QImage/QPixmap size limitations?

Question

Are there any known size/space limitation of QPixmap and/or QImage objects documented? I did not find any useful information regarding this. I'm currently using Qt 4.7.3 on OSX and Windows. Particulary I'm interested in:

• Width/Height limits?
• Limits depending on color format?
• Difference between 32/64 bit machines?
• Difference regarding OS?

I would naively suspect that memory is the only limitation, so one could calculate max size by

width x height x byte_per_pixel

I assume that there is a more elaborate rule of thumb; also 32 bit machines may have addressing problems when you run into GB dimensions.

In the end I want to store multiple RGBA images of about 16000x16000 pixel in size and render them using transparency onto each other within a QGraphicsScene. The workstation available can have a lot of RAM, let's say 16GB.

tl;dr: What size limits of QImage/QPixmap are you aware of, or where can I find such information?

Edit: I'm aware of the tiling approach and I'm fine with that. Still it would be great to know the things described above.

Thanks!

Answer 1

Both are limited to 32767x32767 pixels. That is, you can think of them as using a signed 16-bit value for both the X and Y resolution.

No axis can ever exceed 32767 pixels, even if the other axis is only 1 pixel. Operating system "bitness" does not affect the limitation. The underlying system may run into other limits, such as memory as you mentioned, before such a huge image can be created.

You can see an example of this limitation in the following source code: http://git.zx2c4.com/qt/plain/src/gui/image/qpixmap_x11.cpp

if (uint(w) >= 32768 || uint(h) >= 32768) {
    w = h = 0;
    is_null = true;
    return;
}