Camera app on lock screen
The built-in Camera app (LegacyCamera) does not work on the lock screen. I looked at how Open Camera and the successor of LegacyCamera, Camera2, are doing this and I ended up picking some bits from Camera2 and writing the rest myself.
One part was quite tricky: When the phone wakes up, the Camera goes in an OnResume, OnPause, OnResume cycle and sometimes it ends up in the OnPause state and is unresponsive. Adding the trick with the delay for OnResume from Camera2 worked.
When the thumbnail of the last picture is clicked, the so-called SharePopup comes up and shows the last picture enlarged. As this could be used by someone else to take a closer look at your last pic and as Open Camera and Camera2 disabled similar functionality on the lock screen, I also made the SharePopup inaccessible from the lock screen.
Patch is tested for several days on my Galaxy S3.
Updated by Wolfgang Wiedmeyer about 4 years ago
The view for recording video does also work on the lock screen, but switching to this mode takes a moment and the lock screen is shortly visible.
With the changes applied I described on the mailing list, video recording in the secure mode also works with the S3.