Messages in this thread | | | Date | Wed, 28 Oct 2020 15:12:10 +0000 | From | Daniel Thompson <> | Subject | Re: [PATCH v3 3/3] backlight: pwm_bl: Fix interpolation |
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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.
> 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.
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