I have designed a photo book, initially aimed for print. There are many high resolution large photos. Exporting at 300ppi to epub, it looks glorious on an iPad retina with 2 page spreads. However, in order to upload it to the iTunes store for iBooks at Apple, there is a 4 Mpx limit. So, I have been trying various approaches to staying under the 4 Mpx limit while retaining as good quality as possible for retina displays for which this book is intended. I have examined the resulting epub files by exploding with stuffit expander and then looking at the pixel dimensions of individual images. I tried the following:
1. Export at 150ppi instead of 300ppi. This reduces the pixel size of all images by 1/2. But I am trying to maximize pixels for retina displays and this is a very unsatisfactory solution.
2. Created an alternative layout with a smaller dimensioned (in inches) page with liquid page rule "Scale". Then created an epub at this size at 300 ppi. The pixel dimensions came out larger (!???!) Since images on the alternate dimensions were scaled down in inches, the pixel dimension should be smaller at the same ppi.
3. Tried using Object/Object Export Options, rasterizing both the container and the content in separate trials for individual large images. Then created an epub at 300 dpi. The image sizes were unchanged in both cases. I thought this was how one had control over pixel dimensions of individual images????
Any suggestions as to how to control the pixel dimensions of individual images to keep them under 4 mega pixels (preferably without having to rebuild the whole book from scratch)?
To be clear, I am using InDesign CC 2015 version, latest update.