[Qt-interest] QPainter performance

[Glenn Pierce]

HI

I am new to QT and I have been following the guide at

http://labs.trolltech.com/blogs/2006/05/13/fast-transformed-pixmapimage-drawing/

This is so I can create an QImageView that would allow one to pan and zoom
an image to any size.

I have, I believe, implemented my fast drawing in the same way. I have also
used similar techniques for doing the same with GDI and GTK without
problems.


However, although the code works OK when there is no scaling as soon as I
add painter.scale(2.0, 2.0) the scrolling of the image is very slow.

The image I am testing with is 3888x2592 pixels in size.


My code is

QScrollBar *hs = this->horizontalScrollBar(); QScrollBar *vs =
this->verticalScrollBar();

int horz*pos = hs->value(); int vert*pos = vs->value();

QPainter painter(this->viewport());

painter.scale(2, 2); painter.translate(-horz*pos, -vert*pos);

QRect exposedRect =
painter.matrix().inverted().mapRect(event->rect()).adjusted(-1, -1, 1, 1);

// the adjust is to account for half pixels along edges
painter.drawPixmap(exposedRect, this->pix, exposedRect);


Although I am only drawing only a very small portion of the image using the
scrollbars or panning is dead slow ?

Should this not be fast ?


Any advice is appreciated.

Thank you

Glenn
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