NSImage acting weird

Question

Why is this code setting artistImage to an image with 0 width and 0 height?

NSURL *artistImageURL = [NSURL URLWithString:@"http://userserve-ak.last.fm/serve/252/8581581.jpg"];
NSImage *artistImage = [[NSImage alloc] initWithContentsOfURL:artistImageURL];

Answer 1

NSImage does load this fine for me, but that particular image has corrupt metadata. Its resolution according to the exif data is 7.1999997999228071e-06 dpi.

NSImage respects the DPI info in the file, so if you try to draw the image at its natural size, you'll get something 2520000070 pixels across.

Answer 2

As Ken wrote, the DPI is messed up in this image. If you want to force NSImage to set the real image size (ignoring the DPI), use the method described at http://borkware.com/quickies/one?topic=NSImage:

NSBitmapImageRep *rep = [[image representations] objectAtIndex: 0];
NSSize size = NSMakeSize([rep pixelsWide], [rep pixelsHigh]);
[image setSize: size];

Answer 3

Last I checked, NSImage's -initWithContentsOfURL: only works with file URLs. You'll need to retrieve the URL first, and then use -initWithData:

Answer 4

It is more or less guaranteed that .representations contains NSImageRep* (of course not always NSBitmapImageRep). To be on a safe side for future extensions one can write something like code below. And it also takes into account multiple representation (like in some .icns and .tiff files).

@implementation NSImage (Extension)

- (void) makePixelSized {
    NSSize max = NSZeroSize;
    for (NSObject* o in self.representations) {
        if ([o isKindOfClass: NSImageRep.class]) {
            NSImageRep* r = (NSImageRep*)o;
            if (r.pixelsWide != NSImageRepMatchesDevice && r.pixelsHigh != NSImageRepMatchesDevice) {
                max.width = MAX(max.width, r.pixelsWide);
                max.height = MAX(max.height, r.pixelsHigh);
            }
        }
    }
    if (max.width > 0 && max.height > 0) {
        self.size = max;
    }
}

@end