> Hi,
>
> The VIDIOC_G_JPEGCOMP / VIDIOC_S_JPEGCOMP v4l2 ioctls seem not to be
> used by many drivers / applications. They should!
Unfortunately, these ioctls are completely undocumented. Which might be
the reason why they aren't used :-)
> In some ms-win traces, there are automatic and dynamic adjustments of
> the JPEG quality according to... who knows?
>
> Also, most webcams do not include the quantization tables in the images.
> Then, (in gspca), these tables are added by the subdrivers with a
> quality defined by the testers and according to their taste.
>
> As I understand, the JPEGCOMP ioctls permit to set the JPEG quality and
> to define the content of the JPEG frames.
>
> If I implement these controls in gspca:
>
> - by default, I could not add the quantization and Huffman tables in the
> image frames,
>
> - the quality could be set dynamically, this value being used to load
> the quantization tables in the webcam and also to convert the images.
>
> The questions are:
>
> 1) May the driver refuse to set some values on VIDIOC_S_JPEGCOMP?
> For example, if it cannot add the Huffman table in the frames.
You will have to check what the existing practice is. How to other drivers
handle this?
> 2) Will the VIDIOC_G_JPEGCOMP ioctl be used by the v4l library (for
> conversion purpose)?
>
> 3) Does anybody know a command line or X application which may get/set
> these JPEG parameters?
Support for these ioctls should be added to v4l2-ctl.cpp. It's the right
place for that.
But more important is to document these ioctls in the v4l2 spec. As far as
I can tell these ioctls came from the zoran driver where basically a
private ioctl was elevated to a public ioctl, but with little or no
review.
Do you know enough about these ioctls to update the v4l2 spec? That would
be a great help.
Regards,
Hans
--
Hans Verkuil - video4linux developer - sponsored by TANDBERG
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