How to convert (OS_dispatch_data *) 5018112 bytes into NSData to put into UIImage

Question

I've been looking for an answer to this and some seem like they might be what I need but I'm not sure. I found this Question #9152851 and this Question #2617625 and poked around on a bunch of links but I need some direction here.

Essentially, I'm dispatching an async call to process an image using OpenCV. You can see by the code here that I'm turning it into NSData * before sending it back to my delegate.

NSData *proccessedData = [NSData dataWithBytes:processedImage.data length:(processedImage.rows * processedImage.cols)];        
[self.delegate onProcessedBitmapReady:proccessedData withFocusQuality:focusQuality];

But when I get back to my delegate, my processedBitmap is of type (OS_dispatch_data *) and contains a value of bytes. So, when I try to set the UIImage, it gets set to null.

- (void)onProcessedBitmapReady:(NSData *)processedBitmap withFocusQuality:(double)focusQuality
{
    //Use comverted image from self.captureCommand onComplete
    UIImage *image = [[UIImage alloc] initWithData:processedBitmap];
    [self saveImage:image];
}

Here is a screen capture of the values:

So, how do I convert those bytes (or whatever they are) into something that I can stuff into a UIImage?

Thank you in advance for your help.

------------------------------------------------------- Adding a new image -----------------------------------------

Does this new image help?

Thank you.

Bureaucrat

Does onProcessedBitmapReady retain the NSData object? Is the NSData object retained elsewhere? Are you using ARC? – Skirmish

Looks like reclaimed memory to me, your NSData was not retained properly. – Array

My processedBitmap is my NSData. I see 5018112 bytes in it. The only thing weird is that it is of type OS_dispatch_data *. Does that mean that the memory has been reclaimed? – Bureaucrat

OK. I created a property with (nonatomic, strong) and now I retrieve that value from my object and it still says that it is of type OS_dispatch_data *. Could that just be because I am calling it in an asynch call using GCD? – Bureaucrat

NSData, like many other Foundation classes, is actually a class cluster, meaning that NSData is just providing the interface. OS_dispatch_data sounds like a specialized subclass that is used internally by Apple to transfer NSData stuff around blocks, but I might be wrong though. If it behaves like an NSData instance, it probably is, and the image is just in a format that isn't recognized by UIImage – Toms

@Toms - You are correct! I had to create a CGContextRef using CGBitmapContextCreate and then create a CGImageRef using the CGBitmapContextCreateImage and then create a UIImage using imageWithCGImage. Answer the question and I'll give you the points. – Bureaucrat

@Lucy: Could have made a post with your own answer ? about how did you got around the issue ? – Nubile

@Nubile - I used the CVImageConverter code by Artem Myagkov posted here: code.google.com/p/opencv-on-ios/source/browse/trunk/utilities/… – Bureaucrat

Answer 1

NSData is actually a class cluster which just provides the interface, and there are multiple special implementations for it around. It appears that OS_dispatch_data is such a special implementations made to pass data objects around blocks, especially since your UIImage creation doesn't crash (as it would if you would pass it a non NSData object, or just garbage memory). Instead, it looks like UIImage simply doesn't recognize the format the image is in!

By the way, Apple has a great guide about the concept of class clusters, which can be found here.

Toms

Specifically, OS_dispatch_data is a dispatch_data_t, from libdispatch. In iOS 7 and Mavericks, dispatch datas can be freely cast to NSData and are in fact NSData subclasses (if and only if Foundation is linked in the process of course). – Melyndamem