Is there an HDpiano app?
We don't have plans to release a mobile app as of now and we're not sure that there would be much added functionality with our format as it stands. Have you checked out www.hdpiano.com/resources ? There's a lot you can do with our video player. We would love to hear any feedback about what you're missing from the experience and we can definitely keep your feedback in mind as we continue to improve.
If you're looking for a quicker way to access HDpiano, check out this YouTube video on how to make a shortcut on your home screen.