iOS中图片相关的内容非常多,iOS中的图片相关的API也非常多,从UIKit中的UIImage,到CoreGraphic中CGImage,CoreImage中的CIImage.
通常情况下使用UIImage来缩放图片大小,在UIImage有contentMode属性,会有多个枚举属性.ScaleAspectFit,.ScaleAspectFill,通过赖进行
使图片缩放改变大小
在进行图片的缩放的大小时,首先需要了解到缩放的目标大小.
直接通过缩放因子改变大小
最简单的方式就是使用常量factor,去乘以图像的原来大小.
let size = CGSize(width: image.size.width/2, height: image.size.height/2)
或者直接使用CGAffineTransform改变transform
let size = CGSizeApplyAffineTransform(image.size, CGAffineTransformScale(0.5,0.5))
不改变Aspect Ratio的缩放
如果要将一个图片放到一个rect中,而不改变它原始的aspect ratio.可以使用AVFoundation的AVMakeRectWithAspectRatioInsideRect方法.
import AVFoundation
let rect = AVMakeRectWithAspectRatioInsideRect(image.size, imageView.bounds)
改变图片大小的方法汇总
有多种方法可以改变图片的大小,每种方法的性能不同.
UIGraphicsBeginImageContextWithOptions和UIImage -drawInRect:
通常最高阶的API是UIKit,使用UIImage然后使用Graphic Context实时绘出一个大小不同的图片.可以使用UIGraphicsBeginImageContextWithOptions和UIGraphicsGetImageFromCurrentImageContext获取缩略图.
let image = UIImage(contentsOfFile: self.URL.absoluteString!)
let size = CGSizeApplyAffineTransform(image.size, CGAffineTransformMakeScale(0.5, 0.5))
let hasAlpha = false
let scale: CGFloat = 0.0 // Automatically use scale factor of main screen
UIGraphicsBeginImageContextWithOptions(size, !hasAlpha, scale)
image.drawInRect(CGRect(origin: CGPointZero, size: size))
let scaledImage = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()
UIGraphicsBeginImageContextWithOptions()会创建一个渲染context,然后使用原来的图片draw.
- 第一个参数
size就是缩小以后的图片的大小. - 第二个参数
isOpaque,用来描述图片的alpha通道是否渲染.设置为false表示完全不透明 - 第三个参数
scale,就是dispaly scale factor.如果设置成0.0那么就是main screen使用的(retina是2.0,iphone6p是3.0)
CGBitmapContextCreate和CGContextDrawImage
使用的Core Graphic/Quartz 2D使用的是低阶API,可以使用CGImage,使用CGBitmapContextCreate()和CGBitmapContextCreateImage()获取缩略图.
let cgImage = UIImage(contentsOfFile: self.URL.absoluteString!).CGImage
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)
let context = CGBitmapContextCreate(nil, width, height, bitsPerComponent, bytesPerRow, colorSpace, bitmapInfo.rawValue)
CGContextSetInterpolationQuality(context, kCGInterpolationHigh)
CGContextDrawImage(context, CGRect(origin: CGPointZero, size: CGSize(width: CGFloat(width), height: CGFloat(height))), cgImage)
let scaledImage = CGBitmapContextCreateImage(context).flatMap { UIImage(CGImage: $0) }
CGImageSourceCreateThumbnailAtIndex
也可以使用Image I/O的framework也可以用来缩放图片大小.
import ImageIO
if let imageSource = CGImageSourceCreateWithURL(self.URL, nil) {
let options: [NSString: NSObject] = [
kCGImageSourceThumbnailMaxPixelSize: max(size.width, size.height) / 2.0,
kCGImageSourceCreateThumbnailFromImageAlways: true
]
let scaledImage = CGImageSourceCreateThumbnailAtIndex(imageSource, 0, options).flatMap { UIImage(CGImage: $0) }
}
使用CoreImage进行Lanczos重新采样
CoreImage中内置Lanczos Resampling,具体的函数是CILanczosScaleTransform filter.
let image = CIImage(contentsOfURL: self.URL)
let filter = CIFilter(name: "CILanczosScaleTransform")!
filter.setValue(image, forKey: "inputImage")
filter.setValue(0.5, forKey: "inputScale")
filter.setValue(1.0, forKey: "inputAspectRatio")
let outputImage = filter.valueForKey("outputImage") as! CIImage
let context = CIContext(options: [kCIContextUseSoftwareRenderer: false])
let scaledImage = UIImage(CGImage: self.context.createCGImage(outputImage, fromRect: outputImage.extent()))
在Accelerate中的vImage
使用Accelerat framework包括vImage的图像处理函数.
let cgImage = UIImage(contentsOfFile: self.URL.absoluteString!).CGImage
// create a source buffer
var format = vImage_CGImageFormat(bitsPerComponent: 8, bitsPerPixel: 32, colorSpace: nil,
bitmapInfo: CGBitmapInfo(rawValue: CGImageAlphaInfo.First.rawValue),
version: 0, decode: nil, renderingIntent: CGColorRenderingIntent.RenderingIntentDefault)
var sourceBuffer = vImage_Buffer()
defer {
sourceBuffer.data.dealloc(Int(sourceBuffer.height) * Int(sourceBuffer.height) * 4)
}
var error = vImageBuffer_InitWithCGImage(&sourceBuffer, &format, nil, cgImage, numericCast(kvImageNoFlags))
guard error == kvImageNoError else { return nil }
// create a destination buffer
let scale = UIScreen.mainScreen().scale
let destWidth = Int(image.size.width * 0.5 * scale)
let destHeight = Int(image.size.height * 0.5 * scale)
let bytesPerPixel = CGImageGetBitsPerPixel(image.CGImage) / 8
let destBytesPerRow = destWidth * bytesPerPixel
let destData = UnsafeMutablePointer<UInt8>.alloc(destHeight * destBytesPerRow)
defer {
destData.dealloc(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
let destCGImage = vImageCreateCGImageFromBuffer(&destBuffer, &format, nil, nil, numericCast(kvImageNoFlags), &error)?.takeRetainedValue()
guard error == kvImageNoError else { return nil }
// create a UIImage
let scaledImage = destCGImage.flatMap { UIImage(CGImage: $0, scale: 0.0, orientation: image.imageOrientation) }
各种方式的选择
- UIKit,CoreGraphics以及Image I/O的性能优秀.如果仅仅是缩放的话最好使用CoreImage.
- 日常的图像缩放以后不进行其他操作的话,使用
UIGraphicsBeginImageContextWithOptions是最好的选择. - 如果对图像质量有更高的要求,最好使用
CGBitmapContextCreate和CGContextSetInterpolationQuality. - 如果缩放的目的是显示缩略图,那么最好使用
CGImageSourceCreateThumbnailAtIndex - 缩放不要用
vImage
