[libcamera-devel] [PATCH 10/12] ipa: ipu3: awb: Introduce Black Level Correction

[Laurent Pinchart]

On Tue, Sep 28, 2021 at 05:10:56PM +0100, Kieran Bingham wrote:
> On Thu, Sep 23, 2021 at 10:16:23AM +0200, Jean-Michel Hautbois wrote:
> > The pixels output by the camera normally include a black level, because
> > sensors do not always report a signal level of '0' for black. Pixels at
> > or below this level should be considered black and to achieve that, we
> > need to substract an offset to all the pixels. This can be taken into
> > account by reading the lowest value of a special region on sensors which
> > is not exposed to the lens. This provides a substracting factor to be

s/the lens/light/

> > able to adjust the expected black levels in the resultant images.
> > 
> > For a camera outputting 10-bit pixel values (in the range 0 to 1023) a
> > typical black level might be 64. It is a fixed value, obtained by
> > capturing a raw frame with minimum exposure and gain fixed to 1.0 while
> > covering the sensor (the darker the better). We consider it good enough
> > as a very first approximation, until we measure it during a tuning
> > process and include it in a configuration file
> > 
> > Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois at ideasonboard.com>
> > ---
> >  src/ipa/ipu3/algorithms/awb.cpp | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/src/ipa/ipu3/algorithms/awb.cpp b/src/ipa/ipu3/algorithms/awb.cpp
> > index a5391653..3013870b 100644
> > --- a/src/ipa/ipu3/algorithms/awb.cpp
> > +++ b/src/ipa/ipu3/algorithms/awb.cpp
> > @@ -388,9 +388,17 @@ void Awb::prepare(IPAContext &context, ipu3_uapi_params *params)
> >  	/* The CCM matrix may change when color temperature will be used */
> >  	params->acc_param.ccm = imguCssCcmDefault;
> >  
> > +	/* The Optical Black Level correction values */
> 
> I think I'd like to see a clear todo highlighting that these values are
> ad-hoc:
> 
> 	/*
> 	 * \todo The correction values should come from sensor specific
> 	 * tuning processes. This is a first approximation
> 	 */

Indeed, this is really a hack, and should be documented as such.

> > +	params->obgrid_param.gr = 64;
> > +	params->obgrid_param.r = 64;
> > +	params->obgrid_param.b = 64;
> > +	params->obgrid_param.gb = 64;
> > +
> >  	params->use.acc_awb = 1;
> >  	params->use.acc_bnr = 1;
> >  	params->use.acc_ccm = 1;
> 
> I'd be tempted by a blank line here to split out the obgrid to make it
> clear that it's a separate component.

Why do we set the optical black level compensation parameters in AWB
though ? Shouldn't it be a separate algorithm ?

> With those:
> 
> Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
> 
> > +	params->use.obgrid = 1;
> > +	params->use.obgrid_param = 1;
> >  }
> >  
> >  } /* namespace ipa::ipu3::algorithms */

-- 
Regards,

Laurent Pinchart