, 32 );
fixedImage.setAlphaBuffer( true );
fixedImage.fill( qRgba( 0, 0, 0, 0 ) );  // black transparent.

Saved as PNG, it produces something like this:

Well, some parts of the image are transparent.. But the remaining of it.. juck! Something produces a fuzzy pattern here. Today it even seams to have more noise.

After trying the same code in a stand alone application, I got a perfectly transparent image. When the same code runs in KMess, it doesn't work anymore. That's the weirdest part of it. It can either be in Qt 3.3.8, KDE 3.5.6 or openSUSE 10.2, I really can't tell.

After some debugging, Google codesearch showed how to reset the individual image bits in a for-loop. A bit later I noticed how the image bits are actually one memory block (the Qt API is not really clear about that). So the fill() command can be replaced with memset(), hence the following code fixes the problem:

memset( fixedImage.bits(), 0, size * size * 4 );

This takes advantage of the image data/scanlines exposed by QImage. Pffew

With this fix in place, I'm looking forward to release 1.5-pre2. It should be out really soon, since CVS has some interesting improvements. I'll have to update the TODO and NEWS files, check for any blockers. Small bugs are not one of those, that's for the final 1.5. A next preview needs to be avoid really soon first!