ios - CGImageCreateWithImageInRect 切断

Question

我在 Swift 中使用 CGImageCreateWithImageInRect 在触摸位置创建部分图像的副本。

我的代码运行良好，除非用户触摸屏幕边缘并且矩形大小落在 View 框架之外。它不是返回一个正方形，而是返回一个矩形(我假设)来切断 View 外的数量。形状的变化会偏离复制图像的位置，因此它不会与下方图像对齐。

如何保持正方形/大小并忽略边界，以便我的图像对齐？

override func touchesBegan(touches: Set<UITouch>, withEvent event: UIEvent?) {

    let touch = touches.first!.locationInView(mainImageView?.superview)

    UIGraphicsBeginImageContext(self.view.bounds.size)
    UIApplication.sharedApplication().keyWindow!.layer.renderInContext(UIGraphicsGetCurrentContext()!)
    let screenshot = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()

    let crop = CGRectMake(touch.x - CGFloat(brushWidth) / 2, touch.y - CGFloat(brushWidth) / 2, CGFloat(brushWidth), CGFloat(brushWidth))

    let imageRef = CGImageCreateWithImageInRect(screenshot.CGImage, crop)

    if imageRef != nil {
        let newImage: UIImage = UIImage(CGImage: imageRef!, scale: mainImageView.image!.scale, orientation: mainImageView.image!.imageOrientation)

                let bgImage = UIImageView(image: processedImage)
                bgImage.center = touch
                self.view.addSubview(bgImage)

            }
        }
    }

}

这是它工作的一个例子:

这是一个扭曲图像的例子，因为我触摸了边缘:

Answer 1

CGImageCreateWithImageInRect(screenshot.CGImage, crop) 确实返回了裁剪后的版本。以下是文档中的相关评论:

Quartz performs these tasks to create the subimage:

• Adjusts the area specified by the rect parameter to integral bounds by calling the function CGRectIntegral.

• Intersects the result with a rectangle whose origin is (0,0) and size is equal to the size of the image specified by the image parameter.

• References the pixels within the resulting rectangle, treating the first pixel within the rectangle as the origin of the sub image.

解决此问题的一个简单方法是调整生成的 UIImageView 的位置，使其大小正确。下面是相关的计算:

let screenshotBounds = CGRectMake(0.0, 0.0, screenshot.size.width, screenshot.size.height)
let cropIntegral = CGRectIntegral(crop)
let cropIntersection = CGRectIntersection(cropIntegral, screenshotBounds)
bgImage.center = CGPoint(CGRectGetMidX(cropIntersection), CGRectGetMidY(cropIntersection))

cropIntersection 是我们提取图像的边界矩形(遵循文档中给出的前两个步骤)。因此，我们可以使用它来将 imageView 定位到原始图像中的相同位置。