Hans Verkuil wrote:
On Sunday 15 February 2009 10:08:04 Hans de Goede wrote:
kilgota@xxxxxxxxxxxxxxxxxxxxxx wrote:
On Sat, 14 Feb 2009, Hans Verkuil wrote:
On Saturday 14 February 2009 22:55:39 Hans de Goede wrote:
Adam Baker wrote:
Hi all,
Hans Verkuil put forward a convincing argument that sensor
orientation shouldn't be part of the buffer flags as then it would
be unavailable to clients that use read()
Yes and this is a bogus argument, clients using read also do not get
things like timestamps, and vital information like which field is in
the read buffer when dealing with interleaved sources. read() is a
simple interface for simple applications. Given that the only user of
these flags will likely be libv4l I *strongly* object to having this
info in some control, it is not a control, it is a per frame (on some
cams) information about how to interpret that frame, the buffer flags
is a very
logical place, *the* logical place even for this!
The fact that there is no way to transport metadata about a frame
like flags, but also timestamp and field! Is a problem with the read
interface
in general, iow read() is broken wrt to this. If people care add some
ioctl or something which users of read() can use to get the buffer
metadata from the last read() buffer, stuffing buffer metadata in a
control (barf), because of read() brokenness is a very *bad* idea,
and won't work in general due to synchronization problems.
Doing this as a control will be a pain to implement both at the
driver level, see the discussion this is causing, and in libv4l. For
libv4l this
will basicly mean polling the control. And hello polling is lame and
something from the 1980-ies.
Please just make this a buffer flag.
OK, make it a buffer flag. I've got to agree that it makes more sense
to do
it that way.
Regards,
Hans
--
Hans Verkuil - video4linux developer - sponsored by TANDBERG
Let me take a moment to remind everyone what the problem is that
brought this discussion up. Adam Baker and I are working on a driver
for a certain camera. Or, better stated, for a set of various cameras,
which all have the same USB Vendor:Product number. Various cameras
which all have this ID have different capabilities and need different
treatment of the frame data.
The most particular problem is that some of the cameras require byte
reversal of the frame data string, which would rotate the image 180
degrees around its center. Others of these cameras require reversal of
the horizontal lines in the image (vertical 180 degree rotation of the
image across a horizontal axis).
The point is, one can not tell from the Vendor:Product number which of
these actions is required. However, one *is* able to tell immediately
after the camera is initialized, which of these actions is required.
Namely, one reads and parses the response to the first USB command sent
to the camera.
So, for us (Adam and me) the question is simply to know how everyone
will agree that the information about the image orientation can be sent
from the module to V4L. When this issue is resolved, we can finish
writing the sq905 camera driver. From this rather narrow point of view,
the issue is not which method ought to be adopted. Rather, the issue is
that no method has been adopted. It is rather difficult to write module
code which will obey a non-existent standard.
Ack, but the problem later was extended by the fact that it turns out
some cams have a rotation detection (gravity direction) switch, which
means you can flip the cam on its socket while streaming, and then the
cam will tell you its rotation has changed, that makes this a per frame
property rather then a static property of the cam. Which lead to this
discussion, but we (the 2 Hans 's) agree now that using the flags field
in the buffer struct is the best way forward. So there is a standard now,
simply add 2 buffer flags to videodev2.h, one for content is h-flipped
and one for content is v-flipped and you are done.
Regards,
Hans
I think we should also be able to detect 90 and 270 degree rotations. Or at
the very least prepare for it. It's a safe bet to assume that webcams will
arrive that can detect portrait vs landscape orientation.
Handling those (esp on the fly) will be rather hard as width and height then
get swapped. So lets worry about those when we need to. We will need an
additional flag for those cases anyways.
Regards,
Hans
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