Der Code ist unten:

- (void)drawLayer:(CALayer *)layer inContext:(CGContextRef)ctx {
    CGContextRef context = ctx;
    CGContextRetain(context);
    CGContextSetInterpolationQuality(context, kCGInterpolationNone);
    CGContextSetLineCap(context, kCGLineCapButt);
    CGContextSetLineWidth(context, 1);
    CGContextSetRGBStrokeColor(context, 0, 0, 0, 0.8);

    // draw image
    CGContextSaveGState(context);
    CGAffineTransform flipVertical = CGAffineTransformMake(
                                                           1, 0, 0, -1, 0, layer.bounds.size.height
                                                           );
    CGContextConcatCTM(context, flipVertical);

    CGContextDrawImage(context, layer.bounds, imageRef);
    CGContextRestoreGState(context);

    // draw borders
    CGContextAddRect(context, layer.bounds);
    CGContextDrawPath(context, kCGPathStroke);

    CGFloat apertureSize = scaleFactor * (2 * samplingLevel - 1) - 1;
    CGFloat x = self.bounds.size.width / 2 - apertureSize / 2;
    CGFloat y = self.bounds.size.height / 2 - apertureSize / 2;
    CGRect apertureRect = CGRectMake(x, y, apertureSize, apertureSize);

    // draw the aperture
    CGContextSaveGState(context);
    CGContextSetRGBStrokeColor(context, 0, 0, 0, 1);      
    CGContextSetShouldAntialias(context, NO);
    CGContextStrokeRect(context, apertureRect);
    CGContextRestoreGState(context);

    CGContextRelease(context);
}

und es stürzt ab mit:

(lldb) thread backtrace
* thread #1: tid = 0x1c03, 0x37537a5c CoreGraphics`argb32_image_mark_rgb32 + 800, stop reason = EXC_BAD_ACCESS (code=1, address=0x7314b0)
frame #0: 0x37537a5c CoreGraphics`argb32_image_mark_rgb32 + 800
frame #1: 0x3750e400 CoreGraphics`argb32_image + 5948
frame #2: 0x313c5894 libRIP.A.dylib`ripl_Mark + 16
frame #3: 0x313ca68a libRIP.A.dylib`ripl_BltImage + 998
frame #4: 0x313c2468 libRIP.A.dylib`ripc_RenderImage + 180
frame #5: 0x313be300 libRIP.A.dylib`ripc_DrawImage + 584
frame #6: 0x3750a928 CoreGraphics`CGContextDelegateDrawImage + 48
frame #7: 0x3750a794 CoreGraphics`CGContextDrawImage + 292
frame #8: 0x00166e34 MyApp`-[MyView drawLayer:inContext:] + 380 at MyView.m:43
frame #9: 0x33f054e4 QuartzCore`-[CALayer drawInContext:] + 116
frame #10: 0x33f04b3e QuartzCore`CABackingStoreUpdate_ + 1782
frame #11: 0x33f04334 QuartzCore`CA::Layer::display_() + 956
frame #12: 0x33f03f60 QuartzCore`CA::Layer::display() + 128

Weiß jemand warum? Vielen Dank! Ich denke, es wird entweder von CGImage oder CGContext verursacht, aber beide sind beim Debuggen nicht null. Das Bild, mit dem ich die Ansicht aktualisiert habe, stammt aus der Kameraerfassung.