How to get errors/warnings in [UIImage initWithData:]

Question

i have an MJPEG stream over RTSP/UDP from which i want to generate JPEGs for a UIImageView with [UIImage initWithData:]. Most of the time this works good, but sometimes i get corrupt images and log messages like:

ImageIO: <ERROR> JPEGCorrupt JPEG data: premature end of data segment

My Question is: how can i see (during runtime), that such message occurs? Unfortunatly 'initWithData' has no error output, is there any other way?

Thank you.

Edit: in this case, the initWithData does return a valid UIImage object, not nil!

When you create the image from JPEG data, what does the UIImage look like that's returned? Does it have a bunch of noise? Is it 0x0 pixels? — Alex Nichol, Jul 22 '11 at 13:37

score 2 · Accepted Answer · edited May 23 '17 at 12:14

There is a similar thread to this one on stack overflow: Catching error: Corrupt JPEG data: premature end of data segment.

There solution is to check for the header bytes FF D8 and ending bytes FF D9. So, if you have image data in an NSData, you can check it like so:

- (BOOL)isJPEGValid:(NSData *)jpeg {
    if ([jpeg length] < 4) return NO;
    const char * bytes = (const char *)[jpeg bytes];
    if (bytes[0] != 0xFF || bytes[1] != 0xD8) return NO;
    if (bytes[[jpeg length] - 2] != 0xFF || bytes[[jpeg length] - 1] != 0xD9) return NO;
    return YES;
}

Then, to check if JPEG data is invalid, just write:

if (![self isJPEGValid:myData]) {
    NSLog(@"Do something here");
}

Hope this helps!

Ok, this helps for almost 90% of the frames, should be enough. Thanks — djk, Jul 27 '11 at 08:53

score 1 · Answer 2 · answered Jul 22 '11 at 09:37

1

The initWithData: method should return nil in such cases.

Try :

UIImage *myImage = [[UIImage alloc] initWithData:imgData];
if(!myImage) {
    // problem
}

answered Jul 22 '11 at 09:37

1

thanks for your response, i already use this code. But there are cases when it does return a valid UIImage object but then the displayed image is a crap. – djk Jul 22 '11 at 11:02

score -1 · Answer 3 · answered Nov 23 '13 at 16:54

I've encountered the same problem in this exact situation.

It turned out that I was passing an instance of NSMutableData to global queue for decoding. During decoding the data in NSMutableData was overwritten by next frame received from network.

I've fixed the errors by passing a copy of the data. It might be better to use a buffer pool to improve performance:

    NSData *dataCopy = [_receivedData copy];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    ZAssert([self isJPEGValid:dataCopy], @"JPEG data is invalid"); // should never happen
    UIImage *image = [UIImage imageWithData:dataCopy];
    dispatch_async(dispatch_get_main_queue(), ^{
        // show image
    });
});

How to get errors/warnings in [UIImage initWithData:]

3 Answers3

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