[Interest] Double-buffering QGraphicsItem with QImage on retina display

[Patrick Stinson]

That’s a good setting to know about, so thank you for pointing it out.

Unfortunately, this isn’t what I am trying to accomplish. I have a singleton QGraphicsItem that acts as a canvas and is always resized to fit the view’s viewport. This canvas item paints lines to a QImage buffer with each QPointF that comes in from the pencil or mouse, and each call to paint() just paints the QImage buffer. When the pencil or mouse is lifted, it produces a QPixmap from the boundingRect of the drawn data and creates a QGraphicsPixmapItem to contain it. Each pencil stroke is created in this way which works quite well (undo/move/delete, etc.), except for the low-res rendering.

The canvas item uses a QImage “double-buffer” not to speed up performance but to contain the pixmap data that is used to produce the pencil stroke items.

Make sense?

> On Mar 16, 2017, at 8:21 PM, Ch'Gans <chgans at gna.org> wrote:
> 
> On 17 March 2017 at 16:01, Patrick Stinson <patrickkidd at gmail.com> wrote:
>> Hi there!
>> 
>> I am implementing handwriting with the apple pencil and am double-buffering each stroke on a QImage before passing the resulting pixmap and bounding rect for the stroke to a QGraphicsItem. The problem is that painting to the QImage is not at retina resolution which is what you get when you paint directly to the scene via QGraphicsItem::paint.
>> 
>> Does anyone have a suggestion for how to handle this?
>> 
>> Should I just double the QImage size and then translate the coordinates appropriately? Then half the target rect in QPainter::drawPixmap(const Rect &target, const QPixmap &, const QSize &source)? Or is there a more elegant way to do it?
>> 
> 
> Have you tried QGraphicsItem::DeviceCoordinateCache ?
> 
> My 2 cents
> 
>> Thanks!
>> -P
>> 
>> 
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