Qt-interest Archive, February 2008
Bug in QPainter::drawPixmap
Message 1 in thread
Hi all,
I have found a weird behavior of QPainter::drawPixmap
Qt widgets and painting devices have a coordinate system where a lower y
value corresponds to the top.
Most engineering and math coordinates systems have a coordinate system where
low y is the bottom.
I use a "math type" coordinate system (y rises towards the top) and for this
I use the worldTransformation of the painter:
painter->scale (scale, -scale) //The y axis is inverted
Everything works fine for drawing vector objects, but not for drawing
pixmaps.
I use painter->drawPixmap(targetRect,pixmap,sourceRect) for drawing the
image.
sourceRect is set to QRect(0,0,256,256) to draw the whole image.
If targetRect (the destination rectangle in world coordinates) is
"normalized" , such as QRect(-1,-1,2,2), the pixmap is drawn top-down. This
is an expected behavior, since the target rectangle transformed to device
coordinates is top-down (with negative height)
So I want to correct this. I set targetRect to a top-down rectangle, such as
QRect(-1,1,2,-2).
Then the pixmap is not rendered properly (it looks like a single pixel row
from the source image is stretched to the destination rectangle)
So my conclusion is (I may do something wrong however...)
- It is not possible to draw a pixmap in its original orientation on a
y-flipped worldTransform painter,
- A solution to this consists in pre-flipping the source pixmap. However
this is more a hack than a clean solution.
Thanks
Etienne
Message 2 in thread
Etienne Sandrà wrote:
> So I want to correct this. I set targetRect to a top-down rectangle,
> such as QRect(-1,1,2,-2).
> Then the pixmap is not rendered properly (it looks like a single pixel
> row from the source image is stretched to the destination rectangle)
You probably want to use QRectF instead of QRect. QRect does some really
weird things when its width or height is negative, which according to
the Qt documentation is "for historical reasons."
http://doc.trolltech.com/4.3/qrect.html#coordinates
Search for "historical" on that page. I had a similar problem, and
decided to use QRectF, not because it uses floating point values, but
because it stores x, y, width, height, as opposed to QRect, which stores
x1, y1, x2, y2.
You can use this overload of QPainter::drawPixmap() which takes a QRectF
instead of a QRect:
http://doc.trolltech.com/4.3/qpainter.html#drawPixmap
Happy coding!
--Dave
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