在 Swift 中使用 Core Graphics / Quartz 2D 调整图像大小

Question

我想使用 Core Graphics 来调整一些图像的大小，因为我有一个图像重的应用程序。我在下面使用这种方法：

func createContext(image: UIImage) -> UIImage? {
    let cgImage = image.CGImage

    let width = Int(image.size.width)
    let height = Int(image.size.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: image.size.width, height: image.size.height)), cgImage)

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

    return scaledImage
}

返回的图像在横向图像上看起来很棒，但对于纵向图像，它看起来像这样：

我可以更改代码以引入 UIKit 元素：

func createContext(image: UIImage) -> UIImage? {
    let cgImage = image.CGImage
    let rect = CGRectMake(0, 0, image.size.width, image.size.height)

    let size = image.size
    let scale: CGFloat = 0.0
    let width = CGImageGetWidth(cgImage) / 2
    let height = CGImageGetHeight(cgImage) / 2
    let bitsPerComponent = CGImageGetBitsPerComponent(cgImage)
    let bytesPerRow = CGImageGetBytesPerRow(cgImage)
    let colorSpace = CGImageGetColorSpace(cgImage)
    let bitmapInfo = CGImageGetBitmapInfo(cgImage)

    UIGraphicsBeginImageContextWithOptions(size, false, scale)
    image.drawInRect(CGRect(origin: CGPointZero, size: size))

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

    CGContextSetInterpolationQuality(context, .High)
    let myGeneratedImage = CGBitmapContextCreateImage(context)

    CGContextDrawImage(context, rect, myGeneratedImage)

    let scaledImage = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()

    return scaledImage
}

它看起来像这样，这就是我想要的。

但是，我想尽我所能避免引入 UIKit 并通过仅使用 Core Graphics 来保持代码较低级别，有人可以指出我做错了什么吗？如果您能看到我的错误，请随时用 objC 或 Swift 代码回复。

Answer 1

解决办法如下：

func fixOrientation() -> UIImage {
    if imageOrientation == .Up {
        return self
    }

    var transform = CGAffineTransformIdentity

    switch imageOrientation {
    case .Down, .DownMirrored:
        transform = CGAffineTransformTranslate(transform, size.width, size.height)
        transform = CGAffineTransformRotate(transform, CGFloat(M_PI))
    case .Left, .LeftMirrored:
        transform = CGAffineTransformTranslate(transform, size.width, 0)
        transform = CGAffineTransformRotate(transform, CGFloat(M_PI_2))
    case .Right, .RightMirrored:
        transform = CGAffineTransformTranslate(transform, 0, size.height)
        transform = CGAffineTransformRotate(transform, -CGFloat(M_PI_2))
    case .Up, .UpMirrored:
        break
    }

    switch imageOrientation {
    case .UpMirrored, .DownMirrored:
        transform = CGAffineTransformTranslate(transform, size.width, 0)
        transform = CGAffineTransformScale(transform, -1, 1)
    case .LeftMirrored, .RightMirrored:
        transform = CGAffineTransformTranslate(transform, size.height, 0)
        transform = CGAffineTransformScale(transform, -1, 1)
    case .Up, .Down, .Left, .Right:
        break
    }

    let context = CGBitmapContextCreate(nil, Int(size.width), Int(size.height), CGImageGetBitsPerComponent(CGImage), 0, CGImageGetColorSpace(CGImage), CGImageGetBitmapInfo(CGImage).rawValue)
    CGContextConcatCTM(context, transform)

    switch imageOrientation {
    case .Left, .LeftMirrored, .Right, .RightMirrored:
        CGContextDrawImage(context, CGRectMake(0, 0, size.height, size.width), CGImage)
    default:
        CGContextDrawImage(context, CGRectMake(0, 0, size.width, size.height), CGImage)
        break
    }

    guard let cgImage = CGBitmapContextCreateImage(context) else {
         return UIImage()
    }
    let image = UIImage(CGImage: cgImage)

    return image
}