UIImagePNGRepresentation是否返回空数据？

Question

我正在尝试制作缩略图并保存到文档目录。但问题是，当我试图将缩略图转换为NSData时。它返回nil。

这是我的代码，

  UIImage *thumbNailimage=[image thumbnailImage:40 transparentBorder:0.2 cornerRadius:0.2 interpolationQuality:1.0];
NSData *thumbNailimageData = UIImagePNGRepresentation(thumbNailimage);// Returns nil
[thumbNailimageData writeToFile:[DOCUMENTPATH stringByAppendingPathComponent:@"1.png"] atomically:NO];

那么，问题是什么呢?我也尝试过UIImageJPEGRepresentation，但它对我不起作用。

谢谢。

Answer 1

试试这个：

UIGraphicsBeginImageContext(originalImage.size);
[originalImage drawInRect:CGRectMake(0, 0, originalImage.size.width, originalImage.size.height)];
UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

这将创建原始UIImage的副本。然后，您可以调用UIImagePNGRepresentation，它将正常工作。

Answer 2

试试这段代码，

-(void) createThumbnail
{
   UIImage *originalImage = imgView2.image; // Give your original Image
   CGSize destinationSize = CGSizeMake(25, 25); // Give your Desired thumbnail Size
   UIGraphicsBeginImageContext(destinationSize);
   [originalImage drawInRect:CGRectMake(0,0,destinationSize.width,destinationSize.height)];
   UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
   NSData *thumbNailimageData = UIImagePNGRepresentation(newImage);
   UIGraphicsEndImageContext();
   [thumbNailimageData writeToFile:[NSHomeDirectory() stringByAppendingPathComponent:@"1.png"] atomically:NO];
}

希望这对你有帮助，祝你编码愉快

Answer 3

对于Swift程序员来说，Rickster的回答帮了我很大的忙！UIImageJPEGRepresentation在选择某个图片时会使我的应用崩溃。我正在分享我的UIImage扩展(或者Objective-C术语中的Category )。

import UIKit

extension UIImage {

    /**
     Creates the UIImageJPEGRepresentation out of an UIImage
     @return Data
     */

    func generateJPEGRepresentation() -> Data {

        let newImage = self.copyOriginalImage()
        let newData = UIImageJPEGRepresentation(newImage, 0.75)

        return newData!
    }

    /**
     Copies Original Image which fixes the crash for extracting Data from UIImage
     @return UIImage
     */

    private func copyOriginalImage() -> UIImage {
        UIGraphicsBeginImageContext(self.size);
        self.draw(in: CGRect(x: 0, y: 0, width: self.size.width, height: self.size.height))
        let newImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext();

        return newImage!
    }
}