ios 如何缩小 UIImage 并使其同时变脆/清晰而不是模糊？

How to scale down a UIImage and make it crispy / sharp at the same time instead of blurry?

提问by Proud Member

I need to scale down an image, but in a sharp way. In Photoshop for example there are the image size reduction options "Bicubic Smoother" (blurry) and "Bicubic Sharper".

我需要缩小图像，但要以一种清晰的方式缩小。例如，在 Photoshop 中，有图像尺寸缩小选项“双三次平滑”（模糊）和“双三次锐化”。

Is this image downscaling algorithm open sourced or documented somewhere or does the SDK offer methods to do this?

这个图像缩小算法是开源的还是在某处记录的，或者 SDK 是否提供了这样做的方法？

回答by Dan Rosenstark

Merely using imageWithCGImageis not sufficient. It will scale, but the result will be blurry and suboptimal whether scaling up or down.

仅仅使用imageWithCGImage是不够的。它会缩放，但无论是放大还是缩小，结果都将是模糊和次优的。

If you want to get the aliasing right and get rid of the "jaggies" you need something like this: http://vocaro.com/trevor/blog/2009/10/12/resize-a-uiimage-the-right-way/.

如果您想获得正确的别名并摆脱“锯齿”，您需要这样的东西：http: //vocaro.com/trevor/blog/2009/10/12/resize-a-uiimage-the-right-方式/。

My working test code looks something like this, which is Trevor's solution with one small adjustment to work with my transparent PNGs:

我的工作测试代码看起来像这样，这是 Trevor 的解决方案，通过一个小的调整来处理我的透明 PNG：

- (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;
}

回答by bzmw

For those using Swift here is the accepted answer in Swift:

对于那些使用 Swift 的人，这里是 Swift 中公认的答案：

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
}

回答by Saraz

If someone is looking for Swift version, here is the Swift version of @Dan Rosenstark's accepted answer:

如果有人正在寻找 Swift 版本，这里是@Dan Rosenstark 接受的答案的 Swift 版本：

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
}

回答by Sazzad Hissain Khan

For Swift 3

对于 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
 }

回答by NSExplorer

If you retain the original aspect ratio of the image while scaling, you'll always end up with a sharp image no matter how much you scale down.

如果在缩放时保留图像的原始纵横比，则无论缩小多少，最终都会得到清晰的图像。

You can use the following method for scaling:

您可以使用以下方法进行缩放：

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

回答by Alexander

@YAR your solution is working properly.

@YAR 您的解决方案工作正常。

There is only one thing which does not fit my requirements: The whole image is resized. I wrote a Method which did it like the photos app on iphone. This calculates the "longer side" and cuts off the "overlay" resulting in getting much better results concerning the quality of the image.

只有一件事不符合我的要求：调整了整个图像的大小。我写了一个方法，它像photos app on iphone. 这会计算“长边”并切断“覆盖”，从而在图像质量方面获得更好的结果。

- (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 ;
}

回答by Jovan Stankovic

This extension should scale the image while keeping original aspect ratio. The rest of the image is cropped. (Swift 3)

此扩展应在保持原始纵横比的同时缩放图像。图像的其余部分被裁剪。（斯威夫特 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
    }
}

回答by gabriel_vincent

For swift 4.2:

对于快速 4.2：

extension UIImage {

    func resized(By coefficient:CGFloat) -> UIImage? {

        guard coefficient >= 0 && coefficient <= 1 else {

            print("The coefficient must be a floating point number between 0 and 1")
            return nil
        }

        let newWidth = size.width * coefficient
        let newHeight = size.height * coefficient

        UIGraphicsBeginImageContext(CGSize(width: newWidth, height: newHeight))

        draw(in: CGRect(x: 0, y: 0, width: newWidth, height: newHeight))

        let newImage = UIGraphicsGetImageFromCurrentImageContext()

        UIGraphicsEndImageContext()

        return newImage
    }
}