iOS中5种图片缩略技术及性能的深入探讨

extension UIImage {
 
 //UIKit
 func resizeUI(size: CGSize) -> UIImage? {
 
 let hasAlpha = false
 let scale: CGFloat = 0.0 // Automatically use scale factor of main screen
 
 /**
 创建一个图片类型的上下文。调用UIGraphicsBeginImageContextWithOptions函数就可获得用来处理图片的图形上下文。利用该上下文，你就可以在其上进行绘图，并生成图片
 
 size：表示所要创建的图片的尺寸
 opaque：表示这个图层是否完全透明，如果图形完全不用透明最好设置为YES以优化位图的存储，这样可以让图层在渲染的时候效率更高
 scale：指定生成图片的缩放因子，这个缩放因子与UIImage的scale属性所指的含义是一致的。传入0则表示让图片的缩放因子根据屏幕的分辨率而变化，所以我们得到的图片不管是在单分辨率还是视网膜屏上看起来都会很好
 */
 UIGraphicsBeginImageContextWithOptions(size, !hasAlpha, scale)
 self.draw(in: CGRect(origin: .zero, size: size))
 
 let resizedImage = UIGraphicsGetImageFromCurrentImageContext()
 UIGraphicsEndImageContext()
 return resizedImage!
 }
}

extension UIImage {
 
 //CoreGraphics
 func resizeCG(size:CGSize) -> UIImage? {
 
 guard let cgImage = self.cgImage else { return nil }
 
 let bitsPerComponent = cgImage.bitsPerComponent
 let bytesPerRow = cgImage.bytesPerRow
 let colorSpace = cgImage.colorSpace
 let bitmapInfo = cgImage.bitmapInfo
 
 guard let context = CGContext(data: nil,
  width: Int(size.width),
  height: Int(size.height),
  bitsPerComponent: bitsPerComponent,
  bytesPerRow: bytesPerRow,
  space: colorSpace!,
  bitmapInfo: bitmapInfo.rawValue) else {
 return nil
 }
 
 context.interpolationQuality = .high
 
 context.draw(cgImage, in: CGRect(origin: .zero, size: size))
 
 let resizedImage = context.makeImage().flatMap {
 UIImage(cgImage: $0)
 }
 return resizedImage
 }
}

extension UIImage {
 
 //ImageIO
 func resizeIO(size:CGSize) -> UIImage? {
 
 guard let data = UIImagePNGRepresentation(self) else { return nil }
 
 let maxPixelSize = max(size.width, size.height)
 
 //let imageSource = CGImageSourceCreateWithURL(url, nil)
 guard let imageSource = CGImageSourceCreateWithData(data as CFData, nil) else { return nil }
 
 //kCGImageSourceThumbnailMaxPixelSize为生成缩略图的大小。当设置为800，如果图片本身大于800*600，则生成后图片大小为800*600，如果源图片为700*500，则生成图片为800*500
 let options: [NSString: Any] = [
 kCGImageSourceThumbnailMaxPixelSize: maxPixelSize,
 kCGImageSourceCreateThumbnailFromImageAlways: true
 ]
 
 let resizedImage = CGImageSourceCreateImageAtIndex(imageSource, 0, options as CFDictionary).flatMap{
 UIImage(cgImage: $0)
 }
 return resizedImage
 }
}

extension UIImage {
 
 //CoreImage
 func resizeCI(size:CGSize) -> UIImage? {
 
 guard let cgImage = self.cgImage else { return nil }
 
 let scale = (Double)(size.width) / (Double)(self.size.width)
 
 let image = CIImage(cgImage: cgImage)
 
 let filter = CIFilter(name: "CILanczosScaleTransform")!
 filter.setValue(image, forKey: kCIInputImageKey)
 filter.setValue(NSNumber(value:scale), forKey: kCIInputScaleKey)
 filter.setValue(1.0, forKey:kCIInputAspectRatioKey)
 
 guard let outputImage = filter.value(forKey: kCIOutputImageKey) as? CIImage else { return nil}
 
 let context = CIContext(options: [kCIContextUseSoftwareRenderer: false])
 
 let resizedImage = context.createCGImage(outputImage, from: outputImage.extent).flatMap {
 UIImage(cgImage: $0)
 }
 return resizedImage
 }
}

extension UIImage {
 
 //vImage
 func resizeVI(size:CGSize) -> UIImage? {
 
 guard let cgImage = self.cgImage else { return nil }
 
 var format = vImage_CGImageFormat(bitsPerComponent: 8, bitsPerPixel: 32, colorSpace: nil,
  bitmapInfo: CGBitmapInfo(rawValue: CGImageAlphaInfo.first.rawValue),
  version: 0, decode: nil, renderingIntent: .defaultIntent)
 
 var sourceBuffer = vImage_Buffer()
 defer {
 free(sourceBuffer.data)
 }
 
 var error = vImageBuffer_InitWithCGImage(&sourceBuffer, &format, nil, cgImage, numericCast(kvImageNoFlags))
 guard error == kvImageNoError else { return nil }
 
 // create a destination buffer
 let scale = self.scale
 let destWidth = Int(size.width)
 let destHeight = Int(size.height)
 let bytesPerPixel = cgImage.bitsPerPixel / 8
 let destBytesPerRow = destWidth * bytesPerPixel
 
 let destData = UnsafeMutablePointer<UInt8>.allocate(capacity: destHeight * destBytesPerRow)
 defer {
 destData.deallocate(capacity: destHeight * destBytesPerRow)
 }
 var destBuffer = vImage_Buffer(data: destData, height: vImagePixelCount(destHeight), width: vImagePixelCount(destWidth), rowBytes: destBytesPerRow)
 
 // scale the image
 error = vImageScale_ARGB8888(&sourceBuffer, &destBuffer, nil, numericCast(kvImageHighQualityResampling))
 guard error == kvImageNoError else { return nil }
 
 // create a CGImage from vImage_Buffer
 var destCGImage = vImageCreateCGImageFromBuffer(&destBuffer, &format, nil, nil, numericCast(kvImageNoFlags), &error)?.takeRetainedValue()
 guard error == kvImageNoError else { return nil }
 
 // create a UIImage
 let resizedImage = destCGImage.flatMap {
 UIImage(cgImage: $0, scale: 0.0, orientation: self.imageOrientation)
 }
 
 destCGImage = nil
 return resizedImage
 }
}