Open Radar

 
Summary:
I've just noticed that the official iBooks Asset Guide still specifies a maximum image size of 3 megapixels.  That size was borderline even for a retina iPad from four years ago.  But with the new Retina iMac, it is absurdly small.

If you render an 8.5x11-shaped image in landscape orientation on a Retina iMac, a retina-quality image is somewhere in the neighborhood of 6.5 megapixels.  A truly full-screen image in landscape orientation is 15 megapixels.  Heaven help you if you apply the recommended "1.5x the intended size" rule, because then your image size comes to over 33MP—more than eleven times the maximum pixel count that your rules allow.

That just doesn't work.

An ideal set of rules should tell publishers to use CSS media queries to load images based on resolution and pixel density:

* For non-retina iPhone and iPod Touch screens (by screen size), load a 1MP image, because that's about the biggest that an iPhone 3G can handle.

* Optionally provide a 4 MP size for retina iPhone hardware.

* For all iPad-sized devices, and for all retina iOS devices of any size (-webkit-min-device-pixel-ratio: 2), provide an 8 MP image.

* For everything with a larger screen, provide an image of whatever size you prefer, but no less than 8 MP.

(I wish I could recommend WebKit's srcset functionality, but unfortunately, it fails to take into account scaling performed on the image as a result of CSS and/or SVG manipulation to expand an image to fill the screen.  As a result, on a huge retina iMac screen, you would still get only the 2x version when in practice, to avoid being fuzzy, it would need an 8x version if you make the 1x version iPhone-sized.  See rdar://18775426 "Webkit srcset functionality doesn't handle scaled images well" for details.)

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