按大小压缩图像 - 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，但我只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，只代表压缩质量。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

这里有一些示例代码会尝试为您压缩图像，使其不超过最大压缩或最大文件大小

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

一种方法是循环重新压缩文件，直到找到所需的大小。您可以先找到高度和宽度，然后猜测压缩系数（图像越大压缩越多），然后在压缩后检查大小，然后再次拆分差异。

我知道这不是超级高效，但我不相信有一个单一的调用来实现特定大小的图像。

Answer 3

这里，JPEGRepresentation 非常消耗内存，如果我们在 Loop 中使用，那么它的内存消耗非常高。所以使用下面的代码 & 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

我接受了@kgutteridge 的回答，并使用递归为 Swift 3.0 做了一个类似的解决方案：

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
    }
}

Answer 5

斯威夫特 4：

extension UIImage {
    func compressTo(bytes: Int) -> UIImage {
        var compression: CGFloat = 0.9
        let maxCompression: CGFloat = 0.1
        let maxSize: Int = bytes * 1024

        var imageData = jpegData(compressionQuality: compression)!
        while imageData.count > maxSize && compression > maxCompression {
            compression -= 0.1
            imageData = jpegData(compressionQuality: compression)!
        }

        return UIImage(data: imageData)!
    }
}

Answer 6

做了一些测试后，我能够找到 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，因此我创建了一个算法来尝试始终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 7

- (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 ;
}