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 -- 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
