[Qt-interest] Unnecessary Rounding from QRectF to GCRect?

[Stephen Chu]

My current project requires displaying some huge images. They are 
typically 10s of Ks on both width and height. I need to scale them to 
fit in view and also allow user to zoom in to parts of the image.

QGraphicsView handles most of the task for me nicely. I also only build 
QPixmap at the exact screen resolution to speed up things. Basically 
what I do is in paint(), calculate the current zoom level. Create a 
QPixmap at exact that size. Then build my scaled image into it by 
reading only needed lines from my file. Draw the result pixmap in the 
supplied painter.

It works pretty well. But one issue came up on the Mac version of the 
app: The pixmap is drawn stretched or shrunk by 1 line at times. 
Resulting some annoying interference patterns. This doesn't happen in 
Windows version so I figure it's the difference between raster and Core 
Graphics engines.

I came to the line 971 in qpaintengine_mac.cpp (Qt 4.7.2) and it creates 
a CGRect by rounding the supplied QRectF:

    CGRect rect = CGRectMake(qRound(r.x()), qRound(r.y()), 
qRound(r.width()), qRound(r.height()));

Taking out the qRound calls and my image fits perfectly. Just like the 
Windows version.

So my question is: why is Qt rounding the numbers here? CGRect is made 
up of 4 floats. It's got less precision than the doubles used in QRectF 
but rounding it makes it much worse. What is the reason to round the 
numbers? Should I report this as a bug?

Another (partial) solution is to force graphics engine to "raster" but 
that messes up all widgets. :(

-- 
Stephen Chu