QPixmap not displayed on windows / no proble on Linx -> Qt Bug or not?

[ali99]

To use QPixmap is the only way I know to do what I want to do. That is showing an image in a QScrollView. When the image is zoomed in you can scroll that image to any position wanted. I am not an expert with using Qt, rather an intermediate. And doing that QImage->QPixmap->QLabel - "trick" looked strange to me but that is what I was told to do in order to get the wanted results.
Guido

[wysota]

How about using QPainter::drawImage() instead? And QPainter::scale() for zooming?

If you have a 300x300 pixel image which occupies about 100kB of memory and you zoom it 4 times (making it 1200x1200) you'll use 1.5MB of memory... with 3000x3000 it'll be 9MB, etc. If you use QPainter::scale and QPainter::drawImage, you'll be at 100kB memory usage all the time.

[dimitri]

A QImage lives in your system's main memory and can therefore be quite large.

A QPixmap is handled by the graphics subsystem of your computer and there are limitations on the size of pixmaps that can be handled. This is not a Qt issue.

What you want is to load an image into a QImage (or some other custom specialized structure) in main memory. Then you extract/transform/zoom whatever part of the QImage you need to display, transforming it into a QPixmap and always trying to keep the pixmap a reasonable size. Again, this limitation is not related to Qt.

Now, instead of programming all that by yourself, you could reuse code from existing Qt or KDE high-level widgets that display images and can handle rotations, zooming, shearing, etc.

I don't have links to such programs right now, except maybe GRASS/Qt, but you'll probably be interested in A Zoomable Picture Viewer.