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objective-cmacosretina-displaynsimage

How to get suitable CGImage from combined TIFF for display scale

I have two PNGs in a Mac project. Normal and @2x. Xcode combines these into a single TIFF with the @2x being at index 0 and the @1x at index 1.

What is the suggested approach to get the appropriate image as CGImageRef version (for use with Quartz) for the current display scale?

I can get the image manually via CGImageSource:

NSBundle *mainBundle = [NSBundle mainBundle];
NSURL *URL = [mainBundle URLForResource:@"Canvas-Bkgd-Tile" withExtension:@"tiff"];

CGImageSourceRef source = CGImageSourceCreateWithURL((__bridge CFURLRef)(URL), NULL);
_patternImage = CGImageSourceCreateImageAtIndex(source, 1, NULL);  // index 1 is @1x, index 0 is @2x
CFRelease(source);

I also found this to be working, but I am not certain that this will return the Retina version on a Retina display:

NSImage *patternImage = [NSImage imageNamed:@"Canvas-Bkgd-Tile.tiff"];
_patternImage = [patternImage CGImageForProposedRect:NULL context:nil hints:nil];
CGImageRetain(_patternImage); // retain image, because NSImage goes away

An acceptable answer to this question either provides a solution how to get the CGImage suitable from a combined multi-resolution TIFF, or explains why the second approach here is working. Or what changes are required.

Solution

  • I am opting to answer on "why the second approach here is working".

    In one of the WWDC videos published since 2010, they said that :

    +[NSImage imageNamed:] chooses the best image representation object available for the current display.

    So chances are that you are calling this class method from within a locked focus context (e.g. within a drawRect: method or similar), or maybe you actually called lockFocus yourself. Anyway, the result is that you get the most suitable image. But only when calling +[NSImage imageNamed:].

    EDIT: Found it here: http://adcdownload.apple.com//wwdc_2012/wwdc_2012_session_pdfs/session_213__introduction_to_high_resolution_on_os_x.pdf

    Search for the keyword "best" in the slides: "NSImage automatically chooses best representation […]".

    So, your second version will return the Retina version on a Retina display, you can be certain of it, it is advertised in the documentation[*].

    [*] This will only work if you provide valid artwork.