Does Core Image load image data immediately? - ios

Question

Let's say I want to find out the size of an image, so if a user tries to load a 10,000x10,000 pixel image in my iPad app I can present them with a dialog and not crash. If I do [UIImage imageNamed:] or [UIImage imageWithContentsOfFile:] that will load my potentially large image into memory immediately.
If I use Core Image instead, say like this:
CIImage *ciImage = [CIImage imageWithContentsOfURL:[NSURL fileURLWithPath:imgPath]];
Then ask my new CIImage for its size:
CGSize imgSize = ciImage.extent.size;
Will that load the entire image into memory to tell me this, or will it just look at the metadata of the file to discover the size of the image?

Answer 1

The imageWithContentsOfURL function loads the image into memory, yes.
Fortunately Apple implemented CGImageSource for reading image metadata without loading the actual pixel data into memory in iOS4, you can read about how to use it in this blog post (conveniently it provides a code sample on how to get image dimensions).
EDIT: Pasted code sample here to protect against link rot:
#import <ImageIO/ImageIO.h>
NSURL *imageFileURL = [NSURL fileURLWithPath:...];
CGImageSourceRef imageSource = CGImageSourceCreateWithURL((CFURLRef)imageFileURL, NULL);
if (imageSource == NULL) {
// Error loading image
...
return;
}
NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithBool:NO], (NSString *)kCGImageSourceShouldCache,nil];
CFDictionaryRef imageProperties = CGImageSourceCopyPropertiesAtIndex(imageSource, 0, (CFDictionaryRef)options);
if (imageProperties) {
NSNumber *width = (NSNumber *)CFDictionaryGetValue(imageProperties, kCGImagePropertyPixelWidth);
NSNumber *height = (NSNumber *)CFDictionaryGetValue(imageProperties, kCGImagePropertyPixelHeight);
NSLog(#"Image dimensions: %# x %# px", width, height);
CFRelease(imageProperties);
}
The full API reference is also available here.