CGContextAddArc not respecting clipping rects for open paths

Question

Angus_FX OP

Created Jan ’25

Replies 1

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Participants 2

macOS 15.2, MBP M1, built-in display.

The following code produces a line outside the bounds of my clipping region when drawing to CGLayers, to produce a clockwise arc:

    CGContextBeginPath(m_s.outContext);
    CGContextAddArc(m_s.outContext, leftX + radius, topY - radius, radius, -startRads, -endRads, 1);
    CGContextSetRGBStrokeColor(m_s.outContext, col.fRed(), col.fGreen(), col.fBlue(), col.fAlpha());
    CGContextSetLineWidth(m_s.outContext, width);
    CGContextStrokePath(m_s.outContext);

Drawing other shapes such as rects or ellipses doesn't cause a problem.

I can work around the issue by bringing the path back to the start of the arc:

    CGContextBeginPath(m_s.outContext);
    CGContextAddArc(m_s.outContext, leftX + radius, topY - radius, radius, -startRads, -endRads, 1);
    // add a second arc back to the start
    CGContextAddArc(m_s.outContext, leftX + radius, topY - radius, radius, -endRads, -startRads, 0);
    CGContextSetRGBStrokeColor(m_s.outContext, col.fRed(), col.fGreen(), col.fBlue(), col.fAlpha());
    CGContextSetLineWidth(m_s.outContext, width);
    CGContextStrokePath(m_s.outContext);

But this does appear to be a bug.

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Answer 1

DTS Engineer OP

Apple

Jan ’25

I tried to reproduce the problem you described here, but I could only guess what the values stored in your variables were. Can you provide a version with the values you're using?

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