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QuartzDemo/QuartzClipping.m

/*

     File: QuartzClipping.m

 Abstract: Demonstrates using Quartz for clipping (QuartzClippingView) and masking (QuartzMaskingView).

  Version: 3.0

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*/

#import "QuartzClipping.h"

@implementation QuartzClippingView

    CGImageRef _image;

-(CGImageRef)image

    if (_image == NULL)

        NSString *imagePath = [[NSBundle mainBundle] pathForResource:@"Ship.png" ofType:nil];

        UIImage *img = [UIImage imageWithContentsOfFile:imagePath];

        _image = CGImageRetain(img.CGImage);

    return _image;

-(void)addStarToContext:(CGContextRef)context at:(CGPoint)center radius:(CGFloat)radius angle:(CGFloat)angle

    CGFloat x = radius * sinf(angle * M_PI / 5.0) + center.x;

    CGFloat y = radius * cosf(angle * M_PI / 5.0) + center.y;

    CGContextMoveToPoint(context, x, y);

    for(int i = 1; i < 5; ++i)

        CGFloat x = radius * sinf((i * 4.0 * M_PI + angle) / 5.0) + center.x;

        CGFloat y = radius * cosf((i * 4.0 * M_PI + angle) / 5.0) + center.y;

        CGContextAddLineToPoint(context, x, y);

    // And close the subpath.

    CGContextClosePath(context);

-(void)drawInContext:(CGContextRef)context

    // NOTE

    // So that the images in this demo appear right-side-up, we flip the context

    // In doing so we need to specify all of our Y positions relative to the height of the view.

    // The value we subtract from the height is the Y coordinate for the *bottom* of the image.

    CGFloat height = self.bounds.size.height;

    CGContextTranslateCTM(context, 0.0, height);

    CGContextScaleCTM(context, 1.0, -1.0);

    CGContextSetRGBFillColor(context, 1.0, 0.0, 0.0, 1.0);

    // We'll draw the original image for comparision

    CGContextDrawImage(context, CGRectMake(10.0, height - 100.0, 90.0, 90.0), self.image);

    // First we'll use clipping rectangles to remove the body of the ship.

    // We use CGContextClipToRects() to clip to a set of rectangles.

    CGContextSaveGState(context);

    // For this operation we extract the 35 pixel strip on each side of the source image.

    CGRect clips[] =

        CGRectMake(110.0, height - 100.0, 35.0, 90.0),

        CGRectMake(165.0, height - 100.0, 35.0, 90.0),

};

    // While convinient, this is just the equivalent of adding each rectangle to the current path,

    // then calling CGContextClip().

    CGContextClipToRects(context, clips, sizeof(clips) / sizeof(clips[0]));

    CGContextDrawImage(context, CGRectMake(110.0, height - 100.0, 90.0, 90.0), self.image);

    CGContextRestoreGState(context);

    // You can also clip to aribitrary shapes, which can be useful for special effects.

    // In this case we are going to clip to a star.

    // We will actually clip the image twice, using the different clipping modes.

    [self addStarToContext:context at:CGPointMake(55.0, height - 150.0) radius:45.0 angle:0.0];

    CGContextSaveGState(context);

    // Clip to the current path using the non-zero winding number rule.

    CGContextClip(context);

    // To make the area we draw to a bit more obvious, we'll the image over a red rectangle.

    CGContextFillRect(context, CGRectMake(10.0, height - 190.0, 90.0, 90.0));

    // And finally draw the image

    CGContextDrawImage(context, CGRectMake(10.0, height - 190.0, 90.0, 90.0), self.image);

    CGContextRestoreGState(context);

    [self addStarToContext:context at:CGPointMake(155.0, height - 150.0) radius:45.0 angle:0.0];

    CGContextSaveGState(context);

    // Clip to the current path using the even-odd rule.

    CGContextEOClip(context);

    // To make the area we draw to a bit more obvious, we'll the image over a red rectangle.

    CGContextFillRect(context, CGRectMake(110.0, height - 190.0, 90.0, 90.0));

    // And finally draw the image

    CGContextDrawImage(context, CGRectMake(110.0, height - 190.0, 90.0, 90.0), self.image);

    CGContextRestoreGState(context);

    // Finally making the path slightly more complex by enscribing it in a rectangle changes what is clipped

    // For EO clipping mode this will invert the clip (for non-zero winding this is less predictable).

    [self addStarToContext:context at:CGPointMake(255.0, height - 150.0) radius:45.0 angle:0.0];

    CGContextAddRect(context, CGRectMake(210., height - 190., 90., 90.));

    CGContextSaveGState(context);

    // Clip to the current path using the even-odd rule.

    CGContextEOClip(context);

    // To make the area we draw to a bit more obvious, we'll the image over a red rectangle.

    CGContextFillRect(context, CGRectMake(210.0, height - 190.0, 90.0, 90.0));

    // And finally draw the image

    CGContextDrawImage(context, CGRectMake(210.0, height - 190.0, 90.0, 90.0), self.image);

    CGContextRestoreGState(context);

-(void)dealloc

    CGImageRelease(_image);

@end

#pragma mark - QuartzMaskingView

@implementation QuartzMaskingView

    CGImageRef _maskingImage;

    CGImageRef _alphaImage;

-(void)createImages

    // Load the alpha image, which is just the same Ship.png image used in the clipping demo

    NSString *imagePath = [[NSBundle mainBundle] pathForResource:@"Ship.png" ofType:nil];

    UIImage *img = [UIImage imageWithContentsOfFile:imagePath];

    _alphaImage = CGImageRetain(img.CGImage);

    // To show the difference with an image mask, we take the above image and process it to extract

    // the alpha channel as a mask.

    // Allocate data

    NSMutableData *data = [NSMutableData dataWithLength:90 * 90 * 1];

    // Create a bitmap context

    CGContextRef context = CGBitmapContextCreate([data mutableBytes], 90, 90, 8, 90, NULL, (CGBitmapInfo)kCGImageAlphaOnly);

    // Set the blend mode to copy to avoid any alteration of the source data

    CGContextSetBlendMode(context, kCGBlendModeCopy);

    // Draw the image to extract the alpha channel

    CGContextDrawImage(context, CGRectMake(0.0, 0.0, 90.0, 90.0), _alphaImage);

    // Now the alpha channel has been copied into our NSData object above, so discard the context and lets make an image mask.

    CGContextRelease(context);

    // Create a data provider for our data object (NSMutableData is tollfree bridged to CFMutableDataRef, which is compatible with CFDataRef)

    CGDataProviderRef dataProvider = CGDataProviderCreateWithCFData((__bridge CFMutableDataRef)data);

    // Create our new mask image with the same size as the original image

    _maskingImage = CGImageMaskCreate(90, 90, 8, 8, 90, dataProvider, NULL, YES);

    // And release the provider.

    CGDataProviderRelease(dataProvider);

-(void)drawInContext:(CGContextRef)context

    // NOTE

    // So that the images in this demo appear right-side-up, we flip the context

    // In doing so we need to specify all of our Y positions relative to the height of the view.

    // The value we subtract from the height is the Y coordinate for the *bottom* of the image.

    CGFloat height = self.bounds.size.height;

    CGContextTranslateCTM(context, 0.0, height);

    CGContextScaleCTM(context, 1.0, -1.0);

    CGContextSetRGBFillColor(context, 1.0, 1.0, 1.0, 1.0);

    // Quartz also allows you to mask to an image or image mask, the primary difference being

    // how the image data is interpreted. Note that you can use any image

    // When you use a regular image, the alpha channel is interpreted as the alpha values to use,

    // that is a 0.0 alpha indicates no pass and a 1.0 alpha indicates full pass.

    CGContextSaveGState(context);

    CGContextClipToMask(context, CGRectMake(10.0, height - 100.0, 90.0, 90.0), self.alphaImage);

    // Because we're clipping, we aren't going to be particularly careful with our rect.

    CGContextFillRect(context, self.bounds);

    CGContextRestoreGState(context);

    CGContextSaveGState(context);

    // You can also use the clip rect given to scale the mask image

    CGContextClipToMask(context, CGRectMake(110.0, height - 190.0, 180.0, 180.0), self.alphaImage);

    // As above, not being careful with bounds since we are clipping.

    CGContextFillRect(context, self.bounds);

    CGContextRestoreGState(context);

    // Alternatively when you use a mask image the mask data is used much like an inverse alpha channel,

    // that is 0.0 indicates full pas and 1.0 indicates no pass.

    CGContextSaveGState(context);

    CGContextClipToMask(context, CGRectMake(10.0, height - 300.0, 90.0, 90.0), self.maskingImage);

    // Because we're clipping, we aren't going to be particularly careful with our rect.

    CGContextFillRect(context, self.bounds);

    CGContextRestoreGState(context);

    CGContextSaveGState(context);

    // You can also use the clip rect given to scale the mask image

    CGContextClipToMask(context, CGRectMake(110.0, height - 390.0, 180.0, 180.0), self.maskingImage);

    // As above, not being careful with bounds since we are clipping.

    CGContextFillRect(context, self.bounds);

    CGContextRestoreGState(context);

- (CGImageRef)maskingImage

    if (_maskingImage == NULL)

        [self createImages];

    return _maskingImage;

- (CGImageRef)alphaImage

    if (_alphaImage == NULL)

        [self createImages];

    return _alphaImage;

-(void)dealloc

    CGImageRelease(_maskingImage);

    CGImageRelease(_alphaImage);

@end

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