使用CGContext调整UIImage的大小会使我的图像变得柔和，但像素密度不正确

Question

我正在构建一个图像管理器类，用于处理UIImages的大小调整、裁剪等操作。我之所以使用CGContext，是因为它的速度可以与这里的http://nshipster.com/image-resizing/基准测试中的UIGraphicsContext相媲美。

这是我的经理类：

func resizeImage(exportedWidth width: Int, exportedHeight height: Int, originalImage image: UIImage) -> UIImage? {
    let cgImage = image.CGImage

    let width = width
    let height = height
    let bitsPerComponent = CGImageGetBitsPerComponent(cgImage)
    let bytesPerRow = CGImageGetBytesPerRow(cgImage)
    let colorSpace = CGImageGetColorSpace(cgImage)
    let bitmapInfo = CGImageGetBitmapInfo(cgImage)

    let context = CGBitmapContextCreate(nil, width, height, bitsPerComponent, bytesPerRow, colorSpace, bitmapInfo.rawValue)

    CGContextSetInterpolationQuality(context, .High)

    CGContextDrawImage(context, CGRect(origin: CGPointZero, size: CGSize(width: CGFloat(width), height: CGFloat(height))), cgImage)

    let scaledImage = CGBitmapContextCreateImage(context).flatMap { UIImage(CGImage: $0) }

    return scaledImage
}

没什么特别疯狂的，但是当我把它添加到一个子类的UITextView中时，我遇到了麻烦。我试过将图像大小加倍，然后用UIImageView压缩它，但没有效果。这段代码以正确的大小正确地显示了它，但在我的iPhone SE上，图像像素与屏幕像素的比例似乎是1:1，而不是理想的2:1。

func imagePickerController(picker: UIImagePickerController, didFinishPickingMediaWithInfo info: [String : AnyObject]) {
    if let pickedImage = info[UIImagePickerControllerOriginalImage] as? UIImage {
        let textViewWidth: CGFloat = self.view.frame.size.width - 130
        let percentResize = textViewWidth / pickedImage.size.width
        let toBeExportedHeight = pickedImage.size.height * percentResize
        let resizedImage = ImageManipulationManager.sharedInstance.resizeImage(exportedWidth: Int(textViewWidth),exportedHeight: Int(toBeExportedHeight), originalImage: pickedImage)

        let attachment = NSTextAttachment()
        attachment.image = resizedImage
        let attString = NSAttributedString(attachment: attachment)
        textView.textStorage.insertAttributedString(attString, atIndex: textView.selectedRange.location)
        textInputbar.layoutIfNeeded()
        textView.becomeFirstResponder()
        print(textView.text.characters.count)
    }
    dismissViewControllerAnimated(true, completion: nil)
}

Answer 1

双分辨率屏幕的图像必须是双分辨率的。这意味着它应该是所需实际大小的两倍宽和两倍，并在从CGImage派生时将scale附加到UIImage。