Erase and un erase a image in UIImageVIew with touch using coregraphics

Question

My question is same as mentioned at here. I\'m also using two images in my app and all I need is to erase a top image by touch. Then un-erase (if required) the erased part by to

Answer 1

Your code is quite ambiguous: you're creating a context with imgTop inside, then blending with kCGBlendModeCopy the black color? This would cause the black color to be copied onto imgTop. I assume you wanted to set the layer's contentproperty then?

Anyway this class does what you need. There are only a few interesting methods (they're at the top), the others are only properties or init... routines.

@interface EraseImageView : UIView {
    CGContextRef context;
    CGRect contextBounds;
}

@property (nonatomic, retain) UIImage *backgroundImage;
@property (nonatomic, retain) UIImage *foregroundImage;
@property (nonatomic, assign) CGFloat touchWidth;
@property (nonatomic, assign) BOOL touchRevealsImage;

- (void)resetDrawing;

@end

@interface EraseImageView ()
- (void)createBitmapContext;
- (void)drawImageScaled:(UIImage *)image;
@end

@implementation EraseImageView
@synthesize touchRevealsImage=_touchRevealsImage, backgroundImage=_backgroundImage, foregroundImage=_foregroundImage, touchWidth=_touchWidth;

#pragma mark - Main methods - 

- (void)createBitmapContext
{
    // create a grayscale colorspace
    CGColorSpaceRef grayscale=CGColorSpaceCreateDeviceGray();

    /* TO DO: instead of saving the bounds at the moment of creation,
              override setFrame:, create a new context with the right
              size, draw the previous on the new, and replace the old
              one with the new one.
     */
    contextBounds=self.bounds;

    // create a new 8 bit grayscale bitmap with no alpha (the mask)
    context=CGBitmapContextCreate(NULL,
                                  (size_t)contextBounds.size.width,
                                  (size_t)contextBounds.size.height,
                                  8,
                                  (size_t)contextBounds.size.width,
                                  grayscale,
                                  kCGImageAlphaNone);

    // make it white (touchRevealsImage==NO)
    CGFloat white[]={1., 1.};
    CGContextSetFillColor(context, white);

    CGContextFillRect(context, contextBounds);

    // setup drawing for that context
    CGContextSetLineCap(context, kCGLineCapRound);
    CGContextSetLineJoin(context, kCGLineJoinRound);

    CGColorSpaceRelease(grayscale);
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch *touch=(UITouch *)[touches anyObject];

    // the new line that will be drawn
    CGPoint points[]={
        [touch previousLocationInView:self],
        [touch locationInView:self]
    };

    // setup width and color
    CGContextSetLineWidth(context, self.touchWidth);
    CGFloat color[]={(self.touchRevealsImage ? 1. : 0.), 1.};
    CGContextSetStrokeColor(context, color);

    // stroke
    CGContextStrokeLineSegments(context, points, 2);

    [self setNeedsDisplay];

}

- (void)drawRect:(CGRect)rect
{
    if (self.foregroundImage==nil || self.backgroundImage==nil) return;

    // draw background image
    [self drawImageScaled:self.backgroundImage];

    // create an image mask from the context
    CGImageRef mask=CGBitmapContextCreateImage(context);

    // set the current clipping mask to the image
    CGContextRef ctx=UIGraphicsGetCurrentContext();
    CGContextSaveGState(ctx);

    CGContextClipToMask(ctx, contextBounds, mask);

    // now draw image (with mask)
    [self drawImageScaled:self.foregroundImage];

    CGContextRestoreGState(ctx);

    CGImageRelease(mask);

}

- (void)resetDrawing
{
    // draw black or white
    CGFloat color[]={(self.touchRevealsImage ? 0. : 1.), 1.};

    CGContextSetFillColor(context, color);
    CGContextFillRect(context, contextBounds);

    [self setNeedsDisplay];
}

#pragma mark - Helper methods -

- (void)drawImageScaled:(UIImage *)image
{
    // just draws the image scaled down and centered

    CGFloat selfRatio=self.frame.size.width/self.frame.size.height;
    CGFloat imgRatio=image.size.width/image.size.height;

    CGRect rect={0.,0.,0.,0.};

    if (selfRatio>imgRatio) {
        // view is wider than img
        rect.size.height=self.frame.size.height;
        rect.size.width=imgRatio*rect.size.height;
    } else {
        // img is wider than view
        rect.size.width=self.frame.size.width;
        rect.size.height=rect.size.width/imgRatio;
    }

    rect.origin.x=.5*(self.frame.size.width-rect.size.width);
    rect.origin.y=.5*(self.frame.size.height-rect.size.height);

    [image drawInRect:rect];
}

#pragma mark - Initialization and properties -

- (id)initWithCoder:(NSCoder *)aDecoder
{
    if ((self=[super initWithCoder:aDecoder])) {
        [self createBitmapContext];
        _touchWidth=10.;
    }
    return self;
}

- (id)initWithFrame:(CGRect)frame
{
    if ((self=[super initWithFrame:frame])) {
        [self createBitmapContext];
        _touchWidth=10.;
    }
    return self;
}

- (void)dealloc
{
    CGContextRelease(context);
    [super dealloc];
}

- (void)setBackgroundImage:(UIImage *)value
{
    if (value!=_backgroundImage) {
        [_backgroundImage release];
        _backgroundImage=[value retain];
        [self setNeedsDisplay];
    }
}

- (void)setForegroundImage:(UIImage *)value
{
    if (value!=_foregroundImage) {
        [_foregroundImage release];
        _foregroundImage=[value retain];
        [self setNeedsDisplay];
    }
}

- (void)setTouchRevealsImage:(BOOL)value
{
    if (value!=_touchRevealsImage) {
        _touchRevealsImage=value;
        [self setNeedsDisplay];
    }
}

@end

Some notes:

• This class retains the two images you need. It has a touchRevealsImage property to set the mode to drawing or erasing, and you can set the width of the line.

• At the initialization, it creates a CGBitmapContextRef, grayscale, 8bpp, no alpha, of the same size of the view. This context is used to store a mask, that will be applied to the foreground image.

• Every time you move a finger on the screen, a line is drawn on the CGBitmapContextRef using CoreGraphics, white to reveal the image, black to hide it. In this way we're storing a b/w drawing.

• The drawRect: routine simply draws the background, then creates a CGImageRef from the CGBitmapContextRef and applies it to the current context as a mask. Then draws the foreground image. To draw images it uses - (void)drawImageScaled:(UIImage *)image, which just draws the image scaled and centered.

• If you're planning to resize the view, you should implement a method to copy or to recreate the mask with new size, overriding - (void)setFrame:(CGRect)frame.

• The - (void)reset method simply clears the mask.

• Even if the bitmap context hasn't any alpha channel, the grayscale color space used has alpha: that's why every time a color is set, I had to specify two components.