linux-kernel - Re: [RFC v2 2/2] backlight: pwm_bl: compute brightness of LED linearly to human eye.

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Date:   Mon, 18 Dec 2017 13:33:45 +0100
From:   Pavel Machek <pavel@....cz>
To:     Enric Balletbo Serra <eballetbo@...il.com>
Cc:     Daniel Thompson <daniel.thompson@...aro.org>,
        Doug Anderson <dianders@...gle.com>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        Jingoo Han <jingoohan1@...il.com>,
        Richard Purdie <rpurdie@...ys.net>,
        Jacek Anaszewski <jacek.anaszewski@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Brian Norris <briannorris@...gle.com>,
        Guenter Roeck <groeck@...gle.com>,
        Lee Jones <lee.jones@...aro.org>,
        Alexandru Stan <amstan@...gle.com>, linux-leds@...r.kernel.org,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v2 2/2] backlight: pwm_bl: compute brightness of LED
 linearly to human eye.

On Mon 2017-12-18 11:40:59, Enric Balletbo Serra wrote:
> Hi Pavel,
> 
> 2017-12-15 21:57 GMT+01:00 Pavel Machek <pavel@....cz>:
> > Hi!
> >
> >> Yes, I think that how you describe luminance and lightness is right,
> >> and sounds good improve the doc.
> >>
> >> To be clear the correction table for PWM values can be calculated with
> >> this code.
> >>
> >> OUTPUT_SIZE = 65535      # Output integer size
> >> INPUT_SIZE = 2047
> >>
> >> def cie1931(L):
> >>     L = L*100.0
> >>     if L <= 8:
> >>         return (L/902.3)
> >>     else:
> >>         return ((L+16.0)/116.0)**3
> >>
> >> x = range(0,int(INPUT_SIZE+1))
> >> y = [int(round(cie1931(float(L)/INPUT_SIZE)*(OUTPUT_SIZE))) for L in x]
> >
> > Can we just generate the table on the fly? Should not be hard to do in
> > fixed point, right?
> 
> This was discussed a bit in previous RFC which had the code to
> generate the table on the fly, see [1]. The use of a fixed table or an
> on the fly table is something that I'll let the maintainers to decide.
> I've no strong opinion on use the on the fly table if someone takes
> care to review deeply the fixed point maths :)

You are free to pre-compute the table at boot. And you can even
compare the built-in and pre-computed table at boot, to make sure you
made no mistakes :-).
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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