KDE4 and video acquisition

[Matthias Kretz]

Hi,

On Friday 20 January 2006 21:20, Laurent Pinchart wrote:
> with all the development effort going in KDE4 (and renaming kdemm to Phonon
> must be a sign that developers are active on the project, isn't it ? :-)),
> I thought I would rise the issue of video acquisition.

Thanks for doing so.

[snip motivational part]
> As KDE developers are busy creating an architecture for the multimedia
> framework in KDE4, I was wondering if any of you have thought about webcam
> support. Everything I read about Phonon states that sound will be supported
> through various backends (GStreamer, NMM or MAS), but the video input has
> not been publicly discussed so far. With all the controls that a webcam can
> expose (brightness/contrast/hue/saturation, but also exposure time, gain,
> power line frequency setting, pan/tilt/zoom, iris, ...), the job is not
> trivial.

You are right, that currently there hasn't been any work on a VideoCapture 
interface (only audio). While it should be possible to add this interface in 
a later version of KDE, I'd like to have it done for KDE 4.0, if possible. As 
I have never done any video capture work (on the developers side, as a user 
(on Windoze and Linux) capturing from a DV camera over Firewire I have a 
little experience) I'd be very grateful for helping hands on defining that 
interface.

Here are a few thoughts from the top of my head:
1. it can be a capture source that delivers both audio and video (like a DV 
stream)
2. if it's a video only capture you often want to have the audio signal from 
the soundcard to be in sync with the video signal. So either there needs to 
be an interface for defining the sync between a SoundcardCapture and a 
VideoCapture or there should be only one interface "Capture" where the audio 
and video source can be independently set, and if both are set they're 
implicitly kept in sync.
3. There's a class CaptureSource in SVN that describes the available sources 
to the user. (It's a really simple class, like a struct containing an id, 
name and description, but with users of the API only being able to read, of 
course). This class could just as well be used to describe the available 
video capture sources to the user, or is there any information missing?
4. What properties does the VideoCapture interface need? You already mentioned
- brightness
- contrast
- hue
- saturation
- exposure time
- gain (I only know gain from audio signal processing, is that what you meant 
or is there actually a gain for video signals?)
- power line freq (Is that setting used for filtering? Is it available in the 
hardware interface? Is it available in the mediaframework interfaces?)
- pan
- tilt
- zoom
- iris
As I, like I already said, have no experience with the developer side of video 
capture, I don't know whether these properties are all settings the hardware 
exposes or that need to be done in software. If the latter is needed these 
things should be done in the VideoPath which is meant for processing of the 
video signal.

I'd be thankful if you want to try to write such an interface. We can discuss 
this here on this list.

-- 
C'ya
        Matthias
________________________________________________________
Matthias Kretz (Germany)                            <><
http://Vir.homelinux.org/
MatthiasKretz at gmx.net, kretz at kde.org,
Matthias.Kretz at urz.uni-heidelberg.de
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