Sync intel_backlight to acpi_video0 brightness using gamma.

sync-acpi-intel.sh

#!/bin/sh
# Copyright (c) 2013, Ted Ying
# Permission is hereby granted, without written agreement and without
# license or royalty fees, to use, copy, modify, and distribute this
# software and its documentation for any purpose, provided that the
# above copyright notice and the following two paragraphs appear in
# all copies of this software.
#
# IN NO EVENT SHALL THE COPYRIGHT HOLDER BE LIABLE TO ANY PARTY FOR
# DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES
# ARISING OUT OF THE USE OF THIS SOFTWARE AND ITS DOCUMENTATION, EVEN
# IF THE COPYRIGHT HOLDER HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH
# DAMAGE.
#
# THE COPYRIGHT HOLDER SPECIFICALLY DISCLAIMS ANY WARRANTIES, INCLUDING,
# BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
# FITNESS FOR A PARTICULAR PURPOSE.  THE SOFTWARE PROVIDED HEREUNDER IS
# ON AN "AS IS" BASIS, AND THE COPYRIGHT HOLDER HAS NO OBLIGATION TO
# PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.

backlight_dir=/sys/class/backlight
src_dir=${backlight_dir}/acpi_video0
dst_dir=${backlight_dir}/intel_backlight
gamma=2.5

read src_max < "$src_dir/max_brightness"
read dst_max < "$dst_dir/max_brightness"

update(){
	read brightness < "$src_dir/brightness"
	[ "$brightness" != "$prev_brighness" ] && echo "$brightness" || prev_brightness="$brightness"
}
{
	update
	while inotifywait -qqe modify -t5 "$src_dir/brightness" || :
	do
		update
	done
} | awk "{printf\"%d\\n\",.5+$dst_max*(\$1/$src_max)^$gamma;fflush()}" > "$dst_dir/brightness"