Google Play Books vs Media Overlays

RSS  •  Permalink  •  Created 15 Jun 2014  •  Written by Alberto Pettarin

Prompted by a Twitter conversation, I decided to check whether the Google Play Books Android app supports EPUB 3 Media Overlays, either in reflowable or FXL books. You know, for science.


I tested the Google Play Books Android app (3.1.49) on my Lenovo tablet running Android 4.1.2.

As far as I know, the only way of loading your own EPUB files to Google Play Books is to upload them to "your" library in the cloud, so I uploaded the following two files:

The two files pass EpubCheck validation without warnings or errors, and they can be opened by other EPUB 3 readers, rendering the embedded Media Overlays as expected, that is, reading the embedded SMIL files and playing the corresponding embedded audio files.

After uploading the two files, I opened them in the Google Play Books app, and verified whether the Media Overlays were supported, by navigating each ebook and starting the "read aloud" function.


For both test files, the Google Play Books Android app renders Media Overlays by synthesizing the audio with the OS text-to-speech engine (TTS), ignoring the audio file referenced by the SMIL file embedded inside the EPUB. The interesting thing is that the app highlights the <text> fragments correctly, so the app must have processed the SMIL file.

The same behavior has been independently reported by another person.


This behavior is puzzling because the Google Play Books Android app (3.1.33) is currently reported to support Media Overlays in the EPUBTest results. Specifically, it is reported to pass the following tests:

  • mo-basic-010
  • mo-basic-060
  • mo-audio-010
  • mo-audio-020
  • mo-audio-030
  • mo-audio-040
  • mo-audio-050
  • mo-audio-060
  • mo-chap-020
  • mo-end-010
  • mo-basic-210
  • mo-basic-220
  • mo-basic-250
  • mo-basic-270
  • mo-nav-220
  • mo-nav-230
  • mo-end-210

as you can see from this screenshot of this page.

Please observe that the page has a note saying a cryptic:

This evaluation may not be current.

The "mystery" might be explained by:

  • the fact that the EPUB files were "uploaded" and Google disable reading the embedded audio files (possible);
  • the embedded audio files were MP3 files (possible but unlikely);
  • Google removed support to Media Overlays from v3.1.33 to v3.1.49 (very unlikely); or
  • the Google Play Books Android app is currently still work-in-progress, and always renders the embedded Media Overlays with TTS.

EDIT (2014-06-15 16:52:21+02:00): Gregorio Pellegrino points out that Google Play lets you upload SMIL+audio separately, and Jean Kaplansky confirms that Media Overlays work as expected (reading the provided audio files) only on books bought from Google Play.

This example highlights a potential problem with the way EPUBTest results are currently gathered: are the (often, self-reported) results really accurate? Can a third-party trust them, without re-testing by herself to be reasonably confident that a given reading system actually offers support for the desired feature, as reported in the EPUBTest grid?

Let me remark that I am not criticizing the effort of the organizations or the professionals involved in the EPUBTest project, which I find very useful, especially to improve reading system conformance to the EPUB specification. Indeed, I contributed comments aimed at improving the Media Overlays tests.

The discrepancy between the grid results and the reality might be simply due to a bona fide misinterpretation of the criterion for marking a given test as "passed", or caused by specific settings/requirements of the reading system (EDIT: as in this case), or a trivial mistake in the act of compiling the report. I just want to note that the results of that grid must be read with a pinch of salt.

(I have concerns of other nature about (the usage of) the EPUBTest grid, but this is a story for another post.)

Open problems and future work

  • Do Media Overlays work "as expected" in the Google Play Book Android app for eBooks bought from Google Play Books store? EDIT: confirmed to be the case
  • Do they work if the embedded audio files are in MP4 format? EDIT: immaterial (see above)
  • Is Google going to remove this limitation? Perhaps, even allowing "anonymous" sideloading?
  • What do you think about my observation about the EPUBTest result grid?