Load an Image from URL by many threads - iphone

Question

I'm loading an image from URL into my app. The image size is large (around 1.5Mb). How can I use many threads (ex: 2 threads) to load this image to improve the speed? If using one thread to load this image, it takes me around 5s and I want to reduce this duration.

Answer 1

You are correct. 1.5Mb is a big image and the way to optimise is NOT to use many threads. Although you are on the right track. The technique is called "slicing" and is heavily used on web to load images faster. So take a image and slice it into 3 or 4 smaller pics (and not more) in your server. When rendering call these 4 images all at once. It will load faster than one big pic. Also this lessens the "perceived" latency for the end-user.
Also, when you slice up an image, it makes it easier to reduce the number of colors necessary to display that portion of the image, thus reducing your file size (sometimes fairly significantly).
As an example Google does used to do this for its main logo in its main search page. See 4 split us images of its logo?
The downside of slicing is that it increases maintenance costs. Some one has to maintain these image splits and make sure nothing goes amiss as the app keeps changing.

Answer 2

Please Try the following Code:
//in .h file declare the following objects:
IBOutlet UIImageView *imgTest;
-(IBAction)buttonTapped:(id)sender;
-(void)LoadImage:(NSString *) irlString;
-(void)setImage:(NSData *) imgData;
//in .m file write the following code:
-(IBAction)buttonTapped:(id)sender
{
[self performSelectorOnMainThread:#selector(LoadImage:) withObject:#"http://www.google.com/images/errors/logo_sm.gif" waitUntilDone:NO];
}
-(void)LoadImage:(NSString *) urlString
{
NSURL *imgURL=[NSURL URLWithString:urlString];
NSData *imgData=[NSData dataWithContentsOfURL:imgURL];
[self performSelectorInBackground:#selector(setImage:) withObject:imgData];
}
-(void)setImage:(NSData *) imgData;
{
imgTest.image=[UIImage imageWithData:imgData];
}
you can use activity indicator while loading the image as well. Start it in the buttonTapped method and stop it in the setImage method.
i hope this will help you.