用相同的 CGRect 裁剪相同的 UIImage 会产生不同的结果

Question

我在应用中有以下功能：

1. 用户拍摄（或选择）一张图片（以下简称originalImage）。
2. originalImage 被发送到某个外部 API，它返回我需要添加到 originalImage 的点坐标数组。
3. 由于点始终位于一个区域（面部），我想裁剪靠近面部边界的originalImage，并仅向用户显示裁剪结果。
4. 显示裁剪结果后，我会一个一个地添加点。

这是完成这项工作的代码（除了发送图像，假设它已经发生了）

class ScanResultViewController{

   @IBOutlet weak var scanPreviewImageView: UIImageView!

   let originalImage = ORIGINAL_IMAGE //meaning we already have it

   let scanDots = [["x":123, "y":123], ["x":234, "y":234]]//total 68 coordinates

   var cropRect:CGRect!

   override func viewDidLoad() {
      super.viewDidLoad()

         self.setScanImage()
   }

   override func viewDidAppear(animated: Bool) {
      super.viewDidAppear(animated)

      self.animateScan(0)
   }


   func setScanImage(){

      self.cropRect = self.getCropRect(self.scanDots, sourceImage:self.originalImage)

      let croppedImage = self.originalImage.imageAtRect(self.cropRect)

      self.scanPreviewImageView.image = croppedImage
      self.scanPreviewImageView.contentMode = .ScaleAspectFill

   }


   func animateScan(index:Int){

       let i = index

       self.originalImage = self.addOnePointToImage(self.originalImage, pointImage: GREEN_DOT!, point: self.scanDots[i])

       let croppedImage = self.originalImage.imageAtRect(self.cropRect)

       self.scanPreviewImageView.image = croppedImage
       self.scanPreviewImageView.contentMode = .ScaleAspectFill

       if i < self.scanDots.count-1{

           let delay = dispatch_time(DISPATCH_TIME_NOW, Int64(0.1 * Double(NSEC_PER_SEC)))
           dispatch_after(delay, dispatch_get_main_queue()) {

               self.animateScan(i+1)
           }
       }
   }


   func addOnePointToImage(sourceImage:UIImage, pointImage:UIImage, point: Dictionary<String,CGFloat>)->UIImage{

       let rect = CGRect(x: 0, y: 0, width: sourceImage.size.width, height: sourceImage.size.height)

       UIGraphicsBeginImageContextWithOptions(sourceImage.size, true, 0)
       let context = UIGraphicsGetCurrentContext()

       CGContextSetFillColorWithColor(context, UIColor.whiteColor().CGColor)
       CGContextFillRect(context, rect)

       sourceImage.drawInRect(rect, blendMode: .Normal, alpha: 1)

       let pointWidth = sourceImage.size.width/66.7

       pointImage.drawInRect(CGRectMake(point["x"]!-pointWidth/2, point["y"]!-pointWidth/2, pointWidth, pointWidth), blendMode: .Normal, alpha: 1)

       let result = UIGraphicsGetImageFromCurrentImageContext()
       UIGraphicsEndImageContext()

       return result
   }


   func getCropRect(points: Array<Dictionary<String,CGFloat>>, sourceImage:UIImage)->CGRect{

       var topLeft:CGPoint = CGPoint(x: points[0]["x"]!, y: points[0]["y"]!)
       var topRight:CGPoint = CGPoint(x: points[0]["x"]!, y: points[0]["y"]!)
       var bottomLeft:CGPoint = CGPoint(x: points[0]["x"]!, y: points[0]["y"]!)
       var bottomRight:CGPoint = CGPoint(x: points[0]["x"]!, y: points[0]["y"]!)

       for p in points{

           if p["x"]<topLeft.x {topLeft.x = p["x"]!}
           if p["y"]<topLeft.y {topLeft.y = p["y"]!}

           if p["x"]>topRight.x {topRight.x = p["x"]!}
           if p["y"]<topRight.y {topRight.y = p["y"]!}

           if p["x"]<bottomLeft.x {bottomLeft.x = p["x"]!}
           if p["y"]>bottomLeft.y {bottomLeft.y = p["y"]!}

           if p["x"]>bottomRight.x {bottomRight.x = p["x"]!}
           if p["y"]>bottomRight.y {bottomRight.y = p["y"]!}

       }

       let rect = CGRect(x: topLeft.x, y: topLeft.y, width: (topRight.x-topLeft.x), height: (bottomLeft.y-topLeft.y))

       return rect
   }

}

extension UIImage{

   public func imageAtRect(rect: CGRect) -> UIImage {
       let imageRef: CGImageRef = CGImageCreateWithImageInRect(self.CGImage, rect)!
       let subImage: UIImage = UIImage(CGImage: imageRef)

       return subImage
   }

}

问题是在setScanImage中，所需区域被准确裁剪和显示，但是当animateScan方法被调用时，同一图像的不同区域被裁剪（并显示）尽管@ 987654328@ 是一样的，originalImage 的大小是完全一样的。

有什么想法吗，伙计们？

顺便说一句，如果我显示 originalImage 而不裁剪它，一切都会顺利进行。

Answer 1

最终在大约 10 小时的净时间后（以及 stackoverflow 社区的大量帮助:-)我设法解决了这个问题：

在函数addOnePointToImage中你需要更改以下内容：

在这一行：

UIGraphicsBeginImageContextWithOptions(sourceImage.size, true, 0)

您需要将最后一个参数（代表比例）更改为 1：

UIGraphicsBeginImageContextWithOptions(sourceImage.size, true, 1)

这完全解决了问题。