[libcamera-devel] [PATCH 3/4] libipa: camera_sensor_helper: Add IMX290 helper

[Laurent Pinchart]

Hi Jacopo,

On Mon, Mar 28, 2022 at 03:52:18PM +0200, Jacopo Mondi wrote:
> On Mon, Mar 28, 2022 at 03:03:35PM +0300, Laurent Pinchart via libcamera-devel wrote:
> > The IMX290 is a Sony sensor that expresses its gain in 0.3dB units. It
> > thus maps to the exponential gain model.
> >
> > Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> > ---
> >  src/ipa/libipa/camera_sensor_helper.cpp | 31 +++++++++++++++++++++++++
> >  1 file changed, 31 insertions(+)
> >
> > diff --git a/src/ipa/libipa/camera_sensor_helper.cpp b/src/ipa/libipa/camera_sensor_helper.cpp
> > index 7bb999e19102..136b9f6bc3c5 100644
> > --- a/src/ipa/libipa/camera_sensor_helper.cpp
> > +++ b/src/ipa/libipa/camera_sensor_helper.cpp
> > @@ -332,6 +332,26 @@ std::vector<CameraSensorHelperFactory *> &CameraSensorHelperFactory::factories()
> >
> >  #ifndef __DOXYGEN__
> >
> > +/*
> > + * Helper function to compute the m parameter of the exponential gain model
> > + * when the gain code is expressed in dB.
> > + */
> > +static constexpr double expGainDb(double step)
> > +{
> > +	constexpr double log2_10 = 3.321928094887362;
> > +
> > +	/*
> > +	 * The gain code is expressed in step * dB (e.g. in 0.1 dB steps):
> > +	 *
> > +	 * G_code = G_dB/step = 20/step*log10(G_linear)
> 
> Why was I expecting
>                                 10/step*log10(G_linear)
> 
> As my understanding is that
> 
>                 G_db = 10*log10(G_linear) ?

The joys of dB as a power ratio or an amplitude ratio :-) Here the
linear gain is an amplitude gain, so G_dB = 20*log10(G_linear). Quoting
https://en.wikipedia.org/wiki/Decibel,

"Two principal types of scaling of the decibel are in common use. When
expressing a power ratio, it is defined as ten times the logarithm in
base 10.[5] That is, a change in power by a factor of 10 corresponds to
a 10 dB change in level. When expressing root-power quantities, a change
in amplitude by a factor of 10 corresponds to a 20 dB change in level.
The decibel scales differ by a factor of two, so that the related power
and root-power levels change by the same value in linear systems, where
power is proportional to the square of amplitude."

A 20dB increment will multiple the amplitude by 10 and the power by 100.

> (nit: isn't it more readable with spaces between operands ?)

I can add spaces. The line below may get long though:

	 * G_linear = 10 ^ (step / 20 * G_code) = 2 ^ (log2(10) * step / 20 * G_code)

What do you prefer ? I can also switch to Latex notation:

	 * G_{linear} = 10^{frac{step*G_{code}}{20}} = 2^{log_{2}(10) * frac{step*G_{code}}{20}}

but that's likely not better :-)

> > +	 *
> > +	 * Inverting the formula, we get
> > +	 *
> > +	 * G_linear = 10^(step/20*G_code) = 2^(log2(10)*step/20*G_code)
> > +	 */
> > +	return log2_10 * step / 20;
> > +}
> > +
> >  class CameraSensorHelperImx219 : public CameraSensorHelper
> >  {
> >  public:
> > @@ -354,6 +374,17 @@ public:
> >  };
> >  REGISTER_CAMERA_SENSOR_HELPER("imx258", CameraSensorHelperImx258)
> >
> > +class CameraSensorHelperImx290 : public CameraSensorHelper
> > +{
> > +public:
> > +	CameraSensorHelperImx290()
> > +	{
> > +		gainType_ = AnalogueGainExponential;
> > +		gainConstants_.exp = { 1.0, expGainDb(0.3) };
> > +	}
> > +};
> > +REGISTER_CAMERA_SENSOR_HELPER("imx290", CameraSensorHelperImx290)
> > +
> >  class CameraSensorHelperOv2740 : public CameraSensorHelper
> >  {
> >  public:

-- 
Regards,

Laurent Pinchart