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我的问题与此处提到的相同。我还在我的应用程序中使用了两个图像，我所需要的只是通过触摸擦除顶部图像。然后通过触摸取消擦除（如果需要）已擦除的部分。我正在使用以下代码擦除顶部图像。这种方法也有问题。这就是图像很大，我正在使用 Aspect Fit 内容模式来正确显示它们。当我触摸屏幕时，它会在角落而不是触摸的地方擦除。我认为接触点计算需要一些修复。任何帮助将不胜感激。

第二个问题是如何通过触摸取消擦除已擦除的部分？

UIGraphicsBeginImageContext(self.imgTop.image.size);
[self.imgTop.image drawInRect:CGRectMake(0, 0, self.imgTop.image.size.width, self.imgTop.image.size.height)];
self.frame.size.width, self.frame.size.height)];
CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound); 
GContextSetLineWidth(UIGraphicsGetCurrentContext(), pinSize); 
CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), 0, 0, 0, 1.0);
CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeCopy);

CGContextBeginPath(UIGraphicsGetCurrentContext());
CGContextMoveToPoint(UIGraphicsGetCurrentContext(), lastPoint.x, lastPoint.y);
CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), currentPoint.x, currentPoint.y);
CGContextStrokePath(UIGraphicsGetCurrentContext());
self.imgTop.contentMode = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
4

1 回答 1

9

您的代码非常模棱两可：您正在创建一个内部带有imgTop的上下文，然后与kCGBlendModeCopy黑色混合？这将导致黑色被复制到imgTop上。我假设你想设置图层的content属性呢？

无论如何，这门课可以满足您的需要。只有几个有趣的方法（它们在顶部），其他的只是属性或init...例程。

@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

一些注意事项：

  • 这个类保留了你需要的两个图像。它具有touchRevealsImage将模式设置为绘制或擦除的属性，并且您可以设置线条的宽度。

  • 在初始化时，它会创建一个CGBitmapContextRef与视图大小相同的、灰度、8bpp、无 alpha。此上下文用于存储将应用于前景图像的蒙版。

  • 每次您在屏幕上移动手指时，都会在CGBitmapContextRefCoreGraphics 上绘制一条线，白色显示图像，黑色隐藏图像。通过这种方式，我们存储 ab/w 绘图。

  • 该drawRect:例程简单地绘制背景，然后CGImageRef从 中创建一个CGBitmapContextRef并将其作为掩码应用于当前上下文。然后绘制前景图像。它使用 绘制图像- (void)drawImageScaled:(UIImage *)image，它只是绘制缩放和居中的图像。

  • 如果您打算调整视图大小，您应该实现一种方法来复制或重新创建具有新大小的遮罩，覆盖- (void)setFrame:(CGRect)frame.

  • 该- (void)reset方法只是清除掩码。

  • 即使位图上下文没有任何 alpha 通道，使用的灰度颜色空间也有alpha：这就是为什么每次设置颜色时，我都必须指定两个组件。

<code>EraseImageView</code> 类的示例应用程序

于 2012-11-12T01:08:58.050 回答