[Qt-interest] Difference between QImage and QWidget when drawing a dotted line

[Samuel Rødal]

On 08/05/2010 02:14 PM, ext Ronny Brendel wrote:
> Hi,
>
> I have come across a imo weird behavior.
> When drawing a dotted line on a QImage, the length of a dot is one pixel.
> When in turn drawing on a widget directly the dots have a length of two pixels.
>
> Why is that?
>
> For me it is an inconvenience, because I have to introduce a (not needed)
> qimage in order to achieve similar looks.
>
> Or am I missing something?
>
> Here's an image with both looks of the same painting commands
> http://a.imageshack.us/img375/5499/commitment1.png
>
> ps: please put me on the cc (or reply-all) since I am not subscribed to the
> list.
>
> regards
> Ronny

Different paint engines might be used depending on the platform. For 
QImage the raster paint engine is used, and that is also the default 
widget paint engine on Windows. On Linux the X11 paint engine is used 
for pixmaps and widgets, and on mac the core graphics paint engine is used.

For some painting operations where we use the native APIs to paint we 
don't get the exact same behaviour on all platforms like in this case. 
We're hoping in the future to reduce the number of paint engines and use 
raster as the default on linux and mac too. In the mean-time you can run 
your application with -graphicssystem raster or do 
QApplication::setGraphicsSystem("raster"); before creating the 
QApplication object. Note that the raster graphics system is not that 
stable on mac yet.

--
Samuel