Re: Query regarding the support and testing of MJPEG frame type in the UVC webcam gadget

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Bhupesh,

On Wednesday 01 August 2012 21:29:30 Bhupesh SHARMA wrote:
> On Wednesday, August 01, 2012 6:46 PM Laurent Pinchart wrote:
> > On Wednesday 01 August 2012 14:26:33 Bhupesh SHARMA wrote:
> > > Hi Laurent,
> > > 
> > > I have a query for you regarding the support and testing of MJPEG
> > > frame type in the UVC webcam gadget.
> > > 
> > > I see that in the webcam.c gadget, the 720p and VGA MJPEG uvc formats
> > > are supported. I was trying the same out and got confused because the
> > > data arriving from a real video capture video supporting JPEG will have
> > > no fixed size. We will have the JPEG defined Start-of-Frame and End-of-
> > > Frame markers defining the boundary of the JPEG frame.
> > > 
> > > But for almost all JPEG video capture devices even if we have kept a
> > > frame size of VGA initially, the final frame size will be a compressed
> > > version (with the compression depending on the nature of the scene, so a
> > > flat scene will have high compression and hence less frame size) of VGA
> > > and will not be equal to 640 * 480.
> > > 
> > > So I couldn't exactly get why the dwMaxVideoFrameBufferSize is kept
> > > as 614400 in webcam.c (see [1]).
> > 
> > The dwMaxVideoFrameBufferSize value must be larger than or equal to the
> > largest MJPEG frame size. As I have no idea what that value is, I've
> > kept the same size as for uncompressed frames, which should be big enough
> > (and most probably too big).
> 
> .. Yes, so that means that the user-space application should set the length
> of the buffer being queued at the UVC side equal to the length of the buffer
> dequeued from the V4L2 side, to ensure that varying length JPEG frames are
> correctly handled.

You should copy the bytesused field from the captured v4l2_buffer to the 
output v4l2_buffer. The length field stores the total buffer size, not the 
number of bytes used.

> I will try with these changes in the user-space daemon.

-- 
Regards,

Laurent Pinchart