On Mon, 23 Apr 2012 23:34:05 +0200
Hans de Goede <hdego...@redhat.com> wrote:
> Thanks for your work on this! I've just spend almost 4 days wrestling
> which the Pixart JPEG decompression code to try to better understand
> these cams, and I have learned quite a bit and eventually came up
> with a different approach.
>
> But your effort is appreciated! After spending so much time on this
> myself, I can imagine that it took you quite some time to come up
> with your solution.
>
> Attach is a 4 patch patchset which I plan to push to v4l-utils
> tomorrow (after running some more tests in daylight). I'll also try
> to do some kernel patches tomorrow to match...
Hi Hans,
I tried your patch, but I am not happy with the images I have (pac7302).
You say that the marker cannot be in the range 0..31 (index 0..7), but
I have never seen a value lower than 68 (index 17).
This last marker value (68) is the default when the images have no big
contrasts. With such images / blocks, the standard JPEG quantization
table does not work well. It seems that, for this value, the table
should be full of either 7 or 8 (8 gives a higher contrast).
Here is the sequence which works better (around line 1420 of tinyjpeg.c):
-------------8<--------------
/* And another special Pixart feature, the DC quantization
factor is fixed! */
qt[0] = 7; // 8 gives a higher contrast
// special case for 68
if (marker == 68) {
for (i = 1; i < 64; i++)
qt[i] = 7; // also works with 8
} else {
for (i = 1; i < 64; i++) {
j = (standard_quantization[0][i] * comp + 50) / 100;
qt[i] = (j < 255) ? j : 255;
}
}
build_quantization_table(priv->Q_tables[0], qt);
-------------8<--------------
About the other marker values, it seems also that the quantization
tables are not optimal: some blocks are either too much (small
contrasted lines) or not enough (big pixels) decompressed. As you know,
a finer adjustment would ask for a long test time, so, I think we can
live with your code.
Best regards.
--
Ken ar c'hentañ | ** Breizh ha Linux atav! **
Jef | http://moinejf.free.fr/
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