Ok, so I checked the quantization tables for each of the supported quality factors(Q=20,40,60,80 and 100) and at least for the logitech cameras they are exactly the same, so no compression change. Could the quality factor be relevant only for still image frames ?
Best regards, Paulo 2009/6/2 Laurent Pinchart <[email protected]> > Hi Paulo, > > On Wednesday 03 June 2009 00:04:23 Paulo Assis wrote: > > Hi, > > so here are my findings (using a sphere AF): > > > > I'm now disabling the video stream and re-queueing the buffers, this > > enables VIDIOC_S_JPEGCOMP > > > > below is what I'm getting from the driver and the corresponding log entry > > from dmesg: > > > > setting quality to 60 => > > > > VIDIOC_G_COMP: > > quality: 60 > > APPn: 0 > > APP_len: 0 > > APP_data: > > COM_len: 0 > > COM_data: > > jpeg_markers: 0x10 > > > > "dmesg (with trace=15)" > > [ 9993.399812] uvcvideo: Setting jpeg quality to 6000. > > > > setting quality to 80 => > > > > VIDIOC_G_COMP: > > quality: 80 > > APPn: 0 > > APP_len: 0 > > APP_data: > > COM_len: 0 > > COM_data: > > jpeg_markers: 0x10 > > > > "dmesg (with trace=15)" > > [10001.869273] uvcvideo: Setting jpeg quality to 8200. > > > > In any case the video frames don't change their quality, so I guess the > > camera is not handling this the correct way, > > I haven't seen any visual difference either between Q=20 and Q=100 on my > Logitech Quickcam Pro for Notebooks. However, the average frame size was > ~24k > for Q=20 and ~28k for Q=100, so I guess there's still a difference. > > > I can't also understand the values seen in dmesg, is there any reason > > special reason for this ? > > The quality range in the UVC specification is [0, 10000]. While the V4L2 > specification doesn't mention any specific range, most users are familiar > with > the [0, 100] range. I thus convert between the two ranges in the driver. > > Moreover, the requested quality value is passed to the hardware which is > free > to modify it. This is probably why you get 8200 when you ask for 8000. > > Best regards, > > Laurent Pinchart > >
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