Is UIImageJPEGRepresentation() thread safe?

You can use UIImageJPEGRepresentation() in the background (I'm using it this way in a current project).

However what you can't do is create a UIImage the way you are doing in the background, the [UIImage imagewithCGImage] call must be doing in the main thread (as a rule of thumb all UIKit calls should be done on the main thread).

This seems like a case where you might need nested blocks.

Edit: My own code I have found does call [UIImage imagewithCGImage] while in a background thread, but I am still suspicious that might cause issues in some cases. But my code does work.

Edit2: I just noticed you are resizing the image, UIImage+Resize. There's a very nice class linked to in this post, that has been built to do that in a robust way:

http://vocaro.com/trevor/blog/2009/10/12/resize-a-uiimage-the-right-way/

You should really read that whole page to understand the nuances of resizing images. As I said, I do use that from a background thread even though part of what it does inside is what you were doing.

Edit3: If you are running on iOS4 or later, you may want to look into using the ImageIO framework to output images, which is more likely to be thread safe:

http://developer.apple.com/graphicsimaging/workingwithimageio.html

Example code for that is hard to find, here's a method that saves a PNG image using ImageIO (based on the code in "Programming With Quartz:2D and PDF graphics in Mac OS X):

// You'll need both ImageIO and MobileCoreServices frameworks to have this compile#import <ImageIO/ImageIO.h>#import <MobileCoreServices/MobileCoreServices.h>void exportCGImageToPNGFileWithDestination( CGImageRef image, CFURLRef url){    float resolution = 144;    CFTypeRef keys[2];    CFTypeRef values[2];    CFDictionaryRef options = NULL;    // Create image destination to go into URL, using PNG    CGImageDestinationRef imageDestination = CGImageDestinationCreateWithURL( url, kUTTypePNG, 1, NULL);    if ( imageDestination == NULL )    {        fprintf( stderr, "Error creating image destination\n");        return;    }    // Set the keys to be the X and Y resolution of the image    keys[0] = kCGImagePropertyDPIWidth;    keys[1] = kCGImagePropertyDPIHeight;    // Create a number for the DPI value for the image    values[0] = CFNumberCreate( NULL, kCFNumberFloatType, &resolution );    values[1] = values[0];    // Options dictionary for output    options = CFDictionaryCreate(NULL,                                 (const void **)keys,                                 (const void **)values,                                 2,                                 &kCFTypeDictionaryKeyCallBacks,                                 &kCFTypeDictionaryValueCallBacks);    CFRelease(values[0]);    // Adding the image to the destination    CGImageDestinationAddImage( imageDestination, image, options );    CFRelease( options );    // Finalizing writes out the image to the destination    CGImageDestinationFinalize( imageDestination );    CFRelease( imageDestination );}

Apple's official position is that no part of UIKit is thread-safe. However, the rest of your code appears to be Quartz-based, which is thread-safe when used in the manner you use it.

You can do everything on a background thread, then do the call to UIImageJPEGRepresentation() back on main:

// ...CGImageRef ref = CGBitmapContextCreateImage(bitmap);dispatch_async(dispatch_get_main_queue(), ^ {    NSData *finalData = UIImageJPEGRepresentation([UIImage imageWithCGImage:ref], 1.0);    [self.delegate sendNSDataBack:finalData];});CGContextRelease(bitmap);CGImageRelease(ref);

I think it is thread-safe because I do the similar things to resize an UIImage, or store image data to database in the background thread. And, the main-thread sometimes is named to UI thread. Anything about updating a screen should be executed on the UI thread. But, the UIImage is an object to store image data. It is not sub-classed from UIView. So, it is thread-safe.