On Wed, Oct 21, 2020 at 10:04:45PM -0700, Alexandru Stan wrote:
The previous behavior was a little unexpected, its properties/problems:
- It was designed to generate strictly increasing values (no repeats)
- 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.