图片展现、缩放、移动及裁减的实现

首先，读取一个图片文件到UIImage对象中，添加到一个UIImageView视图中。UIImageView的size设置成和图片的size一样。保证图片能全部展现。
// Create the image from a png file
UIImage *image = [UIImage imageNamed:@"prgBinary.jpg"];
UIImageView *imageView = [[UIImageView alloc] initWithImage:image];
 
// Get size of current image
CGSize size = [image size];
 
// Frame location in view to show original image
[imageView setFrame:CGRectMake(0, 0, size.width, size.height)];
[[self view] addSubview:imageView];
[imageView release];    
 
这样UIImageView视图可以通过放大、缩小、移动的方式查看超出屏幕范围的部分图。
将图放大或者缩小到合适尺寸，移动需要裁减的图的一部分到屏幕中，再进行裁减。这样保证裁减到的部分是需要进行图像识别的文字，从而提升识别效率。

使用pinch手势和pan手势实现图像缩放和图像移动操作，将合适的内容完全拉入屏幕中。

使用pan手势移动uiimageview视图。

- (void)pan:(UIPanGestureRecognizer *)gesture
{
  if ((gesture.state == UIGestureRecognizerStateChanged) ||
      (gesture.state == UIGestureRecognizerStateEnded)) {

    CGPoint location = [gesture locationInView:[self superview]];

    [self setCenter:location];
  }
}

使用pinch手势缩放uiimageview视图。
-(void)handlePinch:(UIPinchGestureRecognizer*)sender {
         NSLog(@"latscale = %f",mLastScale);

         mCurrentScale += [sender scale] - mLastScale;
         mLastScale = [sender scale];
        
         if (sender.state == UIGestureRecognizerStateEnded)
         {
                  mLastScale = 1.0;
         }
        
         CGAffineTransform currentTransform = CGAffineTransformIdentity;
         CGAffineTransform newTransform = CGAffineTransformScale(currentTransform, mCurrentScale, mCurrentScale);
         mGestureView.transform = newTransform;
        
}

缩放后的图片，裁减是还是针对了原来的图。因此需要将屏幕进行照相，生成一个图，供裁减操作。因为裁减只能在可看到的区域里进行。
- (UIImage *)imageWithUIView:(UIView *)view
{
    CGSize screenShotSize = view.bounds.size;

    UIImage *img;

    UIGraphicsBeginImageContext(screenShotSize);
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSaveGState(context);

    [view.layer renderInContext:UIGraphicsGetCurrentContext()];
    img = UIGraphicsGetImageFromCurrentImageContext();

    CGContextRestoreGState(context);
    UIGraphicsEndImageContext();

    return img;
}

采用手指滑动画线成矩形框的方式，指定裁减的区域，而后确认。

1、可以用UIBezierPath画一个矩形。它的实现方法大概这样，

UIBezierPath*    aPath = [UIBezierPath bezierPath];
// Set the starting point of the shape.
[aPath moveToPoint:CGPointMake(100.0, 0.0)];
// Draw the lines
[aPath addLineToPoint:CGPointMake(200.0, 0.0)];
[aPath addLineToPoint:CGPointMake(200, 140)];
[aPath addLineToPoint:CGPointMake(0.0, 140)];
[aPath closePath];

参考一个uiview的子类，它使用touchMove调用UIBezierPath画图。

@implementation MyLineDrawingView

- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
        // Initialization code
                
        self.backgroundColor=[UIColor clear];
        myPath=[[UIBezierPath alloc]init];
        myPath.lineCapStyle=kCGLineCapRound;
        myPath.miterLimit=0;
        myPath.lineWidth=10;
        brushPattern=[UIColor redColor];
    }
    return self;
}


// Only override drawRect: if you perform custom drawing.
// An empty implementation adversely affects performance during animation.
- (void)drawRect:(CGRect)rect
{
    [brushPattern setStroke];
    [myPath strokeWithBlendMode:kCGBlendModeNormal alpha:1.0];
    // Drawing code
    //[myPath stroke];
}

#pragma mark - Touch Methods
-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{   
    UITouch *mytouch=[[touches allObjects] objectAtIndex:0];
    [myPath moveToPoint:[mytouch locationInView:self]];
}
-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch *mytouch=[[touches allObjects] objectAtIndex:0];
    [myPath addLineToPoint:[mytouch locationInView:self]];
    [self setNeedsDisplay];
}
-(void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
    [myPath closePath];
    [self setNeedsDisplay];
}

2、如果不用UIBezierPath的话，还可以直接使用 Core Graphics 框架的API进行画线。

@implementation GestureView

{
    CGPoint _originOfTouchPoint; // your fist touch detected in touchesBegan: method
    CGPoint _currentFingerPositionPoint; // the position you have dragged your finger to
    CGFloat _strokeWidth; // the width of the line you wish to draw
    id _touchStartedObject; // the object(UIView) that the first touch was detected on
}

// If you use Interface Builder to design your interface, Objects in a nib file are reconstituted and then initialized using
// their initWithCoder: method
- (id)initWithCoder:(NSCoder *)decoder
{
    self = [super initWithCoder:decoder];
    if (self) {
        // Initialization code
        _originOfTouchPoint = CGPointMake( 0.0, 0.0 );
        _currentFingerPositionPoint = CGPointMake( 100.0, 100.0 );
        _strokeWidth = 2.0;
    }
    return self;
}

// Only override drawRect: if you perform custom drawing.
// An empty implementation adversely affects performance during animation.
- (void)drawRect:(CGRect)rect
{
    CGContextRef context    = UIGraphicsGetCurrentContext();
    CGContextSetStrokeColorWithColor( context, [UIColor blueColor].CGColor );
    CGContextSetLineWidth( context, _strokeWidth );
    // fisrt point of line
    CGContextMoveToPoint( context, _originOfTouchPoint.x, _originOfTouchPoint.y );
    // last point of line
    CGContextAddLineToPoint( context, _currentFingerPositionPoint.x, _currentFingerPositionPoint.y );
    // draw the line
    CGContextStrokePath( context );
}

#pragma mark touches

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    // get starting point and first view touched (if you need to send that view messages)
    _originOfTouchPoint = [[touches anyObject] locationInView:self];
    _touchStartedObject = [[touches anyObject] view];
}
- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    CGPoint movedToPoint = [[touches anyObject] locationInView:self];
    // if moved to a new point redraw the line
    if ( CGPointEqualToPoint( movedToPoint, _currentFingerPositionPoint ) == NO )
    {
        _currentFingerPositionPoint = movedToPoint;
        // calls drawRect: method to show updated line
        [self setNeedsDisplay];
    }
}
- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
    // reset values
    _originOfTouchPoint = CGPointZero;
    _currentFingerPositionPoint = CGPointZero;
    _touchStartedObject = nil;
}

@end

 怎么获取所画的矩形的frame值呢？

 取所涉及的点的最大和最小的X和Y的值，保存到一个数组中。在touch结束之后，重新画矩形。
 这样获得图像上的矩形区域和frame值，根据这个frame值，再使用CGImageCreateWithImageInRect取获得屏幕中截图。 将截图结果即一个UIImage对象换到UIImageView中。验证截图操作是否正确。
 
// Create rectangle that represents a cropped image  
// from the middle of the existing image
CGRect rect = CGRectMake(size.width / 4, size.height / 4 ,
    (size.width / 2), (size.height / 2));
 
// Create bitmap image from original image data,
// using rectangle to specify desired crop area
CGImageRef imageRef = CGImageCreateWithImageInRect([image CGImage], rect);
UIImage *img = [UIImage imageWithCGImage:imageRef];
CGImageRelease(imageRef);
 
// Create and show the new image from bitmap data
imageView = [[UIImageView alloc] initWithImage:img];
[imageView setFrame:CGRectMake(0, 200, (size.width / 2), (size.height / 2))];
[[self view] addSubview:imageView];

[imageView release];

注意，以上所有的代码都是设想和查资料所得的参考，未经编码测试，请读者谨慎使用。