QPainter::drawLine(): straight lines look aliased

[xqrp]

I'm trying to draw two single pixel lines onto a QImage with the following code:

QPainter linePainter(currentImage);
linePainter.setRenderHint(QPainter::Antialiasing, false);
linePainter.setPen(QPen(Qt::red,1));
linePainter.drawLine(xSize/2, 0, xSize/2, ySize);
linePainter.drawLine(0, ySize/2, xSize, ySize/2);

QPainter linePainter(currentImage);
linePainter.setRenderHint(QPainter::Antialiasing, false);
linePainter.setPen(QPen(Qt::red,1));
linePainter.drawLine(xSize/2, 0, xSize/2, ySize);
linePainter.drawLine(0, ySize/2, xSize, ySize/2);

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The image is displayed on a QLabel like this:

ccdDisplayLabel->setPixmap(QPixmap::fromImage(*currentImage));

ccdDisplayLabel->setPixmap(QPixmap::fromImage(*currentImage));

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However, as you can see from the attached image, somewhere along the line something bad happens. The lines should be one pixel wide and bright red, however, the vertical line looks weird. How can I fix this?

CcdDialogCrosshairs.png

[wysota]

Please provide a minimal compilable example reproducing the problem.

[ChrisW67]

If you are doing the data rendering with anti-aliasing on after drawing the red line, and it is directly adjacent to your red line, then the anti-aliasing may be overlaying the red line. Try turning off the data rendering... does the red line look right then? What happens if you render the data without anti-aliasing?

[xqrp]

Thanks for your replies. It seems the problem is due to Windows' remote desktop, not Qt. If I run the program while I'm in front of the actual computer, the lines appear as they're supposed to.

[wysota]

You probably have a loosy compression scheme (like JPEG) for your remote desktop enabled. So it's not antialiasing, it's just simplified data.