[libcamera-devel] Colour Matrix control proposal

[Laurent Pinchart]

Hi Dave,

On Thu, Jul 02, 2020 at 04:19:20PM +0100, David Plowman wrote:
> Hi everyone
> 
> One thing we've overlooked is the ability to return the colour matrix
> used by the pipeline. This is particularly important for people
> wanting to process raw files themselves, as we can then arrange for it
> to be written into DNG files. (You might allow the matrix to be
> written as well as returned; I think both are fine.)
> 
> I'd therefore like to propose a new control called ColourMatrix:

Bikeshedding on the name, this is usually referred to as CCM, Colour
Correction Matrix. Would ColourCorrectionMatrix make sense a a control
name, or do you think it's unnecessary long ?

> * It would hold an array of 9 floats representing a 3x3 matrix, listed
> in normal "reading order".
> 
> * It is the cameraRGB -> sRGB matrix used by the ISP (you can find
> them in Raspberry Pi camera tuning files under "rpi.ccm"). To be extra
> clear, this is the matrix normally applied *after* white balancing,
> and *before* the gamma transform.

By white balancing, I assume you mean the colour gains, as reported by
ColourGains ?

Looking at the IPA implementation, a CCM is picked from the
configuration file based on the colour temperature (with interpolation),
and the saturation is then applied to it. Do you think there would be a
use case for letting the application set the CCM directly ? Or maybe
provide a matrix to replace the Y2RGB * S * RGB2Y term in
apply_saturation(), keeping the base CCM interpolation in the IPA ?

> I'm hoping this is relatively uncontroversial (??) so I'm happy to
> create a patch. If people have other ideas, I'm of course delighted to
> discuss and re-think.

I think it's a good idea.

-- 
Regards,

Laurent Pinchart