[Development] Retina display support

[Sorvig Morten]

On 21 Sep 2012, at 15:30, P Bai <apbai at yahoo.com>
 wrote:

> Thank you, Eike. From what I read in the codereview, a function to get the scale factor qt_mac_get_scalefactor() is available as a patch, but QPixmap/QImage HiDPI support is still a WIP. Does that mean even if I were able to detect HiDPI mode, I still can't do anything until QPixmap/QImage support becomes available. Is that right?

The patches are still not ready, but I can give you an idea of how it's going to work:

As you know there is a point/pixel split. Screen/window/widget/event geometry is in points. QPixmap and QImage sizes are in pixels. On high-dpi screens there is a 1:2 mapping between points and pixels - point geometry does not change. You can mostly keep working in points as before and ignore the scale factor, except when dealing with raster graphics.

There will be a (public) function for accessing the scale factor, most likely qt_mac_get_scalefactor() in Qt 4 and perhaps a QScreen/QWindow property in Qt 5. I say perhaps because it's complicated: get_scalefactor needs to know the screen in question, but we don't always have that information in Qt (for example deep in style code).

For QPixmap/QImage my current approach is to add a dpiScaleFactor property to both classes, which is typically set to 2.0 to indicate high-dpi raster content. I've experimented with adopting the "@2x" file name convention when loading images from files, this seems to work well. QIcon::pixmap() will also be modified to produce a high-dpi pixmap when appropriate. 

The plan is that Qt 4 will support painting OF high-dpi images to screen, and that Qt 5 will (in addition) support painting ON high-dpi images using a scaled painter. The latter is required since Qt 5 uses the raster graphics system.

There is one behaviour change to be aware of: With the addition of dpiScaleFactor it is now an error to calculate layouts directly from pixmap and image sizes. The correct way is to divide the size by the scale factor. For example, a 2x 64x64 pixmap should be 32x32 points on screen. This is a behaviour change wich has the potential to break user code, so I think we'll need to have an opt-in mechanism here.

For OpenGL you can request a 2x pixel buffer by setting a flag. Qt should provide a similar option to apps.

- Morten