Python and webcam capture delay?

[jack catcher (nick)]

Nobody kirjoitti:
> On Mon, 06 Jul 2009 20:41:03 +0300, jack catcher (nick) wrote:
> 
>>> Does the webcam just deliver frames, or are you getting frames out of
>>> a decoder layer?  If it's the latter, you want to distribute the encoded
>>> video, which should be much lower bandwidth.  Exactly how you do that
>>> depends a bit on what format the webcam claims to deliver.
>> Well, getting already encoded video from the webcam sounds almost like a 
>> free lunch (which it probably is not). At least I wouldn't want to get 
>> too long a delay because of the encoding.
>>
>> I haven't got the webcam(s) yet, and I guess I can basically purchase 
>> any ones I find suitable for getting the job done. Any recommendations?
> 
> The webcam is bound to do some encoding; most of them use USB "full speed"
> (12Mbit/sec), which isn't enough for raw 640x480x24bpp at 30fps data.
> 
> Chroma subsampling and JPEG compression will both reduce the bandwidth
> without introducing noticable latency (the compression time will be no
> more than one frame).
> 
> Temporal compression will reduce the bandwidth further. Using B-frames
> (frames which contain the differences from a predicted frame which is
> based upon both previous and subsequent frames) will provide more 
> compression, but increases the latency significantly. For this reason, the
> compression built into video cameras normally only uses P-frames (frames
> which contain the differences from a predicted frame which is based only
> upon previous frames).
> 
> The biggest issue is likely to be finding latency specifications, followed
> by finding a camera which meets your latency requirement.

Thanks for the comments. Unfortunately, such specifications aren't easy 
to find, even in reviews. Fortunately several newer webcams seem at 
least to use usb2.

Regarding Python, I'll have to check whether the ImageCapture extension 
works at all, as someone suggested earlier, and the possibility of using 
DirectShow. I'm still considering Matlab as an option for Python here.