iBooks Image Limit Increased To 3.2 Million Pixels

Apple today released an updated version of the iBooks Asset Guide 5.1 - dubbed "Revision 2" - which includes two significant changes, along with a variety of minor corrections and clarifications.
The main focus of this revision is aimed at preparing content for the impending launch of iBooks on the OSX platform, which of course requires mouse-click interactivity rather than touch events as found in iBooks for iOS devices thus far.
To this end, several new additions have been included in the section on interactive elements that explain or describe the new requirements. In addition, to support the necessary coding changes, a new updated fixed-layout epub file has also been released - as version 3.1 - which contains examples of the modified code. Both the guide and sample file can be downloaded from your iTunes Connect page, but are not otherwise available to those who don't have an account.
More important to my mind is a single line in the "Optimizing Images" section (page 61) which has been changed to read:
"Images inside Fixed Layout books must not exceed 3.2 million pixels."
A further clarification of this change is given in the "What's New in Revision 2" section, once again as just a single line with no further explanation or expansion:
"The pixel limit for the interior cover, as with all interior book images, has been increased to 3.2 million pixels from 2 million pixels."
And that is all that is said on the subject. However, the significance of this cannot be understated. With display resolutions increasing at a nearly exponential rate it is crucial that the software operating on that device support it, particularly if its native to the device platform. After all, there is little point in buying a high quality device if high quality images cannot be displayed on it!
Now, granted, iBooks is far from being the sole, or even primary, app in use on iOS devices. But it is a staple of Apple's flagship tablet (and essentially the only one I use), and with all the effort that has been poured into the iBooks platform with regard to authoring complex, interactive content - not to mention the increased resolution of the retina display itself - one would think the ability to display that content at high resolution would have been a priority from the start.
This is made all the more clear if one takes a quick look at some numbers:
DEVICE
Resolution
Pixels (ppi / total)
Original Kindle
800 x 600
167ppi = 480,000 pixels
iPad 2 / iPad Mini
1024 x 768
132ppi = 786,432 pixels
Kindle Fire HD8.9
1920 x 1200
254ppi = 2,304,000 pixels
iPad 3 & 4
2048 x 1536
264ppi = 3,145,728 pixels
Nexus 10
2560 x 1600
300ppi = 4,096,000 pixels
As you can see, the total number of pixels available on device displays has been expanding at a rather rapid rate. And it will not stop here. The 2nd generation Fire HD8.9 is rumored to boast the same 2560x1600 resolution as the Nexus 10, but in an 8.9" screen, pushing the pixel-per-inch density to somewhere around 340, a point at which individual pixels cannot be distinguished by the human eye.
As for the iPad, the new 3.2 million pixel limit for the first time allows images to be inserted into iBooks that can actually take advantage of the retina display. If we do a little simple math we discover that the largest image that could be inserted up to now was roughly 1600x1200 in size (at the iPad's 4:3 aspect ratio), or more specifically:
1632 x 1224 = 1,997,568 pixels
Beyond this you would exceed the 2 million pixel limit previously imposed. But, of course, looking at the chart above we see that the retina display hosts 3,145,728 pixels - just shy of ... what? ... 3.2 million pixels - the new limit. About a year and a half late, unfortunately, as the retina display made its appearance on the iPad 3 in March of 2012. Thus, for eighteen months iBooks has been handicapped on its own home turf.
Now, however, we can actually have images in iBooks that match the iPad's resolution! And for the record, the new absolute upper limit for an image in the iPad's 4:3 aspect ratio would be:
2065 x 1549 = 3,198,685 pixels
And because Apple allows up to 50 megabyte file sizes, these are not unreasonable dimensions for high quality art in image-heavy ebooks.
Meanwhile, Amazon has capped image file size at a measly 800kb for comics, and 256kb for all other fixed layout content, which reaches absurdly poor quality at anything over 1200x800 in size, depending on the image content. Expect to see an increase in the Kindle's file size restrictions with the release of the next generation Fire line in a few months.
Hopefully, Amazon won't take as long as Apple to allow images that can actually take advantage of the device they're intended to be viewed on.

Published on August 30, 2013 18:05
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