On Sat, 28 Feb 2009 16:11:36 +0100
Anders Blomdell <[email protected]> wrote:
> Jean-Francois Moine wrote:
> > Thomas Champagne (See To:) was also writing a driver for this
> > webcam. Maybe you may merge your codes...
> Thomas, if you have DQT/Huffman tables for this camera, please drop
> me a note.
>
> > About the JPEG images, the Huffman table is always the same
> Does this mean that it's the same for all JPEG images or only for one
> camera?
>
> If it's the same for all images, it should mean that I have a way to
> determine how much I have to chop off after the 0xfffe tag (no
> illegal huffman codes -> possibly chop at the correct position).
>
> Comments anyone?
I already explained it:
- when a packet starts with '55 ff d8', it is the first part of the
image. This one should start at the offset 8 of the packet.
~~~~~~~~
> > and the
> > quantization tables depend on the compression quality.
> >
> > From the USB trace I had from Thomas, I saw that:
> >
> > - when a packet starts with '55 ff d8', it is the first part of the
> > image. This one should start at the offset 8 of the packet.
> >
> > - when a packet starts with 'cc', it is the next part of the image.
> This is even in the docs, and is implemented in the driver.
>
> > In the function pkt_scan, when finding the image start, you must add
> > the JPEG header: 'ff d8', DQT, huffman table, SOF0 and SOS.
> OK, will see if I can find the DQT (and possibly the Huffman table)
> in the windows driver (as suggested by Thomas Kaiser).
See below.
> > As we don't know the quality used by the webcam, in my test
> > repository, I added a control for that: the JPEG header is created
> > at streamon time, and the quantization tables may be modified by
> > the control on the fly (have a look at stk014.c for an example).
> >
> > This solution is not the right one: the JPEG quality must be set by
> > the VIDIOC_S_JPEGCOMP ioctl instead of VIDIOC_S_CTRL. I think I
> > will update the concerned subdrivers next week.
> I'll look into that monday.
In gspca, the file "jpeg.h" contains all the material to build a
complete JPEG header. With the new mechanism of my test repository, you
must:
- at streamon time (function sd_start):
- allocate a buffer for the JPEG header,
- set the resolution and number of samplesY (0x22 or 0x21) by
the function jpeg_define,
- set the quantization tables by the function jpeg_set_qual,
the quality being in percent (15..95).
- when getting the start of a new image in sd_pkt_scan, add the JPEG
header as the first packet.
- at streamoff time (function sd_stop0), free the JPEG header.
- on VIDIOC_S_JPEGCOMP (function sd_set_jcomp which you must create),
redefine the quantization tables by jpeg_set_qual.
Cheers.
--
Ken ar c'hentañ | ** Breizh ha Linux atav! **
Jef | http://moinejf.free.fr/
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