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.
Signed-off-by: Rasmus Villemoes linux@rasmusvillemoes.dk --- 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));