Applying alpha outside intersection of 2 CGPath objects

问题

I have a CGPathRef for an oddly shaped polygon. I need to apply an alpha to the area outside of this path. This is pretty straightforward.

CGContextAddPath(context, crazyPolygon);
CGContextSetFillColor(context, someAlphaColor);
CGContextEOFillPath(context);

I need to do something similar with a circle, also straightforward.

CGMutablePathRef circlePath = CGPathCreateMutable();
CGPathAddRect(circlePath, NULL, rect);
CGPathAddEllipseInRect(circlePath, NULL, circleBox);
CGContextAddPath(context, circlePath);
CGContextSetFillColor(context, someAlphaColor);
CGContextEOFillPath(context);

The problem arises when I try to intersect the two shapes. I want to apply alpha to any pixel that isn't inside both shapes.

If the point is in the circle but not within the polygon, apply the alpha.
If it is in the polygon but not in the circle, apply the alpha.
If it is in both the polygon and the circle, the pixel should be fully transparent.

I've tried a bunch of different approaches. None have worked. The most promising was to create a mask with the polygon and using CGContextClipToMask to bound the drawing of the circle. The full circle was drawn with no clipping though.

回答1:

After a couple hours of more trial and error, I finally figured it out.

// Set up
CGContextRef context = UIGraphicsGetCurrentContext();
CGFloat outOfAreaColor[4] = { 0.0, 0.0, 0.0, OUT_OF_AREA_ALPHA };
CGContextSetFillColor(context, outOfAreaColor);

// Path for specifying outside of polygons
CGMutablePathRef outline = CGPathCreateMutable();
CGPathAddRect(outline, NULL, rect);
CGPathAddPath(outline, NULL, path);

// Fill the area outside of the path with an alpha mask
CGContextAddPath(context, outline);
CGPathRelease(outline);
CGContextEOFillPath(context);

// Add the inside path to the context and clip the context to that area
CGContextAddPath(context, insidePolygon);
CGContextClip(context);

// Create a path defining the area to draw outside of the circle
// but within the polygon
CGRect circleBox = CGRectMake(0, 0, circleRadius * 2.0, circleRadius * 2.0);
CGMutablePathRef darkLayer = CGPathCreateMutable();
CGPathAddRect(darkLayer, NULL, rect);
CGPathAddEllipseInRect(darkLayer, NULL, circleBox);
CGContextAddPath(context, darkLayer);
CGContextEOFillPath(context);
CGPathRelease(darkLayer);

When using CGPathAddEllipseInRect, the center of the circle/ellipse is circleBox.origin.x + circleBox.size.width / 2.0, circleBox.origin.y + circleBox.size.height / 2.0, not 0.0, 0.0. The documentation makes this very clear, but it's annoying to have to figure out when positioning your shapes.

来源：https://stackoverflow.com/questions/7015667/applying-alpha-outside-intersection-of-2-cgpath-objects

标签

iphone

core-graphics

cgpath