[Interest] double-buffering, quality drop

[Michael Sué]

> This was what we did before. But for big data sets (say > 50k points) painting
> of the lines and symbols is slow. 
I understand completely.

> > 1) Does your QPixmap look correct if you save it to a file?
> No.
Here I do not mean the image you attached to your first mail. I mean the QPixmap you create in Curve::updatePixmap(), which is the one that then gets scaled afterwards, which itself is the JPG you attached, or not? If the QPixmap of Curve::updatePixmap() is wrong the further processing can, of course, not improve it.

On the other hand, you can try to solve the issue pragmatically: if the drawing works correct if done directly in Curve::paint((QPainter* painter, const QStyleOptionGraphicsItem*, QWidget*) then you can create the QPixmap there (if an update is needed) and draw it or just draw it if no update is needed.

The difference has to lie in the preparation (by Qt) of QPainter before Curve::paint is called.

- Michael.