Messages in this thread | | | From | Alexandru M Stan <> | Date | Mon, 2 Nov 2020 10:52:49 -0800 | Subject | Re: [PATCH v3 3/3] backlight: pwm_bl: Fix interpolation |
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On Wed, Oct 28, 2020 at 8:12 AM Daniel Thompson <daniel.thompson@linaro.org> wrote: > > On Wed, Oct 21, 2020 at 10:04:45PM -0700, Alexandru Stan wrote: > > The previous behavior was a little unexpected, its properties/problems: > > 1. It was designed to generate strictly increasing values (no repeats) > > 2. It had quantization errors when calculating step size. Resulting in > > unexpected jumps near the end of some segments. > > > > Example settings: > > brightness-levels = <0 1 2 4 8 16 32 64 128 256>; > > num-interpolated-steps = <16>; > > > > Whenever num-interpolated-steps was larger than the distance > > between 2 consecutive brightness levels the table would get really > > discontinuous. The slope of the interpolation would stick with > > integers only and if it was 0 the whole line segment would get skipped. > > > > The distances between 1 2 4 and 8 would be 1 (property #1 fighting us), > > and only starting with 16 it would start to interpolate properly. > > > > Property #1 is not enough. The goal here is more than just monotonically > > increasing. We should still care about the shape of the curve. Repeated > > points might be desired if we're in the part of the curve where we want > > to go slow (aka slope near 0). > > > > Problem #2 is plainly a bug. Imagine if the 64 entry was 63 instead, > > the calculated slope on the 32-63 segment will be almost half as it > > should be. > > > > The most expected and simplest algorithm for interpolation is linear > > interpolation, which would handle both problems. > > Let's just implement that! > > > > Take pairs of points from the brightness-levels array and linearly > > interpolate between them. On the X axis (what userspace sees) we'll > > now have equally sized intervals (num-interpolated-steps sized, > > as opposed to before where we were at the mercy of quantization). > > > > END > > INTERESTING. > > I guess this a copy 'n paste error from some internal log book? > Better removed... but I won't lose sleep over it.
Sorry! Yeah, I mistakenly duplicated the "END" line in patman.
> > > > Signed-off-by: Alexandru Stan <amstan@chromium.org> > > I've waited a bit to see how strong the feelings were w.r.t. getting rid > of the division from the table initialization. It was something I was > aware of during an earlier review but it was below my personal nitpicking > threshold (which could be badly calibrated... hence waiting). However > it's all been quiet so: > > Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org> > > > Daniel.
Alexandru Stan (amstan)
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