Hi Laurent, > -----Original Message----- > From: Laurent Pinchart [mailto:laurent.pinchart@xxxxxxxxxxxxxxxx] > Sent: Friday, August 03, 2012 1:45 PM > To: Bhupesh SHARMA > Cc: linux-usb@xxxxxxxxxxxxxxx; linux-media@xxxxxxxxxxxxxxx > Subject: Re: Query regarding the support and testing of MJPEG frame > type in the UVC webcam gadget > > 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. > Yes, you are right. It should be bytesused field instead of the length field. Regards, Bhupesh -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
