libcamera: software_isp: Apply black level compensation

Black may not be represented as 0 pixel value for given hardware, it may
be higher.  If this is not compensated then various problems may occur
such as low contrast or suboptimal exposure.

The black pixel value can be either retrieved from a tuning file for the
given hardware, or automatically on the fly.  The former is the right
and correct method, while the latter can be used when a tuning file is
not available for the given hardware.  Since there is currently no
support for tuning files in software ISP, the automatic, hardware
independent way, is always used.  Support for tuning files should be
added in future but it will require more work than this patch.

The patch looks at the image histogram and assumes that black starts
when pixel values start occurring on the left.  A certain amount of the
darkest pixels is ignored; it doesn't matter whether they represent
various kinds of noise or are real, they are better to omit in any case
to make the image looking better.  It also doesn't matter whether the
darkest pixels occur around the supposed black level or are spread
between 0 and the black level, the difference is not important.

An arbitrary threshold of 2% darkest pixels is applied; there is no
magic about that value.

The patch assumes that the black values for different colors are the
same and doesn't attempt any other non-primitive enhancements.  It
cannot completely replace tuning files and simplicity, while providing
visible benefit, is its goal.  Anything more sophisticated is left for
future patches.

A possible cheap enhancement, if needed, could be setting exposure +
gain to minimum values temporarily, before setting the black level.  In
theory, the black level should be fixed but it may not be reached in all
images.  For this reason, the patch updates black level only if the
observed value is lower than the current one; it should be never
increased.

The purpose of the patch is to compensate for hardware properties.
General image contrast enhancements are out of scope of this patch.

Stats are still gathered as an uncorrected histogram, to avoid any
confusion and to represent the raw image data.  Exposure must be
determined after the black level correction -- it has no influence on
the sub-black area and must be correct after applying the black level
correction.  The granularity of the histogram is increased from 16 to 64
to provide a better precision (there is no theory behind either of those
numbers).

Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Milan Zamazal <mzamazal@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Milan Zamazal <mzamazal@redhat.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

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