On Tue, 2010-03-02 at 18:23 -0300, Mauro Carvalho Chehab wrote:
> Devin Heitmueller wrote:
> > On Tue, Mar 2, 2010 at 3:28 PM, Hans Verkuil <hverkuil@xxxxxxxxx> wrote:
>
> > I had some extended discussion with Mauro on this yesterday on
> > #linuxtv, and he is now in favor of introducing a standard user
> ===
> > control for chroma gain, as opposed to doing a private control at all.
>
> To be clear: I was never against ;)
>
> It is worthy to summarize the discussions we have and the rationale to
> create another control for it.
>
> I've checked the datasheets of some chipsets, and the chroma gain is
> different than the saturation control: the gain control (chroma or luma)
> are applied at the analog input (or analog input samples) before the color
> decoding, while the saturation is applied to the U/V output levels (some
> datasheets call it as U/V output gain
Yes, that is correct.
> - causing some mess on the interpretation
> of this value).
AFAICT, the effect of chroma gain is not really different from a
saturation control that scales both the U & V components by the same
factor.
_
A color vector A can be expressed as
_ _ _
A = YW + C
_
Where YW is a white vector that has a luminance component of magnitude
Y.
_
C is the chrominace vector in a constant luuminance plane. Its phase is
the hue, and its magnitude is the saturation.
_
Adjusting the magnitude of C in the analog domain will change the
saturation.
_ _
Adjusting C's U & V components will adjust only the magnitude of C, if U
& V are adjusted by the same scale factor. (You can tweak both the hue
and saturation by adjusting U & V by different scale factors.)
> The API spec patch should clearly state that Saturation is for the U/V output
> level, while gain is for the analog input gain.
That makes sense.
Since we're thinking about what to name controls, I will note the
CX25843, doesn't quite fit the current discussion of an analog "chroma
gain" independent from "luma gain":
1. the CX25843 has at least 3 front end gains well before U/V
separation:
a +12 dB analog boost
a analog coarse gain (controlled by an AGC),
a digital fine gain (also has an AGC).
The +12 dB analog boost can be applied separately for Y, C, Pb and/or
Pr, but the other analog and digital gains cannot. They will be applied
to all video signal inputs the same.
2. the CX25843 U and V saturation scale factors can be set
independently, if desired.
Regards,
Andy
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