[libcamera-devel] CCM from CCT: why not mired?

[David Plowman]

Thanks for the comments.

Indeed, looking at the matrices in apply_saturation, I actually have no
recollection where those numbers come from. Actually I suspect it makes no
difference mathematically (unless saturation were changed by different
amounts on the U and V axes), but I agree it would make sense to use the
"official" numbers there.

As regards the interpolation, the behaviour is intended to match the way
the AWB works (so that the two algorithms "agree" about exactly where on
the CT curve they are, rather than caring particularly about the reported
value). That said, the AWB might do better to work in mireds too, and then
this would be changed to match. It would make no difference to the actual
white point chosen (in terms of red/blue gains), but the colour temperature
number reported in between the calibrated locations might be less
approximate. I'll put it on the list of things to look at!

Thanks again and best regards

David

On Mon, 8 Jun 2020 at 07:35, Jack Hogan <jack.hogan77 at gmail.com> wrote:

> Thanks Laurent, I look forward to hearing their thoughts.
>
> Jack
>
> On Sun, Jun 7, 2020 at 3:01 AM Laurent Pinchart <
> laurent.pinchart at ideasonboard.com> wrote:
>
>> Hi Jack,
>>
>> On Sat, Jun 06, 2020 at 04:58:00PM +0200, Jack Hogan wrote:
>> > Hello,
>> >
>> > I am not a programmer but I was perusing the calculate_ccm module in
>> ccm.ccp
>> > and I noticed that the interpolation factor lambda is computed directly
>> off
>> > correlated color temperature values in deg K.  Normally this is done in
>> mired
>> > units, that is the interpolation factor is computed using one over each
>> of the
>> > cct's, see for instance the DNG SDK.  Is there a reason why it was not
>> done
>> > this way?
>> >
>> > Also the matrices in apply_saturation the coefficients look incorrect
>> (from the
>> > 601 standard instead of 709).
>> >
>> > Hope it helps.
>>
>> Thanks a lot for the feedback. I've added David and Naush in CC, as the
>> authors of the code I'm sure they know better than I do :-)
>>
>> --
>> Regards,
>>
>> Laurent Pinchart
>>
>
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