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@xxxxxxxxxxxxxxxxxx>
---
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
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