Re: V4L2_PIX_FMT_RAW
- Date: Thu, 21 Feb 2008 21:12:35 +0100
- From: "H. Willstrand" <h.willstrand@xxxxxxxxx>
- Subject: Re: V4L2_PIX_FMT_RAW
On Thu, Feb 21, 2008 at 7:55 PM, Thomas Kaiser
<linux-dvb@xxxxxxxxxxxxxxx> wrote:
>
> H. Willstrand wrote:
> > On Thu, Feb 21, 2008 at 1:00 PM, Thomas Kaiser
> > <linux-dvb@xxxxxxxxxxxxxxx> wrote:
> >> H. Willstrand wrote:
> >> > On Thu, Feb 21, 2008 at 2:20 AM, Daniel Glöckner <daniel-gl@xxxxxxx> wrote:
> >> >> On Thu, Feb 21, 2008 at 01:02:39AM +0100, H. Willstrand wrote:
> >> >> > What's the problem with having a name of the formalized data in the
> >> >> > video stream? ie raw do not mean undefined.
> >> >>
> >> >> I thought you wanted to avoid having to define V4L2_PIX_FMT_x for an
> >> >> exploding number of proprietary formats that are quite similar but still
> >> >> incompatible. It makes sense for formats that are used by more than one
> >> >> driver.
> >> >
> >> > Correct, the number of unique pixel formats should be kept down.
> >> > Again, comparing with digital cameras there are >200 proprietary
> >> > formats and there is a "clean-up" on-going where the "market" is
> >> > aiming for a OpenRAW.
> >> >
> >> > However, by declaring a generic RAW format (which is then driver
> >> > specific) doesn't help the user mode app developers. Calling a
> >> > multitude of libraries to see if you get lucky might not be a good
> >> > idea.
> >> >
> >> > Still, I'm suspectious about the definition "raw" used here.
> >> > RAW should mean unprocessed image data:
> >> > * no white balance adjustment
> >> > * no color saturation adjustments
> >> > * no contrast adjustments
> >> > * no sharpness improvements
> >> > * no compression with loss
> >>
> >> Yes, raw means "as it is" no stripping, decoding or removing of SOF headers are
> >> done in the driver. May be V4L2_PIX_FMT_AII (AII -> As It Is) is the better name?
> >>
> >
> > I struggle with the probability to find several CCD's having similar
> > formats. There aren't so many manifactors of CCD's but they truelly
> > can generate divergeting formats. Worst case scenario means >200
> > V4L2_PIX_FMT_RAW_...
> >
> > I think RAW is a OK name, the question is if the subcomponents of the
> > RAW formats has similarities, if so they might be standardized.
> > Looking into different Sony CCD's it's clearly possible, but after the
> > CCD the data has to be buffered, packaged and transmitted which of
> > course can be done in several ways...
> >
> > Cheers,
> > Harri
> >
> >> >
> >> > So, by looking for similarities in the "raw" formats where available
> >> > there should be a potential to consolidate them.
> >> >
> >> >>
> >> >> > I don't see how separate RAW ioctl's will add value to the V4l2 API,
> >> >> > it fits into the current API.
> >> >>
> >> >> Yes, it does. Each driver having multiple raw formats just needs a
> >> >> private control id to select one.
> >> >>
> >> > I was more thinking about the VIDIOC_S_RAW stuff, a VIDIOC_S_FMT
> >> > should do the job.
> >> > I.e. I think there should be strong reasons to break V4L2 API behavior.
> >> >
> >> > Harri
>
> Actually, in a webcam you have the image sensor and a usb bridge. Usually, the
> sensor capture a picture in Bayer pattern. This gets forwarded to the usb
> bridge. The usb bridge may or may not transfer the picture to an other format
> and/or compress it with a standard compression algo or a proprietary compression
> algo. The resulting data stream will be transmitted over the usb interface.
>
Yes, the USB bridge buffers, packages and transmits.
> I just would like to get this resulting stream to user space without
> manipulation/conversion/decoding of the stream in the kernel module.
>
> That means we don't know what the format is in this data which comes trough the
> usb interface. That's way I call it raw.
>
> At the moment with V4L2, I have to forward a stream to user space which is in a
> format v4l2 knows. That means I have sometimes to do heavy data processing in
> the kernel module to decode/convert the data from the usb stream to a known v4l2
> video format.
>
Drivers should not do any decoding / converting, it's not allowed in
kernel mode.
But you are right, there are a number of V4L1 exceptions:
AR M64278 (arv.c) converts YUV422 to YUV422P
QuickCam (bw-qcam.c) converts RAW to a useful format :)
CPiA (cpia.c) converts 420 to different RGB formats
OmniVision (ov511.c) converts from YUV4:0:0
PWC (V4L2) does decoding
...
However, the Webcams provides only a limited set of formats and the
"raw" are usually available. New drivers with proprietary "raw"
formats should be added to videodev2.h
> That's way I want a official way to forward the untouched usb stream to user space!
>
> How the user space application has to react on this stream is an other story, I
> think. But there will be some way to tell the usespace application what to do
> with this "unknown" stream, I am sure.
>
> Thomas
>
Cheers,
Harri
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