如何縮小UIImage並使其變得脆脆/銳利，而不是模糊？

Question

我需要縮小圖像的大小，但方式很敏銳。在Photoshop中，例如，圖像尺寸縮小選項「Bicubic Smoother」（模糊）和「Bicubic Sharper」。

此圖像縮小算法是開源還是在某處存在文檔，或者SDK是否提供了執行此操作的方法？

Answer 1

僅僅使用imageWithCGImage是不夠的。它會擴大規模，但結果將會模糊和不理想，無論是向上還是向下。

如果你想得到別名的權利，擺脫「鋸齒」你需要這樣的事情：http://vocaro.com/trevor/blog/2009/10/12/resize-a-uiimage-the-right-way/。

我的工作測試代碼看起來是這樣的，這是Trevor的一個小的調整解決方案與我透明的PNG工作：

- (UIImage *)resizeImage:(UIImage*)image newSize:(CGSize)newSize { 
    CGRect newRect = CGRectIntegral(CGRectMake(0, 0, newSize.width, newSize.height)); 
    CGImageRef imageRef = image.CGImage; 

    UIGraphicsBeginImageContextWithOptions(newSize, NO, 0); 
    CGContextRef context = UIGraphicsGetCurrentContext(); 

    // Set the quality level to use when rescaling 
    CGContextSetInterpolationQuality(context, kCGInterpolationHigh); 
    CGAffineTransform flipVertical = CGAffineTransformMake(1, 0, 0, -1, 0, newSize.height); 

    CGContextConcatCTM(context, flipVertical); 
    // Draw into the context; this scales the image 
    CGContextDrawImage(context, newRect, imageRef); 

    // Get the resized image from the context and a UIImage 
    CGImageRef newImageRef = CGBitmapContextCreateImage(context); 
    UIImage *newImage = [UIImage imageWithCGImage:newImageRef]; 

    CGImageRelease(newImageRef); 
    UIGraphicsEndImageContext();  

    return newImage; 
} 

Answer 2

如果在縮放時保留圖像的原始縱橫比，則無論縮小多少，總是會得到清晰的圖像。

您可以用下面的方法進行縮放：

+ (UIImage *)imageWithCGImage:(CGImageRef)imageRef scale:(CGFloat)scale orientation:(UIImageOrientation)orientation 

Answer 3

@YAR您的解決方案是否工作正常。

只有一件事不符合我的要求：整個圖像被調整大小。我寫了一個類似photos app on iphone的方法。 這會計算「長邊」並切斷「疊加」，從而獲得有關圖像質量的更好結果。

- (UIImage *)resizeImageProportionallyIntoNewSize:(CGSize)newSize; 
{ 
    CGFloat scaleWidth = 1.0f; 
    CGFloat scaleHeight = 1.0f; 

    if (CGSizeEqualToSize(self.size, newSize) == NO) { 

     //calculate "the longer side" 
     if(self.size.width > self.size.height) { 
      scaleWidth = self.size.width/self.size.height; 
     } else { 
      scaleHeight = self.size.height/self.size.width; 
     } 
    }  

    //prepare source and target image 
    UIImage *sourceImage = self; 
    UIImage *newImage = nil; 

    // Now we create a context in newSize and draw the image out of the bounds of the context to get 
    // A proportionally scaled image by cutting of the image overlay 
    UIGraphicsBeginImageContext(newSize); 

    //Center image point so that on each egde is a little cutoff 
    CGRect thumbnailRect = CGRectZero; 
    thumbnailRect.size.width = newSize.width * scaleWidth; 
    thumbnailRect.size.height = newSize.height * scaleHeight; 
    thumbnailRect.origin.x = (int) (newSize.width - thumbnailRect.size.width) * 0.5; 
    thumbnailRect.origin.y = (int) (newSize.height - thumbnailRect.size.height) * 0.5; 

    [sourceImage drawInRect:thumbnailRect]; 

    newImage = UIGraphicsGetImageFromCurrentImageContext(); 
    UIGraphicsEndImageContext(); 

    if(newImage == nil) NSLog(@"could not scale image"); 

    return newImage ; 
} 

Answer 4

對於這裏使用雨燕的是斯威夫特接受的答案：

func resizeImage(image: UIImage, newSize: CGSize) -> (UIImage) { 
    let newRect = CGRectIntegral(CGRectMake(0,0, newSize.width, newSize.height)) 
    let imageRef = image.CGImage 

    UIGraphicsBeginImageContextWithOptions(newSize, false, 0) 
    let context = UIGraphicsGetCurrentContext() 

    // Set the quality level to use when rescaling 
    CGContextSetInterpolationQuality(context, kCGInterpolationHigh) 
    let flipVertical = CGAffineTransformMake(1, 0, 0, -1, 0, newSize.height) 

    CGContextConcatCTM(context, flipVertical) 
    // Draw into the context; this scales the image 
    CGContextDrawImage(context, newRect, imageRef) 

    let newImageRef = CGBitmapContextCreateImage(context) as CGImage 
    let newImage = UIImage(CGImage: newImageRef) 

    // Get the resized image from the context and a UIImage 
    UIGraphicsEndImageContext() 

    return newImage 
} 

Answer 5

如果有人找斯威夫特的版本，這裏是雨燕版本@亞爾的接受的答案：

func resizeImage(image: UIImage, newHeight: CGFloat) -> UIImage { 
    let scale = newHeight/image.size.height 
    let newWidth = image.size.width * scale 
    UIGraphicsBeginImageContext(CGSizeMake(newWidth, newHeight)) 
    image.drawInRect(CGRectMake(0, 0, newWidth, newHeight)) 
    let newImage = UIGraphicsGetImageFromCurrentImageContext() 
    UIGraphicsEndImageContext() 

    return newImage 
} 

Answer 6

For Swift 3

func resizeImage(image: UIImage, newSize: CGSize) -> (UIImage) { 

    let newRect = CGRect(x: 0, y: 0, width: newSize.width, height: newSize.height).integral 
    UIGraphicsBeginImageContextWithOptions(newSize, false, 0) 
    let context = UIGraphicsGetCurrentContext() 

    // Set the quality level to use when rescaling 
    context!.interpolationQuality = CGInterpolationQuality.default 
    let flipVertical = CGAffineTransform(a: 1, b: 0, c: 0, d: -1, tx: 0, ty: newSize.height) 

    context!.concatenate(flipVertical) 
    // Draw into the context; this scales the image 
    context?.draw(image.cgImage!, in: CGRect(x: 0.0,y: 0.0, width: newRect.width, height: newRect.height)) 

    let newImageRef = context!.makeImage()! as CGImage 
    let newImage = UIImage(cgImage: newImageRef) 

    // Get the resized image from the context and a UIImage 
    UIGraphicsEndImageContext() 

    return newImage 
} 

Answer 7

此擴展應該在保持原始寬高比的同時縮放圖像。圖像的其餘部分被裁剪。 （SWIFT 3）

extension UIImage {  
    func thumbnail(ofSize proposedSize: CGSize) -> UIImage? { 

     let scale = min(size.width/proposedSize.width, size.height/proposedSize.height) 

     let newSize = CGSize(width: size.width/scale, height: size.height/scale) 
     let newOrigin = CGPoint(x: (proposedSize.width - newSize.width)/2, y: (proposedSize.height - newSize.height)/2) 

     let thumbRect = CGRect(origin: newOrigin, size: newSize).integral 

     UIGraphicsBeginImageContextWithOptions(proposedSize, false, 0) 

     draw(in: thumbRect) 

     let result = UIGraphicsGetImageFromCurrentImageContext() 

     UIGraphicsEndImageContext() 

     return result 
    } 
}