The QPicture paint device appear to always have the system DPI (screen DPI). When painting they always scale the image by the ratios between the system and destination DPI.

This behaviour is pretty horrid for a number of reasons:

• If the system DPI doesn't have the same X and Y DPI the images will often get squashed (as drawing code assumes equal X and Y, e.g. for drawing circles and text)
• Makes it difficult to use an intermediate drawing format to pass rendered output between parts of the program. If you know your final output will have a certain DPI, you want to render everything with this DPI.
• If a system has a different screen DPI then the final output from painting the QPicture will be different from another system (due to the scaling commands in the output). This breaks output file consistency between different systems.
• Might cause the problems in PDF character spacing: QTBUG-4974 and differences between integer and floating point coordinates.

I'm not suggesting that the default should be changed, but it would be easy to fix these probklems by:

• have a setResolution method like QPrinter to change the apparent DPI of the QPicture to users drawing onto it
• have an option to switch off DPI scaling when rendering a QPicture to a QPainter.

This is probably a few line patch which wouldn't break any existing code.