Re: [PATCH v2 1/3] backlight: pwm_bl: Fix interpolation
From: Geert Uytterhoeven <geert@linux-m68k.org>
Date: 2020-10-15 06:55:12
Also in:
dri-devel, linux-pwm, lkml
Hi Alexandru, On Tue, Oct 13, 2020 at 1:57 PM Alexandru Stan [off-list ref] wrote:
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.
Example settings:
brightness-levels = <0 1 2 4 8 16 32 64 128 256>;
num-interpolated-steps = <16>;
The distances between 1 2 4 and 8 would be 1, and only starting with 16
it would start to interpolate properly.
Let's change it so there's always interpolation happening, even if
there's no enough points available (read: values in the table would
appear more than once). This should match the expected behavior much
more closely.
Signed-off-by: Alexandru Stan <redacted>Thanks for your patch!
quoted hunk ↗ jump to hunk
--- a/drivers/video/backlight/pwm_bl.c +++ b/drivers/video/backlight/pwm_bl.c@@ -327,24 +324,25 @@ static int pwm_backlight_parse_dt(struct device *dev, table = devm_kzalloc(dev, size, GFP_KERNEL); if (!table) return -ENOMEM; - - /* Fill the interpolated table. */ - levels_count = 0; - for (i = 0; i < data->max_brightness - 1; i++) { - value = data->levels[i]; - n = (data->levels[i + 1] - value) / num_steps; - if (n > 0) { - for (j = 0; j < num_steps; j++) { - table[levels_count] = value; - value += n; - levels_count++; - } - } else { - table[levels_count] = data->levels[i]; - levels_count++; + /* + * Fill the interpolated table[x] = y + * by draw lines between each (x1, y1) to (x2, y2). + */ + dx = num_steps; + for (i = 0; i < num_input_levels - 1; i++) { + x1 = i * dx; + x2 = x1 + dx; + y1 = data->levels[i]; + y2 = data->levels[i + 1]; + dy = (s64)y2 - y1; + + for (x = x1; x < x2; x++) { + table[x] = y1 + + div_s64(dy * ((s64)x - x1), dx);
Yummy, 64-by-32 divisions. Shouldn't this use a rounded division? Nevertheless, I think it would be worthwhile to implement this using a (modified) Bresenham algorithm, avoiding multiplications and divisions, and possibly increasing accuracy as well. https://en.wikipedia.org/wiki/Bresenham%27s_line_algorithm
}
}
- table[levels_count] = data->levels[i];
+ /* Fill in the last point, since no line starts here. */
+ table[x2] = y2;
/*
* As we use interpolation lets remove current
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds