QPainter / QPixmap : problem erasing antialiased lines

marievin

Hello,

I would like to erase a previously drawn line in a QPixmap. Here is little sample code to demonstrate the problem:

void Widget::paintEvent(QPaintEvent *)
{
    QPainter p(this);
    QPixmap px(size());

    px.fill(Qt::transparent);

    QPainter p2(&px);
    p2.setRenderHint(QPainter::Antialiasing, true);
    p2.setPen(Qt::red);
    p2.drawLine(100, 100, 200, 200);
    p2.setCompositionMode(QPainter::CompositionMode_Clear);
    p2.drawLine(100, 100, 200, 200);

    p2.end();

    p.drawPixmap(0, 0, px);
    // no line should be drawn in the widget
}

When I run this code, the widget contains a semi transparent red line.
Here is the screenshot

If I deactivate the antialiasing, everything works fine : the line is properly erased from the pixmap.

 p2.setRenderHint(QPainter::Antialiasing, false);

Anybody can tell me what I'm doing wrong ?

Thank you in advance,
Best regards
Vincent

m.sue

Hi,
probably it should work as you expects. But as it does not I would resort to the usual way to do this:
if you want the transparent line removed from the widget simply draw the whole widget anew without the line.
-Michael.

mrjj

Hi
Not tested myself but did you try with
setCompositionMode(QPainter::RasterOp_SourceXorDestination);

m.sue

Yes, Xor-mode is the other way (a fast way) to do this. Then you would already draw the original line in Xor-mode and redraw it in Xor-mode in exactly the same way as before to get rid of it again.
But beware: lines drawn in Xor-mode look in a way your users have to get accustomed to :-)

marievin

Thank you for your help,

@m-sue
For performance reasons, I cannot redraw the entire widget. I draw a lot of line in a pixmap (representing the curve of an analogic signal over time : it can be one hour of data). I optimise my code to only draw needed lines but sometimes I have to redraw a line to reflect signal changes.

@mrjj
As Qt doc says turning on the QPainter::Antialiasing render hint will effectively disable the RasterOp modes. So I cannot use this flag to erase the line

Regards
Vincent

mrjj

Oh, Its simply not supported to
mix Antialiasing and xor style.
Ok no use then. Was just a thought.
Good you do in fact read the docs :)

If full repaint is too expensive, what about dirty rect ? If you need to redraw a line, delete bounding box
then you redraw all lines affected. (also if partial in .)