Python and webcam capture delay?

[Rhodri James]

On Mon, 06 Jul 2009 07:10:38 +0100, jack catcher (nick) <nomail at thank.you>  
wrote:

> Tim Roberts kirjoitti:
>> "jack catcher (nick)" <nomail at thank.you> wrote:
>>> I'm thinking of using Python for capturing and showing live webcam  
>>> stream simultaneously between two computers via local area network.  
>>> Operating system is Windows. I'm going to begin with VideoCapture  
>>> extension, no ideas about other implementation yet. Do you have any  
>>> suggestions on how short delay I should hope to achieve in showing the  
>>> video? This would be part of a psychological experiment, so I would  
>>> need to deliver the video stream with a reasonable delay (say, below  
>>> 100ms).
>>  You need to do the math on this.  Remember that a full 640x480 RGB  
>> stream
>> at 30 frames per second runs 28 megabytes per second.  That's more than
>> twice what a 100 megabit network can pump.
>>  You can probably use Python to oversee this, but you might want to  
>> consider
>> using lower-level code to control the actual hardware.  If you are
>> targeting Windows, for example, you could write a DirectShow graph to  
>> pump
>> into a renderer that transmits out to a network, then another graph to
>> receive from the network and display it.
>>  You can manage the network latency by adding a delays in the local  
>> graph.
>
> Thanks Tim, you're correct about the math. What is your main point about  
> DirectShow: that it is generally faster and more reliable than doing the  
> job high-level, or that one could use coding/decoding in DirectShow to  
> speed up the transmission? I think the latter would be a great idea if  
> the latency were tolerable. On the other hand, I'd like to keep things  
> simple and do all the programming in Python. I've got no experience with  
> DirectShow, but I guess the filters need to be programmed in C++ and  
> called from Python?
>
> Another option might be to use resolution 320x240 at 15fps.

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.

-- 
Rhodri James *-* Wildebeest Herder to the Masses