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I'm building an 80 page, fixed layout, image heavy EPUB for a client. When exported as a low quality PDF in its current form (A3 design from InDesign), the PDF total size is around: 500MB.

The EPUB will be viewed via an iPad for an important presentation.

I'm very concerned about performance for such a large EPUB. To ensure the end-user isn't waiting for the whole presentation to load, is it possible to implement lazy loading?

In an ideal world, I'd like to load 5 pages when the presentation is launched. When a page is turned, it should load additional pages.

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  • Do you mind making an edit and clarifying by what you mean as "80 page, image heavy EPUB" as you dont mention if this is fixed layout or re-flowable.
    – user9447
    Jun 23, 2016 at 11:42
  • I've updated my question. Is this enough information?
    – Sam
    Jun 23, 2016 at 12:16
  • ya I still would advise attacking the ePub from a scripting level after the export from InDesign. Doing it that way you can modify the HTML, CSS and the images to increase performance. If this is for one isolated event and not distribution then I would separate into individual HTML files but you would need to test. You have to understand that ePub has very limited capabilities because that want you to develop in an app. Apple alone has a JavaScript library that restricts you to only animate with, what I recall, 25 events and image loading wasn't one of them.
    – user9447
    Jun 23, 2016 at 12:20

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No this is not possible due to the application control of the title and that most ebook software restricts the usage of JavaScript. I would advise using a script to unpack the ePub and use something like Imagemagick or sips (for mac) to resize the images to around 650px width but that depends on the image overal size.

One issue you will face with InDesign is that if you use the same image, say like a chapter ornament, the application doesn't consider you using it multiple times but generates the same image for every instance it is found. I would advise you to do what you can and then on export unzip and cleanup the images. As far as lazy loading most applications load on chapter increments so make sure to separate the content by chapter levels so it will not effect the user.

One thing I would point out is if you have a ton of text as images I would reconsider that approach as text in an image is considered bad and tends to be rejected by a lot of areas.

For performance if you used paragraph styles you could use InDesign to export to individual HTML files based on the InDesign style on the ePub export. That may increase performance for you but in the end I would still advise unpacking the ePub, as it is technically a zipped file, and cleaning the HTML, CSS and images.

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  • Excellent answer, thank you. I think un-packing the epub to optimise the HTML/CSS is a good move. I'm a front-end developer, so hopefully I can work some magic.
    – Sam
    Jun 23, 2016 at 12:28

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