图像压缩的大小 - iPhone SDK

Question

我想压缩图像（相机/照片库），然后将其发送到服务器。我知道我可以按高度和宽度进行压缩，但我想按大小将图像压缩到固定大小（200 KB），并保持原始高度和宽度。 JPEGRepresentation中的比例因子不代表大小，仅代表压缩质量。如何在不使用任何第三方库的情况下实现这一点（压缩到固定大小）？ 感谢您的帮助。

Answer 1

继承人一些示例代码，将尝试压缩您的图像，使其不超过或者是最大压力或最大文件大小

CGFloat compression = 0.9f; 
CGFloat maxCompression = 0.1f; 
int maxFileSize = 250*1024; 

NSData *imageData = UIImageJPEGRepresentation(yourImage, compression); 

while ([imageData length] > maxFileSize && compression > maxCompression) 
{ 
    compression -= 0.1; 
    imageData = UIImageJPEGRepresentation(yourImage, compression); 
} 

Answer 2

做到这一点的一种方法是在循环中重新压缩文件，直到找到所需的大小。您可以先查找高度和宽度，然后猜测压缩因子（更大图像更多压缩），然后在压缩它之后，检查大小并再次分割差异。

我知道这不是超高效的，但我不相信有一个电话来实现特定大小的图像。

Answer 3

这里，JPEGRepresentation是相当耗费内存，如果我们使用在循环中，所以它是非常高的内存消耗。因此，使用下面的代码& ImageSize不会超过200KB。

UIImage* newImage = [self captureView:yourUIView]; 


- (UIImage*)captureView:(UIView *)view { 
CGRect rect = view.bounds; 
UIGraphicsBeginImageContext(rect.size); 
CGContextRef context = UIGraphicsGetCurrentContext(); 

[view.layer renderInContext:context]; 
UIImage* img = [UIImage alloc]init]; 
img = UIGraphicsGetImageFromCurrentImageContext(); 
UIGraphicsEndImageContext(); 
NSLog(@"img=%@",img); 
return img; 
} 

Answer 4

- (UIImage *)resizeImageToSize:(CGSize)targetSize 
{ 
    UIImage *sourceImage = captureImage; 
    UIImage *newImage = nil; 

    CGSize imageSize = sourceImage.size; 
    CGFloat width = imageSize.width; 
    CGFloat height = imageSize.height; 

    CGFloat targetWidth = targetSize.width; 
    CGFloat targetHeight = targetSize.height; 

    CGFloat scaleFactor = 0.0; 
    CGFloat scaledWidth = targetWidth; 
    CGFloat scaledHeight = targetHeight; 

    CGPoint thumbnailPoint = CGPointMake(0.0,0.0); 

    if (CGSizeEqualToSize(imageSize, targetSize) == NO) { 

     CGFloat widthFactor = targetWidth/width; 
     CGFloat heightFactor = targetHeight/height; 

     if (widthFactor < heightFactor) 
      scaleFactor = widthFactor; 
     else 
      scaleFactor = heightFactor; 

     scaledWidth = width * scaleFactor; 
     scaledHeight = height * scaleFactor; 

     // make image center aligned 
     if (widthFactor < heightFactor) 
     { 
      thumbnailPoint.y = (targetHeight - scaledHeight) * 0.5; 
     } 
     else if (widthFactor > heightFactor) 
     { 
      thumbnailPoint.x = (targetWidth - scaledWidth) * 0.5; 
     } 
    } 

    UIGraphicsBeginImageContext(targetSize); 
    CGRect thumbnailRect = CGRectZero; 
    thumbnailRect.origin = thumbnailPoint; 
    thumbnailRect.size.width = scaledWidth; 
    thumbnailRect.size.height = scaledHeight; 

    [sourceImage drawInRect:thumbnailRect]; 
    newImage = UIGraphicsGetImageFromCurrentImageContext(); 
    UIGraphicsEndImageContext(); 

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

    return newImage ; 
} 

Answer 5

做了一些测试后，我能找到 A relationship between image size and compression value。由于这种关系对于压缩小于1的所有值都是线性的，因此我创建了一种算法来尝试将图像压缩到某个值。

//Use 0.99 because at 1, the relationship between the compression and the file size is not linear 
NSData *image = UIImageJPEGRepresentation(currentImage, 0.99); 
float maxFileSize = MAX_IMAGE_SIZE * 1024; 

//If the image is bigger than the max file size, try to bring it down to the max file size 
if ([image length] > maxFileSize) { 
    image = UIImageJPEGRepresentation(currentImage, maxFileSize/[image length]); 
} 

Answer 6

我把@kgutteridge的答案，并提出了用递归雨燕3.0类似的解决方案：

extension UIImage { 
    static func compress(image: UIImage, maxFileSize: Int, compression: CGFloat = 1.0, maxCompression: CGFloat = 0.4) -> Data? { 

     if let data = UIImageJPEGRepresentation(image, compression) { 

      let bcf = ByteCountFormatter() 
      bcf.allowedUnits = [.useMB] // optional: restricts the units to MB only 
      bcf.countStyle = .file 
      let string = bcf.string(fromByteCount: Int64(data.count)) 
      print("Data size is: \(string)") 

      if data.count > (maxFileSize * 1024 * 1024) && (compression > maxCompression) { 
       let newCompression = compression - 0.1 
       let compressedData = self.compress(image: image, maxFileSize: maxFileSize, compression: newCompression, maxCompression: maxCompression) 
       return compressedData 
      } 

      return data 
     } 

     return nil 
    } 
}