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

[Stephen Chu]

Just find out this has much more impact than I originally thought.

One part of my project is using QGraphicsScene with inches as the unit. 
That is QPointF(1, 0) is 1 inch to the right of the origin. This makes 
conversion between pixels and physical length simple since all 
conversions are done inside of the items.

That all works great until I tried it on the Mac. This rounding is 
causing error up to 0.5 inch which in my case is typically 1200 pixels.

I did file a bug for this and hope for a quick solution: 
<http://bugreports.qt.nokia.com/browse/QTBUG-18416>

Right now I am testing for graphics engine at runtime and switch to 
QImage if it's Core Graphics. QImage doesn't have this rounding.

In article 
<stephen-3EFC07.11212328032011 at r186-50-238-87.dialup.adsl.anteldata.net.
uy>,
 Stephen Chu <stephen at ju-ju.com> wrote:

> 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