iphone stroke path used for clipping UIImage

问题:

I'm very new to CoreGraphics (although I've been doing iphone programming for a while)

Question, I have this snippet that scales proportionally and clips an UIImage to a circle:

-(UIImage *)counterpartImageForSchedule:(Schedule *)counterpartSchedule inSize:(CGSize)size
{
    // This function returns a newImage, based on image, that has been:
    // - scaled to fit in (CGRect) rect
    // - and cropped within a circle of radius: rectWidth/2

    UIImage *image=[UIImage imageNamed:@"fakeuser1.png"];

    // when kendy sends hash, check that this image is not on the cache, otherwise download, clip & stylized
    UIGraphicsBeginImageContextWithOptions(CGSizeMake(size.width, size.height), NO, 0.0);
    CGContextRef context = UIGraphicsGetCurrentContext();


                //Get the width and heights
    CGFloat imageWidth = image.size.width;
    CGFloat imageHeight = image.size.height;
    CGFloat rectWidth = size.width;
    CGFloat rectHeight = size.height;

    //Calculate the scale factor
    CGFloat scaleFactorX = rectWidth/imageWidth;
    CGFloat scaleFactorY = rectHeight/imageHeight;

    if (scaleFactorX>scaleFactorY) {
        scaleFactorY=scaleFactorX;
    } else
    {
        scaleFactorX=scaleFactorY;
    }


    //Calculate the centre of the circle
    CGFloat imageCentreX = rectWidth/2;
    CGFloat imageCentreY = rectHeight/2;

    // Create and CLIP to a CIRCULAR Path
    // (This could be replaced with any closed path if you want a different shaped clip)
    CGFloat radius = rectWidth/2;
    CGContextBeginPath (context);
    CGContextAddArc (context, imageCentreX, imageCentreY, radius, 0, 2*M_PI, 0);


    CGContextClosePath (context);

    CGContextClip (context);

    //Set the SCALE factor for the graphics context
    //All future draw calls will be scaled by this factor
    CGContextScaleCTM (context, scaleFactorX, scaleFactorY);    
    // the stroke

    // Draw the IMAGE
    CGRect myRect = CGRectMake(0, 0, imageWidth, imageHeight);
    [image drawInRect:myRect];

    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();



    return newImage;
}

Question is, how can I stroke the path I'm using to clip the image (say, 4 pixels with black or white color)? do i need to draw a new one?

any help is much appreciated!

回答1:

A path can be an object like any other - in this case, a CGPath (CGPathRef). Before using the path for anything, encapsulate it as a CGPath (in this case, probably a CGMutablePathRef, or perhaps you can call CGContextCopyPath before using the path to clip to). Now you can reuse that path.

Here's an example from one of my apps where I form a path, then stroke it, and then clip to the same path (c is the graphics context):

CGMutablePathRef path = CGPathCreateMutable();
CGPathMoveToPoint(path, nil, CGRectGetMaxX(r) - radius, ins);
CGPathAddArc(path, nil, 
             radius+ins, radius+ins, radius, -M_PI/2.0, M_PI/2.0, true);
CGPathAddArc(path, nil, 
             CGRectGetMaxX(r) - radius, radius+ins, radius, M_PI/2.0, -M_PI/2.0, true);
CGPathCloseSubpath(path);
CGContextAddPath(c, path);
CGContextSetLineWidth(c, 2);
CGContextStrokePath(c);
CGContextAddPath(c, path);
CGContextClip(c);
CGPathRelease(path);

Another possibility is to use UIBezierPath - a full-fledged Objective-C object, instead of CGContext functions. It encapsulates a CGPath and you can reuse that path - clip to it, then stroke it. Or the other way round.