Re: [RFC] Motion Detection API

Hans Verkuil
2013-05-06T13:42:05Z

On Mon April 29 2013 22:52:31 Laurent Pinchart wrote:
> Hi Hans,
>
> Sorry for the late reply.
>
> On Friday 12 April 2013 17:36:16 Hans Verkuil wrote:
> > This RFC looks at adding support for motion detection to V4L2. This is the
> > main missing piece that prevents the go7007 and solo6x10 drivers from being
> > moved into mainline from the staging directory.
> >
> > Step one is to look at existing drivers/hardware:
> >
> > 1) The go7007 driver:
> >
> > - divides the frame into blocks of 16x16 pixels each (that's 45x36 blocks
> > for PAL)
> > - each block can be assigned to region 0, 1, 2 or 3
> > - each region has:
> > - a pixel change threshold
> > - a motion vector change threshold
> > - a trigger level; if this is 0, then motion detection for this
> > region is disabled
> > - when streaming the reserved field of v4l2_buffer is used as a bitmask:
> > one bit for each region where motion is detected.
> >
> > 2) The solo6x10 driver:
> >
> > - divides the frame into blocks of 16x16 pixels each
> > - each block has its own threshold
> > - the driver adds one MOTION_ON buffer flag and one MOTION_DETECTED
> > buffer flag.
> > - motion detection can be disabled or enabled.
> > - the driver has a global motion detection mode with just one threshold:
> > in that case all blocks are set to the same threshold.
> > - there is also support for displaying a border around the image if
> > motion is detected (very hardware specific).
> >
> > 3) The tw2804 video encoder (based on the datasheet, not implemented in the
> > driver):
> >
> > - divides the image in 12x12 blocks (block size will differ for NTSC vs
> > PAL)
> > - motion detection can be enabled or disabled for each block
> > - there are four controls:
> > - luminance level change threshold
> > - spatial sensitivity threshold
> > - temporal sensitivity threshold
> > - velocity control (determines how well slow motions are detected)
> > - detection is reported by a hardware pin in this case
> >
> > Comparing these three examples of motion detection I see quite a lot of
> > similarities, enough to make a proposal for an API:
> >
> > - Add a MOTION_DETECTION menu control:
> > - Disabled
> > - Global Motion Detection
> > - Regional Motion Detection
> >
> > If 'Global Motion Detection' is selected, then various threshold controls
> > become available. What sort of thresholds are available seems to be quite
> > variable, so I am inclined to leave this as private controls.
> >
> > - Add new buffer flags when motion is detected. The go7007 driver would need
> > 4 bits (one for each region), the others just one. This can be done by
> > taking 4 bits from the v4l2_buffer flags field. There are still 16 bits
> > left there, and if it becomes full, then we still have two reserved fields.
> > I see no reason for adding a 'MOTION_ON' flag as the solo6x10 driver does
> > today: just check the MOTION_DETECTION control if you want to know if
> > motion detection is on or not.
>
> We're really starting to shove metadata in buffer flags. Isn't it time to add
> a proper metadata API ? I don't really like the idea of using (valuable)
> buffer flags for a feature supported by three drivers only.

There are still 18 (not 16) bits remaining, so we are hardly running out of bits.
And I feel it is overkill to create a new metadata API for just a few bits.

It's actually quite rare that bits are added here, so I am OK with this myself.

I will produce an RFCv2 though (the current API doesn't really extend to 1080p motion
detection due to the limited number of blocks), and I will take another look
at this. Although I don't really know off-hand how to implement it. One idea that
I had (just a brainstorm at this moment) is to associate V4L2 events with a buffer.
So internally buffers would have an event queue and events could be added there
once the driver is done with the buffer.

An event buffer flag would be set, signalling to userspace that events are
available for this buffer and DQEVENT can be used to determine which events
there are (e.g. a motion detection event).

This makes it easy to associate many types of event with a buffer (motion detection,
face/smile/whatever detection) using standard ioctls, but it feels a bit convoluted
at the same time. It's probably worth pursuing, though, as it is nicely generic.

An alternative might be to set a 'DETECT' buffer flag, and rename 'reserved2'
to 'detect_mask', thus having up to 32 things to detect. The problem with that
is that we have no idea yet how to do face detection or any other type of
detection since we have no experience with it at all. So 32 bits may also be
insufficient, and I'd rather not use up a full field.

Regards,

Hans