Quartz drawing curve - Weirdness

Question

I've got a weird issue with quartz. The first time I use this piece of code, the line will be drawn like the line to the right in the image. The second time (and every time after) these lines are executed, the line will look like the line to the left. Why does it look like that? It's all jaggy and dark. If you'd draw the curve with opacity, it would be rather bright the first time, and very dark the second.
CGContextSetRGBFillColor(context, 0.0,0.0,0.0,0.0);
CGContextClearRect(context, CGRectMake( 0, 0, 320, 480));
CGContextSetRGBFillColor(context, 0, 0, 0, 0);
CGContextSetBlendMode(context, kCGBlendModeNormal);
CGContextSetRGBStrokeColor(context, 1.0, 1.0, 1.0, 1.0);
CGContextSetLineWidth(context, 4.0);
CGContextMoveToPoint(context, p1.x, p1.y);
CGContextAddCurveToPoint(context, cp1.x, cp1.y, cp2.x, cp2.y, p2.x, p2.y);
CGContextDrawPath(context, kCGPathStroke);
!http://pici.se/pictures/NPTbVloLR.png!
Message was edited by: rawgrarr

Answer 1

Being a total newb, I cannot help noticing you set the fill color to black in your first line.
Would that not imply Quartz tries to antialias the white line to black?
(Although that does not explain why it's correct the first way around.)