There is no way to distinguish an unknown format from Mono8 because Mono8 is zero. The current CL fixes this issue by starting the format constants from iota + 1. Change-Id: I8c709cdaff3bc46bcc15ab770cfcfd04c186034d Reviewed-on: https://go-review.googlesource.com/12386 Reviewed-by: Burcu Dogan <jbd@google.com>