[libdc1394-devel] RE: DMA Buffers

[Andrew H. <ho...@cs...>]

Hi Chris,

> Yep,
> when I originally wrote the capture code for dc1394, I was thinking about
> real-time vision applications, so It was written to only return the most
> recent image.  I warning about this:  you don't necessarily get synchronized
> video, unless you use some kind of triggering mechanism.

What type of triggering mechanism?  Should I be using signals or ?????

I thought that the VIDEO1394_SYNC_FRAMES flag meant that the video coming
from the ohci cards were synchronized?

Thanks,

-- Andrew





>
> Chris
>
> The Robotics Institute
> Carnegie Mellon University
> (412)-268-3978
>
> > -----Original Message-----
> > From: lin...@li...
> > [mailto:lin...@li...]On Behalf Of Dan
> > Dennedy
> > Sent: Tuesday, July 23, 2002 10:12 PM
> > To: Andrew Hogue
> > Cc: linux1394-devel; libdc1394-devel
> > Subject: Re: DMA Buffers
> >
> >
> > On Tue, 2002-07-23 at 10:29, Andrew Hogue wrote:
> > > Hi,
> > >
> > > I have another question about the DMA buffer strategy that you employ in
> > > the dc1394 library...
> > >
> > > I can see how that would be beneficial if you always wanted a
> > smooth image
> > > sequence and are grabbing a video but in my case (realtime image
> > > processing) the most recent frame is wanted rather than some
> > frame in the
> > > buffer somewhere... in my case the only place I can see the use of a
> >
> > Oh great, the last release 0.8.3 does exactly what you want, but its
> > multicapture functionality does not span ports. We have since "enhanced"
> > the lib to perform the image buffering attempting to preserve each
> > frame.
> >
> > I can easily make the change, but it will change the parameter list to
> > dc1394_dma_setup_capture() to add the option. What do others think?
> >
> > In the meantime, I suppose in your program you could cycle through the
> > "extra" buffers without any processing after calling dma_multi_capture:
> >
> > dc1394_dma_multi_capture( cameras, numCameras );
> > for (i = 0; i < numCameras; i++) {
> >
> > 	/* process the frame at cameras[i].capture_buffer */
> >
> > 	dc1394_dma_done_with_buffer( &cameras[i] );
> >
> > 	/* discard the extra frames */
> > 	while (cameras[i].dma_extra_count > 0) {
> > 		dc1394_dma_single_capture( &cameras[i] );
> > 		dc1394_dma_done_with_buffer( &cameras[i] );
> > 	}
> > }
> >
> > ---------
> > I am committing a version of dc1394_multiview.c that uses a #define
> > DROP_FRAMES to enable this behavior. This is not optimal as is still
> > allocates the extra memory and has some memcpy() overhead, but it might
> > be satisfactory. The difference is quite noticeable visibly even with 2
> > cams on 2 ports (one per port).
> >
> >
> >
> >
> >
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