Invert or XOR a line in Qt

Question

I can draw a solid-color line easily in Qt, but now I need to draw a line by inverting the original pixels, or perhaps XOR-ing the QBrush.

I'm asking for the equivalent of SetROP2(R2_NOT) or SetROP2(R2_XORPEN) in Win32.

Is that possible using Qt? I want to do it so I can easily "unpaint" it later.

What is the inverted color of Rgb(10, 10, 10)? Please clarify. — vahancho, Dec 04 '14 at 08:48
Well, when mentioned alongside XOR-ing, it's flipping every bit (bitwise inversion, or the ~ operator in C). — sashoalm, Dec 04 '14 at 08:52

score 10 · Accepted Answer · answered Dec 04 '14 at 08:59

You can do it like this:

QPainter dc(widget);

dc.setCompositionMode(QPainter::RasterOp_SourceXorDestination);
dc.setPen(QColor(0xff, 0xff, 0xff));

dc.drawEllipse(center, r1, r2);

Other composition modes are described in the QPainter documentation. To undo individual graphics operations, this one should be exactly what you want, though -- just set the same pen you used in the first place and redraw the shape.

Invert or XOR a line in Qt

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