Question

为了防止我的应用程序滞后，我正在尝试压缩大于1 MB的图像（主要用于从iphone普通相机拍摄的照片。

UIImage *image = [info objectForKey:UIImagePickerControllerOriginalImage];
NSData *imageSize = UIImageJPEGRepresentation(image, 1);
NSLog(@"original size %u", [imageSize length]);

 UIImage *image2 = [UIImage imageWithData:UIImageJPEGRepresentation(image, 0)];
    NSData *newImageSize = UIImageJPEGRepresentation(image2, 1);
    NSLog(@"new size %u", [newImageSize length]);
    UIImage *image3 = [UIImage imageWithData:UIImageJPEGRepresentation(image2, 0)];
    NSData *newImageSize2 = UIImageJPEGRepresentation(image3, 1);
    NSLog(@"new size %u", [newImageSize2 length]);

picView = [[UIImageView alloc] initWithImage:image3] ;

然而，NSLog我得到的东西是

 original size 3649058
 new size 1835251
 new size 1834884

第一次和第二次压缩之间的差异几乎可以忽略不计。我的目标是使图像大小低于1 MB。我是否忽视了某些事情/有没有其他方法来实现这一目标？

编辑：如果可能的话，我想避免缩放图像的高度和宽度。

Answer 1

有几点想法：

UIImageJPEGRepresentation功能不会返回“原始”图像。例如，如果您使用compressionQuality 1.0，从技术上讲，它不返回“原始”图像，而是返回图像的JPEG再现，其中compressionQuality为最大值。这实际上可以产生比原始资产更大的对象（至少如果原始图像是JPEG）。您还要在此过程中丢弃所有元数据（有关拍摄图像的位置，相机设置等的信息）。

如果您需要原始资源，则应使用PHImageManager：

NSURL *url = [info objectForKey:UIImagePickerControllerReferenceURL];

PHFetchResult *result = [PHAsset fetchAssetsWithALAssetURLs:@[url] options:nil];
PHAsset *asset = [result firstObject];

PHImageManager *manager = [PHImageManager defaultManager];
[manager requestImageDataForAsset:asset options:nil resultHandler:^(NSData *imageData, NSString *dataUTI, UIImageOrientation orientation, NSDictionary *info) {
    NSString *filename = [(NSURL *)info[@"PHImageFileURLKey"] lastPathComponent];
    // do what you want with the `imageData`
}];

在8之前的iOS版本中，您必须使用assetForURL类的ALAssetsLibrary：

NSURL *url = [info objectForKey:UIImagePickerControllerReferenceURL];
ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
[library assetForURL:url resultBlock:^(ALAsset *asset) {
    ALAssetRepresentation *representation = [asset defaultRepresentation];

    NSLog(@"size of original asset %llu", [representation size]);

    // I generally would write directly to a `NSOutputStream`, but if you want it in a
    // NSData, it would be something like:

    NSMutableData *data = [NSMutableData data];

    // now loop, reading data into buffer and writing that to our data strea
    NSError *error;
    long long bufferOffset = 0ll;
    NSInteger bufferSize = 10000;
    long long bytesRemaining = [representation size];
    uint8_t buffer[bufferSize];
    NSUInteger bytesRead;
    while (bytesRemaining > 0) {
        bytesRead = [representation getBytes:buffer fromOffset:bufferOffset length:bufferSize error:&error];
        if (bytesRead == 0) {
            NSLog(@"error reading asset representation: %@", error);
            return;
        }
        bytesRemaining -= bytesRead;
        bufferOffset   += bytesRead;
        [data appendBytes:buffer length:bytesRead];
    }

    // ok, successfully read original asset; 
    // do whatever you want with it here

} failureBlock:^(NSError *error) {
    NSLog(@"error=%@", error);
}];

请注意，此assetForURL以异步方式运行。

如果您想要NSData进行压缩，则可以UIImageJPEGRepresentation使用compressionQuality小于1.0。您的代码实际上使用compressionQuality 0.0执行此操作，该代码应提供最大压缩。但是你不保存NSData，而是用它来创建UIImage，然后你得到UIImageJPEGRepresentation compressionQuality 1.0的新// a UIImage of the original asset (discarding meta data) UIImage *image = [info objectForKey:UIImagePickerControllerOriginalImage]; // this may well be larger than the original asset NSData *jpgDataHighestCompressionQuality = UIImageJPEGRepresentation(image, 1.0); [jpgDataHighestCompressionQuality writeToFile:[docsPath stringByAppendingPathComponent:@"imageDataFromJpeg.jpg"] atomically:YES]; NSLog(@"compressionQuality = 1.0; length = %u", [jpgDataHighestCompressionQuality length]); // this will be smaller, but with some loss of data NSData *jpgDataLowestCompressionQuality = UIImageJPEGRepresentation(image, 0.0); NSLog(@"compressionQuality = 0.0; length = %u", [jpgDataLowestCompressionQuality length]); UIImage *image2 = [UIImage imageWithData:jpgDataLowestCompressionQuality]; // ironically, this will be larger than jpgDataLowestCompressionQuality NSData *newImageSize = UIImageJPEGRepresentation(image2, 1.0); NSLog(@"new size %u", [newImageSize length]);，因此失去了你最初实现的大部分压缩。

请考虑以下代码：
```
compressionQuality
```
除了前面提到的JPEG压缩质量之外，您还可以resize the image。您也可以将其与JPEG {{1}}结合使用。

Answer 2

您无法反复压缩图像。如果是这样，一切都可以一次又一次地压缩。那么你认为它会有多小？

使图像变小的一种方法是改变图像的大小。例如，将640X960更改为320X480。但是你会失去品质。

Answer 3

我是第一个实现UIImageJPEGRepresentation（图像，0.75），然后改变大小。也许是图像的宽度和高度的三分之二或一半。

多次调用imageWithData：UIImageJPEGRepresentation（）只会在第一次压缩图像

3 个答案: