Fastest way to handle UIImagePickerController compression

Question

What's the fastest way of getting a picture into a SQLite data store for compression, so I can return control to the user?

• I'm using UIImagePickerController to take pictures in my application. The problem is using the picture is quite slow, because of the speed of UIImageJPEGRepresentation.
• I want to push JPEG compression into a background thread, but before trying this I need to satisfy myself that I can persist the picture in a way that will survive across runs. That means either a blob in SQLite or a file. Which as far as I can tell, takes me right back to doing slow picture encoding right away.

What I want to achieve is speed fast enough that it feels instant to the user.

How should I be handling this? Is there anything else I should know?

Answer 1

You should not save a picture in the database, unless it's very small in size. The threshold which determines if the picture is small enough is, of course, highly subjective. In my humble opinion (and experience on the iPhone), it should not exceed one megabyte. Therefore, you should only save in the database small sized images, such as icons, thumbnails etc. For images beyond one megabytes you should simply store them as files in the filesystem, and put the filename (the image path) in the database. By the way, storing the image on the filesystem and its pathname in the database is extremely fast.

About compression: you can certainly compress the image using another thread, but consider whether or not it's really worth doing this. You can use a thread to save the image to a file, save the pathname in the database and return immediately the control to your user. You have (usually) plenty of space, but a very small computational power, even on the latest iPhone 3GS. Also, you should verify (I really do not know this) whether or not loading a compressed image through UIImageView requires more time w.r.t. a non compressed one such as a PNG. If your application will incur an additional overhead when loading a compressed image, then it may definitely not worth compressing your images. It's basically a tradeoff between space and speed. Hope this helps to decide.

Answer 2

Based on comments and tests, here's what I'm currently doing:

When I get the image from the UIImageController, I retain it in a class ivar and dismiss the image picker. I show a view that blocks my main view and schedule a NSTimer event to do the compression in a second, then return to the caller.

This lets the animation run that dismisses the image controller. My blocker view is revealed under it.

(The blocker view fills the entire content area of the navigation controller, and is solid black with a UIActivityIndicatorView.)

- (void)imagePickerController: (UIImagePickerController *)picker
        didFinishPickingImage: (UIImage *)selectedImage
                  editingInfo: (NSDictionary *)editingInfo;
{
    busyView.userInteractionEnabled = YES;
    busyView.alpha = 0.7f;
    mainView.userInteractionEnabled = NO;
    [self dismissModalViewControllerAnimated: YES];
    [NSTimer scheduledTimerWithTimeInterval: 1.0f
                                     target: self
                                   selector: @selector(compress:)
                                   userInfo: selectedImage
                                    repeats: NO];
}

When the timer fires, I compress the image using JPEG (because it's faster than PNG, despite intuition) and fade away the blocker view.

- (void)compress: (NSTimer *)inTimer;
{
    [self gotJPEG: UIImageJPEGRepresentation( inTimer.userInfo, 0.5f )];
    [UIView beginAnimations: @"PostCompressFade" context: nil];
    [UIView setAnimationDuration: 0.5];
    busyView.userInteractionEnabled = NO;
    busyView.alpha = 0.0f;
    [UIView commitAnimations];
    mainView.userInteractionEnabled = YES;
}

Although this adds a second do the processing, it gets the image picker out of the way faster so it no longer feels like my application has frozen. The animation from the UIActivityIndicatorView does run while UIImageJPEGRepresentation is working.

A better answer than using the NSTimer with 1 second delay would be to get an event when the animation from dismissModalViewControllerAnimated: finishes, but I'm not sure how to do this.

(I don't consider this solved yet.)