Hi, On Thu, Nov 30, 2017 at 3:27 AM, Daniel Thompson <daniel.thompson@xxxxxxxxxx> wrote: > > > On 30/11/17 00:44, Doug Anderson wrote: >> >> Hi, >> >> On Thu, Nov 16, 2017 at 6:11 AM, Enric Balletbo i Serra >> <enric.balletbo@xxxxxxxxxxxxx> wrote: >>> >>> When you want to change the brightness using a PWM signal, one thing you >>> need to consider is how human perceive the brightness. Human perceive the >>> brightness change non-linearly, we have better sensitivity at low >>> luminance than high luminance, so to achieve perceived linear dimming, >>> the >>> brightness must be matches to the way our eyes behave. The CIE 1931 >>> lightness formula is what actually describes how we perceive light. >>> >>> This patch adds support to compute the brightness levels based on a >>> static >>> table filled with the numbers provided by the CIE 1931 algorithm, for now >>> it only supports PWM resolutions up to 65535 (16 bits) with 1024 steps. >>> Lower PWM resolutions are implemented using the same curve but with less >>> steps, e.g. For a PWM resolution of 256 (8 bits) we have 37 steps. >> >> >> Your patch assumes that the input to your formula (luminance, I think) >> scales linearly with PWM duty cycle. I don't personally know this, >> but has anyone confirmed it's common in reality, or at least is a >> close enough approximation of reality? > > > Isn't this the loop we went round for v1? > > We do know that its not linear, however the graphs from a couple of example > devices didn't look too scary and nobody has proposed a better formula. > > At this point the linear interpolation code in patch 1 allows people with > especially alinear devices to express suitable brightness curves. > > However we also know that many DT authors choose not to create good > brightness tables for their devices... and we'd rather they used allowed the > kernel to choose a model than to use no model at all. OK, cool. I didn't remember anyone actually confirming that they had checked that this was the case, but that's probably just my bad memory and failures at searching through history. I don't have any objections to the idea if people are convinced it's a good enough approximation. :) It would be kinda nice if something could go in the commit message, like: This method will work in any cases where linearly scaling the PWM duty cycle causes a roughly linear scaling of the luminance of the backlight. :) -Doug -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
