On Thu, Sep 19, 2019 at 04:06:16PM +0200, Rasmus Villemoes wrote: > The "break-even" point for the two formulas is L==8, which is also > what the code actually implements. [Incidentally, at that point one > has Y=0.008856, not 0.08856]. > > Moreover, all the sources I can find say the linear factor is 903.3 > rather than 902.3, which makes sense since then the formulas agree at > L==8, both yielding the 0.008856 figure to four significant digits.
Indeed. Interestingly the following doc (with a high search rank in Google) has exactly this inconsistency and uses different values at different times: http://www.photonstophotos.net/GeneralTopics/Exposure/Psychometric_Lightness_and_Gamma.htm > > Signed-off-by: Rasmus Villemoes <li...@rasmusvillemoes.dk> Reviewed-by: Daniel Thompson <daniel.thomp...@linaro.org> > --- > drivers/video/backlight/pwm_bl.c | 12 +++++++++--- > 1 file changed, 9 insertions(+), 3 deletions(-) > > diff --git a/drivers/video/backlight/pwm_bl.c > b/drivers/video/backlight/pwm_bl.c > index 2201b8c78641..be36be1cacb7 100644 > --- a/drivers/video/backlight/pwm_bl.c > +++ b/drivers/video/backlight/pwm_bl.c > @@ -155,8 +155,8 @@ static const struct backlight_ops pwm_backlight_ops = { > * > * The CIE 1931 lightness formula is what actually describes how we perceive > * light: > - * Y = (L* / 902.3) if L* ≤ 0.08856 > - * Y = ((L* + 16) / 116)^3 if L* > 0.08856 > + * Y = (L* / 903.3) if L* ≤ 8 > + * Y = ((L* + 16) / 116)^3 if L* > 8 > * > * Where Y is the luminance, the amount of light coming out of the screen, > and > * is a number between 0.0 and 1.0; and L* is the lightness, how bright a > human > @@ -169,9 +169,15 @@ static u64 cie1931(unsigned int lightness, unsigned int > scale) > { > u64 retval; > > + /* > + * @lightness is given as a number between 0 and 1, expressed > + * as a fixed-point number in scale @scale. Convert to a > + * percentage, still expressed as a fixed-point number, so the > + * above formulas can be applied. > + */ > lightness *= 100; > if (lightness <= (8 * scale)) { > - retval = DIV_ROUND_CLOSEST_ULL(lightness * 10, 9023); > + retval = DIV_ROUND_CLOSEST_ULL(lightness * 10, 9033); > } else { > retval = int_pow((lightness + (16 * scale)) / 116, 3); > retval = DIV_ROUND_CLOSEST_ULL(retval, (scale * scale)); > -- > 2.20.1 _______________________________________________ dri-devel mailing list dri-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/dri-devel
