Phil, I didn't mean, that you should debug someones code. Believe me its completely impossible to understand ( I never tried myself ) the graphics pipeline of Qt without spending a lot of time before.
What you know is that Qt is running into an assertion in qpaintengine_raster.cpp, line 4286. Obviously CreateCompatibleDC(displayDC); returns NULL, what is the return value, when the method fails. Maybe its worth to read the MSDN docs (or google) about CreateCompatibleDC.
The assertion might have to do with the warnings before. If those are written immediately before the assertion ( do they happen in all Qwt examples too) try to find the code line, that writes them.
If you have a support license this should be enough information to contact the TrollTech support.
Uwe
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