Hi Sakari :-)
On 05/08/2013 06:26 PM, Sakari Ailus wrote:
On Tue, May 07, 2013 at 04:04:10PM +0200, Sylwester Nawrocki wrote:
On 05/07/2013 02:35 PM, Hans Verkuil wrote:
A metadata plane works well if you have substantial amounts of data (e.g.
histogram
data) but it has the disadvantage of requiring you to use the MPLANE buffer
types,
something which standard apps do not support. I definitely think that is
overkill
for things like this.
Standard application could use the MPLANE interface through the libv4l-mplane
plugin [1]. And meta-data plane could be handled in libv4l, passed in raw form
from the kernel.
There can be substantial amount of meta-data per frame and we were considering
e.g. creating separate buffer queue for meta-data, to be able to use mmaped
buffer at user space, rather than parsing and copying data multiple times in
the kernel until it gets into user space and is further processed there.
What kind of metadata do you have?
At least I can tell of three kinds of meta-data at the moment:
a) face/smile/blink detection markers (rectangles), see struct
is_face_marker
in file [1] in the media tree for more details; these markers can be
available after an image frame is dequeued AFAIK, i.e. not immediately
together with the image data,
b) EXIF tags (struct exif_attribute in file [1]), it's a preprocessed by
the ISP metadata appended to each buffer,
c) the object detection bitmap, and this one can have size comparable to
the actual image frame; I didn't see how it works in practice yet
though.
For b) I have been re-considering using EXIF standard (chapter 4.6, [2]) to
create some sane interface for the ISP driver.
From performance POV only c) would need a meta-data specific buffer
queue, as
such data has similar characteristics to the actual image data and a DMA
engine
is used to capture those bitmaps.
As far as we're not copying megabytes of data by CPU there should be no big
issues, I guess couple pages per frame is fine.
I'm actually not sure if performance is a real issue here, were are talking
of 1.5 KiB order amounts of data per frame. Likely on x86 desktop machines
it is not a big deal, for ARM embedded platforms we would need to do some
profiling.
I'm not sure myself yet how much such motion/object detection data should be
interpreted in the kernel, rather than in user space. I suspect some generic
API like in your $subject RFC makes sense, it would cover as many cases as
possible. But I was wondering how much it makes sense to design a sort of
raw interface/buffer queue (similar to raw sockets concept), that would allow
user space libraries to parse meta-data.
This was proposed as one possible solution in the Cambourne meeting.
<URL:http://permalink.gmane.org/gmane.linux.drivers.video-input-infrastructure/36587>
Oh, thanks for bringing up those meeting minutes.
I'm in favour of using a separate video buffer queue for passing low-level
metadata to user space.
Sure. I certainly see a need for such an interface. I wouldn't like to
see it
as the only option, however. One of the main reasons of introducing MPLANE
API was to allow capture of meta-data. We are going to finally prepare some
RFC regarding usage of a separate plane for meta-data capture. I'm not sure
yet how it would look exactly in detail, we've just discussed this topic
roughly with Andrzej.
The format of meta-data could for example have changed after switching to
a new version of device's firmware. It might be rare, I'm just trying to say
I would like to avoid designing a kernel interface that might soon become a
limitation.
On some devices it seems the metadata consists of much higher level
information.
Indeed. It seems in case of devices like OMAP3 ISP we need to deal
mostly with
raw data from a Bayer sensor, while for the Exynos ISP I would need to
expose
something produced by the standalone ISP from such a raw metadata.
[1] drivers/media/platform/exynos4-is/fimc-is-param.h
[2] http://www.exif.org/Exif2-2.PDF
--
Regards,
Sylwester
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