[Nottingham] Webcam snapshots

[James Moore]

On 23/08/2011 10:41, Martin wrote:
> On 23 August 2011 10:27, James Moore<jmthelostpacket at googlemail.com>  wrote:
>> On 18/08/2011 12:24, Martin wrote:
>>> Folks,
>>>
>>> Anyone know of how to, or any utilities, that can take a single frame
>>> snapshot from a webcam *without* having the webcam running
>>> continuously?
>>>
>>> That is: How can you use one of the new fangled HD-quality webcams as
>>> a 'camera'?
>>>
>>> (This is also to get around the problem that you cannot use multiple
>>> HD webcams on one USB2 hub...)
>>>
>>>
>> um... you can... I have.
>> There's a Mandriva-based ISO around called Zoneminder, which allows you to
>> capture video or stills on a timebase or a motion-detect basis, from any
>> number of cameras, USB or network (LAN or WAN), into a single location. If
> Had a brief look at that. The problem there is a single HD-webcam
> grabs the entire bandwidth available on a USB2 root hub. Hence, unless
> you start adding additional USB2 interface cards so that you can have
> one root hub per HD-webcam, you can't concurrently run multiple
> webcams.
>
> Unless... You could only ever have just one of them active at any instant.
>
>
> I'm wanting to experiment with 3d reconstruction and want to switch
> between 4 webcams. It's rather too tedious (and too slow) to manually
> individually select between them in sequence with such a camorama.
>
> Hence... Any tricks to automate selecting each webcam in sequence to
> grab the 3rd frame of 3 frames of a video stream?
>
> Or indeed use them as a snapshot camera in the first place?
>
> (3rd frame of a 3 frames video streaming needed to allow the camera
> settings to settle... First two frames usually have the wrong
> exposure.)
>
>
> Anyone any ideas? I think this has got to be one of those things that
> has already been done...
>
IME Zoneminder grabs frames on a round-robin basis, which has the added 
bonus of complete system failure if one of the cameras fail(!) since it 
would just sit there and wait until the camera comes back online. 
Probably not the best thing to use if you're using remote cameras over 
the internet. So basically what you would have on a four camera setup is 
every fourth frame is grabbed from each camera, in sequence, or a net of 
7fps per camera. Good if you're doing bullet time, not so good if you're 
doing what I think you're trying to do - simultaneous grabbing from all 
four units?