[Development] High-dpi Qt best practices

Ziller Eike Eike.Ziller at digia.com
Tue Oct 9 13:15:12 CEST 2012

On 9 Oct 2012, at 12:50, Sorvig Morten <Morten.Sorvig at digia.com>

> On Oct 9, 2012, at 12:21 PM, Olivier Goffart <olivier at woboq.com>
> wrote:
>> On Tuesday 09 October 2012 09:46:37 Sorvig Morten wrote:
>>> On Oct 9, 2012, at 11:32 AM, Olivier Goffart <olivier at woboq.com>
>>> wrote:
>>>> But QSize is already the size in pixel.
>>>> Or do you mean that QIcon::pixmap could return a pixmap that is larger
>>>> than
>>>> the given QSize, scaled with some magic heuristics.  That is not really
>>>> intuitive. (and violate the current documentation)
>>> I'm re-defining it to be the size in points. I think this intuitive, you are
>>> asking QIcon::pixmap() for pixmap suitable for covering this many units on
>>> screen - not for a pixmap of a specific size.
>> What is an "unit on screen"?  Is it not a pixel?
> In this context it is a point - the unit that does not change when you enable high-dpi mode. 
> Apple has quite comprehensive documentation about this topic: http://tinyurl.com/8me4yzz is recommended background material for those who are interested.
>> When you have do QImage(QSize),  the image has hight*width pixel, right?
>> Same for QPixmap?  or has a pixmap actually more pixal than its size? (as 
>> returned by QPixmap::size/width/height)
> Pixmaps and images _always_ have exactly QPixmap::size() pixels.
>> Maybe we can introduce QSizePt
>> Code would look like
>> QPixmap p = QIcon::pixmap(QSizePt(20,20).toSize(window));
>> (but then again, i'd say it is bad practice to even hardcode pt size)
> I don't know.. to me QSize is just a pair of ints, it does not have a unit.

It certainly doesn't have a clearly defined mapping to "screen pixels", if you remember this old video :)

Wolfenstein Qt. Qt widgets in a 3D environment

>> I think there should not be that may use of QIcon::pixmap anyway.  It is 
>> mainly going to be used handled QToolBar and similar.
> Then it has to work for QToolBar. But I think it's used a fair bit in Qt:
> widgets msorvig$ git grep QIcon | wc
>     668    3666   59429
> Morten
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Eike Ziller, Senior Software Engineer - Digia, Qt
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