[libdc1394-devel] Re: DMA Buffers

[Dan D. <da...@de...>]

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).