This is a continuation of pre-recording, as suggested by @Mathew Kelly in pull request #728. I think I've found a way that would cover a wide range of uses:
start recording without the additional delay (required for creating the video file, setting up the raw backend, allocating memory etc)
start recording with an additional remote
group short bursts of action into a single clip
interface with external triggers (using the half-shutter signals)
interface with Lua scripts (using half-shutter - not tested, may require merging with lua_fix)
interfacing with intervalometer or motion detection (not implemented, but very easy after this change)
The half-shutter triggers can be used as standalone, or alongside with pre-recording. Internally, they use the pre-recording backend to pause and resume the video (therefore skipping the frames that were not triggered).
The post-recording feature suggested by @Mathew Kelly can be achieved in the "Half-shutter hold" mode, by holding the half-shutter as long as needed.
In all cases, you still have to press the usual REC button to start/stop a clip. Then, half-shutter (in the 3 different modes) should be used to start/pause recording.
I'm a little late to the party. It looks like this has been merged with the crop_rec branch so I'm using a crop_rec test build. I can think of so many uses for this but thought I'd start out with the note on the film scanning project where the EOSM captures one frame at a time using AUDIO_REMOTE_SHOT.
I started out setting the Pre-record for 1 second and the Reg trigger to Half-shut:1 frame. Every half-shutter recorded a second instead of just 1 frame.
Ok--don't really need pre-record for this so I turned that off and now I'm getting 2 frames per half-shutter press. Not sure if I'm doing something wrong but it looks like all the settings are telling me that it should record only 1 frame.
We'd like to use an EOSM on the film scanner but it doesn't have a wired remote so I tried Audio RemoteShot. It works to get the camera into record mode but when trying to shoot a single frame it returns an "Already recording" error on the screen.
The option to record 1 frame at a time was really a cool feature. Looks like commit fc4f20b did away with that. Is there a possibility of getting 1-frame-at-a-time recording to work properly? Yeah, the silent module can do this but it doesn't have all the goodies that are in mlv_lite.
Ok--took me a while but got it working, 1-frame at a time, I think. Will have to do further testing. So the counter is counting in 1/10th of second instead of in frames? On the EOSM I need to do 3 half-shutter presses for the counter to increment to 1, three more to get to 2, etc. ?
Submitted pull request #823 to change display from showing fractional seconds to frames. This should help when shooting one-frame-at-a-time because you can see the counter increment on every frame instead of every tenth second.