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...
Signed-off-by: Rasmus Villemoes linux@rasmusvillemoes.dk
Reviewed-by: Daniel Thompson daniel.thompson@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.*/
retval = DIV_ROUND_CLOSEST_ULL(lightness * 10, 9023);
retval = DIV_ROUND_CLOSEST_ULL(lightness * 10, 9033);-- 2.20.1