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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 的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] ;

但是，我得到的 NSLog 输出的内容类似于

 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。我是否忽略了某些事情/是否有替代方法可以实现这一目标？

编辑：如果可能的话，我想避免缩放图像的高度和宽度。

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3 回答 3

17

几个想法：

  1. 该UIImageJPEGRepresentation函数不返回“原始”图像。例如，如果您使用compressionQualityof 1.0，从技术上讲，它不会返回“原始”图像，而是返回具有compressionQuality最大值的图像的 JPEG 再现。这实际上可以产生比原始资源更大的对象（至少如果原始图像是 JPEG）。在此过程中，您还将丢弃所有元数据（有关图像拍摄位置、相机设置等的信息）。

  2. 如果你想要原始资产，你应该使用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该类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，这assetForURL是异步运行的。

  3. 如果你想要一个NSDatawith 压缩，你可以使用UIImageJPEGRepresentationwith a compressionQualityless than 1.0。您的代码实际上使用compressionQualityof来执行此操作0.0，它应该提供最大的压缩。但是您不保存它NSData，而是使用它来创建 a UIImage，然后您会得到一个UIImageJPEGRepresentation带有 a的新compressionQuality的1.0，从而失去了您最初实现的大部分压缩。

    考虑以下代码：

    // 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]);

  4. 除了前面提到的 JPEG 压缩质量外，您还可以调整图像大小。您也可以将它与 JPEG 结合使用compressionQuality。

于 2013-06-09T10:47:22.203 回答
1

您不能一次又一次地压缩图像。如果是这样，一切都可以一次又一次地压缩。那你觉得它会小到什么程度呢？

使图像更小的一种方法是更改​​其大小。例如将 640X960 更改为 320X480。但是你会失去质量。

于 2013-06-09T01:57:45.313 回答
-1

我是先实现UIImageJPEGRepresentation(image,0.75)，然后改变大小。也许图像的宽度和高度是三分之二或一半。

于 2013-06-09T02:42:56.107 回答