On Sun, 15 Feb 2009, Trent Piepho wrote:
On Sun, 15 Feb 2009, Hans de Goede wrote:
Trent Piepho wrote:
On Sun, 15 Feb 2009, Hans de Goede wrote:
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:
OK, make it a buffer flag. I've got to agree that it makes more sense
to do
it that way.
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.
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.
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.
Why would you need to worry about width and height getting swapped?
Meta-data about the frame would indicate it's now in portrait mode vs
landscape mode, but the dimentions would be unchanged.
Yes, unless ofcourse you want to display a proper picture and not one on its
side, when the camera is rotated 90 degrees, so somewere you need to rotate the
picture 90 degrees, and the lower down in the stack you do that, the bigger the
chance you do not need to duplicate the rotation code in every single app.
however the app will mostlikely become unhappy when you start out pushing
frames whith a changed width / height.
It seems that image rotation, like format conversion, is something that is
best done in userspace. It could be done in hardware with opengl or faster
software using MMX or SSE based code that can't be used in the kernel.
My impression is that nobody here disagrees with this. Certainly, I do
not. As I understand, there is a general intention to avoid writing new
modules which do such things and to try to rewrite old ones. The reasons
are, presumably, very similar to the reasons you give.
Therefore,
1. Everyone seems to agree that the kernel module itself is not going to
do things like rotate or flip data even if a given supported device
always needs that done.
However, this decision has a consequence:
2. Therefore, the module must send the information about what is needed
out of the module, to whatever place is going to deal with it. Information
which is known to the module but unknown anywere else must be transmitted
somehow.
Now there is a further consequence:
3. In view of (1) and (2) there has to be a way agreed upon for the module
to pass the relevant information onward.
It is precisely on item 3 that we are stuck right now. There is an
immediate need, not a theoretical need but an immediate need. However,
there is no agreed-upon method or convention for communication.
Some might argue that it is sufficient to know some ID of the device (USB
Vendor:Product number, for example), but it is not:
4. The idea of relying on the USB ID of the supported device to decide
what should be done with the frame data is inadequate. Sometimes,
preliminary communication with the device is the only possible way to
learn what is needed. Again, this is not a theoretical problem. It is an
actual problem. A known device exists. Go back to item (3).
There are of course related problems. But it strikes me that the related
problems are not so very related at all. As I understand, it is visualized
that a camera could be put on a pivot, with control mechanism which would
permit various rotations and then the question becomes how to support a
camera and to make the stream come out "right" no matter which way the
camera is pointed. A far-seeing project design will certainly think of
things like that before they happen and will try to anticipate what to do.
Why do I say then that these problems are not related at all to the
present problem? Because we are dealing with cameras that always present
the data upside-down or mirrored, unless corrective action is taken.
So, again, either the module has to do the correction inside itself (and
everyone agrees that it should not!) or it has to have a standard protocol
to pass that information onward. to be dealt with appropriately. It would
seem to me best to separate a problem like this from discussions about
tilting or physically rotating the camera.
Theodore Kilgore
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