In an earlier post on Object Export Options, I had shown you how to specify Alt-text. In this post, we’ll see how Object Export options help us in the EPUB and HTML workflows. You can use the Object Export options to create different conversion settings on each object, with special attention spent on settings useful for different screens sizes and pixel densities (ppi).
Unlike alt text, which is supported in all three major export formats—PDF, EPUB, and HTML— the EPUB and HTML tab in the Object Export dialog box represents conversion and formatting options unique to EPUB and HTML.
The main purpose for this tab is to set image conversion properties on an object by object basis. This enables you to apply different degrees of quality on each individual object. If not specified, the global conversion options defined by EPUB or HTML export is used. To enable per object conversion settings, the “Custom Rasterization” box must first be checked.
Below is a description for the major features. (The rest are easy to understand).
You can choose between fixed size or the new relative to page width size. Using Fixed results in an image with static height and width pixel dimensions based on the size of the object used in the InDesign document. “Relative to page width” setting sets a % value based on the width of the image relative to the InDesign page width. The % value enables the image to resize automatically based on the screen size of the device or the size of the browser window. “Relative” is recommended when producing EPUBs that are intended to be viewed on different devices.
PNG is now supported, in addition to GIF and JPEG. PNG is a lossless format and also supports transparency. When PNG is selected, compression options are dimmed out.
While operating systems have standardized on either 72 or 96 ppi, mobile devices range from 132 ppi (iPad), to 172 ppi (Sony Reader), to over 300 ppi (iPhone 4). New in CS5.5 is the ability to specific a ppi value for each object selected. Values include 72, 96, 150 (average of all ebook devices today), and 300.