That screen really is too bright sometimes

Since we have chronic light sensitivity as part of our ME/CFS, I decided to investigate whether there is any software way to make the screen of our phone running SailfishOS consistently darker without disabling automatic brightness control.

Unfortunately the minimum allowed by the “Brightness Base Level” slider under Settings → Look and Feel → Display is still someway too high after all, so there is no user-friendly way to lower it further, but there did turn out to be a hackey way to force a lower brightness response by editing system files which in turn requires enabling root access if you don’t already have that…

Changing the system brightness profile

Note that some of these steps may need to be repeated after a system upgrade.

Getting root access

  1. Enable “Developer Mode” under Settings → System → Developer tools
  2. Set an SSH password, this will be also your root password
  3. Either:
  4. Also enable SSH by checking “Remote connection”, then running ssh defaultuser@<ADDRESS> (where <ADDRESS> is the “WLAN IP address” listed) on your computer’s terminal window and entering the set password (recommended if you have SSH on a computer as using the “Terminal” app is pretty annoying)
  5. Close settings and launch the “Terminal” from the app drawer
  6. Enter devel-su on the computer with SSH logged in or in the “Terminal” app
  7. Type in the password set in step 2
  8. Make sure nano is installed by typing in pkcon install nano and pressing enter

There is also an official help guide with pictures for this at https://docs.sailfishos.org/Support/Help_Articles/Enabling_Developer_Mode/

Replacing brightness profiles

System brightness profiles are stored at /etc/mce/20als-defaults.ini on the device (ALS = ambient light sensor). Since you probably already selected the lowest profile in system settings and its still too bright, we have to update the Ambient Light level to screen brightness mapping to be even less sensitive.

  1. Run nano /etc/mce/20als-defaults.ini to edit the MCE Ambient Light Sensor (ALS) preset/defaults file.
  2. Use the arrow keys to navigate to the end of the [BrightnessDisplay] section (just above [BrightnessLed]).
  3. Remove all the existing lines starting with LimitsProfile… and LevelsProfile… by going to the start of the line and repeatedly pressing Ctrl+K (cut line)

  4. Copy and paste the following lines containing the updated profiles:

    LimitsProfile0=1;2;4;6;11;19;34;61;109;195;350;511;608;724;862;1026;1221;1454;1731;2060;2453
LevelsProfile0=1;1;1;1;1;1;1;1;1;1;1;10;20;30;40;50;60;70;80;90;100

LimitsProfile1=1;2;4;6;11;19;34;61;109;195;350;490;584;695;827;985;1172;1396;1662;1978;2355
LevelsProfile1=1;1;1;1;1;2;2;2;3;3;4;14;24;33;43;52;62;72;81;91;100

LimitsProfile2=1;2;4;6;11;19;34;61;109;195;350;471;560;667;794;945;1125;1340;1595;1899;2261
LevelsProfile2=1;1;2;2;3;4;5;5;6;7;8;18;27;36;45;54;64;73;82;91;100

LimitsProfile3=1;2;4;6;11;19;34;61;109;195;350;452;538;640;762;908;1080;1286;1531;1823;2170
LevelsProfile3=1;2;3;4;5;6;7;8;9;10;12;21;30;39;48;56;65;74;83;92;100

LimitsProfile4=1;2;4;6;11;19;34;61;109;195;350;434;516;615;732;871;1037;1235;1470;1750;2083
LevelsProfile4=1;2;4;5;7;8;10;11;13;14;16;25;33;42;50;58;67;75;84;92;100

LimitsProfile5=1;2;4;6;11;19;34;61;109;195;350;416;496;590;703;836;996;1185;1411;1680;2000
LevelsProfile5=1;3;5;7;9;10;12;14;16;18;20;28;36;44;52;60;68;76;84;92;100

LimitsProfile6=1;2;4;6;11;19;34;61;109;195;350;400;476;567;674;803;956;1138;1355;1613;1920
LevelsProfile6=2;4;6;8;10;13;15;17;19;21;24;32;40;47;55;62;70;78;85;93;100

LimitsProfile7=1;2;4;6;11;19;34;61;109;195;350;384;457;544;648;771;918;1092;1301;1548;1843
LevelsProfile7=2;5;7;10;12;15;17;20;22;25;28;36;43;50;57;64;72;79;86;93;100

LimitsProfile8=1;2;4;6;11;19;34;61;109;195;350;368;439;522;622;740;881;1049;1249;1486;1769
LevelsProfile8=2;5;8;11;14;17;20;23;26;29;32;39;46;53;60;66;73;80;87;94;100

LimitsProfile9=1;2;4;6;11;19;34;61;109;195;350;354;421;501;597;710;846;1007;1199;1427;1699
LevelsProfile9=3;6;9;13;16;19;22;26;29;32;36;43;49;56;62;68;75;81;88;94;100

LimitsProfile10=1;2;4;6;11;19;34;61;109;195;350;340;404;481;573;682;812;967;1151;1370;1631
LevelsProfile10=3;7;10;14;18;21;25;29;32;36;40;46;52;58;64;70;76;82;88;94;100

LimitsProfile11=1;2;4;6;11;19;34;61;109;195;350;326;388;462;550;655;779;928;1105;1315;1566
LevelsProfile11=4;8;12;16;20;24;28;32;36;40;44;50;56;61;67;72;78;84;89;95;100

LimitsProfile12=1;2;4;6;11;19;34;61;109;195;350;313;373;443;528;629;748;891;1060;1262;1503
LevelsProfile12=4;8;13;17;21;26;30;34;39;43;48;54;59;64;69;74;80;85;90;95;100

LimitsProfile13=1;2;4;6;11;19;34;61;109;195;350;300;358;426;507;603;718;855;1018;1212;1443
LevelsProfile13=4;9;14;18;23;28;33;37;42;47;52;57;62;67;72;76;81;86;91;96;100

LimitsProfile14=1;2;4;6;11;19;34;61;109;195;350;288;343;409;487;579;690;821;977;1163;1385
LevelsProfile14=5;10;15;20;25;30;35;40;45;50;56;61;65;70;74;78;83;87;92;96;100

LimitsProfile15=1;2;4;6;11;19;34;61;109;195;350;277;330;392;467;556;662;788;938;1117;1330
LevelsProfile15=5;10;16;21;27;32;38;43;49;54;59;64;68;72;76;80;84;88;92;96;100

LimitsProfile16=1;2;4;6;11;19;34;61;109;195;350;266;316;377;448;534;636;757;901;1072;1276
LevelsProfile16=5;11;17;23;29;34;40;46;52;58;64;68;72;75;79;82;86;90;93;97;100

LimitsProfile17=1;2;4;6;11;19;34;61;109;195;350;255;304;362;430;512;610;726;865;1029;1225
LevelsProfile17=6;12;18;24;30;37;43;49;55;61;68;72;75;78;81;84;88;91;94;97;100

LimitsProfile18=1;2;4;6;11;19;34;61;109;195;350;245;292;347;413;492;586;697;830;988;1176
LevelsProfile18=6;13;19;26;32;39;45;52;58;65;72;75;78;81;84;86;89;92;95;98;100

LimitsProfile19=1;2;4;6;11;19;34;61;109;195;350;235;280;333;397;472;562;669;797;949;1129
LevelsProfile19=6;13;20;27;34;41;48;55;62;69;76;79;81;84;86;88;91;93;96;98;100

LimitsProfile20=1;2;4;6;11;19;34;61;109;195;350;226;269;320;381;453;540;643;765;911;1084
LevelsProfile20=7;14;21;29;36;43;50;58;65;72;80;82;84;86;88;90;92;94;96;98;100

  5. Press Ctrl+X, then Y, then Enter to save and close the editor.

  6. Enter systemctl restart mce or just restart your device to load the new values.

This replaces the default profiles with ones that starting from about the same maximum brightness curves will go continously lower with all settings below about 24% in setting selecting curves that were previously unreachable.

Background

How I found about this

TL;DR: Some educated guesswork, intimiate Linux system knowledge and a little help from the MCE source code.

To find out which SailfishOS system component was responsible for managing the backlight, I knew I had find which program writes to /sys/class/backlight/<DEVNAME> (the Linux kernel interface for controlling backlight devices). So after looking up how to find our which process writes to a file, I compiled fanotify-example on the device using gcc (GNU C Compiler), then ran it as ./fanotify-example /sys/class/backlight/panel0-backlight. This revealed that the brightness value of the main panel is controlled by /usr/sbin/mce (MCE).

Knowing that it is customary for Linux system services to have configuration files in /etc, I then checked the directory /etc/mce and found a file named 20als-defaults.ini in there, which I correctly guessed would stand for Ambient Light Sensor defaults.

Given that that file is both documented and rather obvious I than started looking for a way to select a different display backlight profile. For this I played around a bit with the mce command and found a debug logging switch that revealed that there are apparently some “GConf” values influencing its operation in addition to the files at /etc/mce and that it reads the file /var/lib/mce/builtin-gconf.values to find these values. The name GConf indeed turned out related to the old GNOME 2.x configuration system, but GConf isn’t actually used anymore. It’s just that MCE contains a GConf compatible configuration parser, which made assume that nothing in the system apparently ever writes that format anymore – this turned out to be a wrong assumption as that file is actually rewritten by system settings when you update any system-wide values.

Since I assumed that that file is shipped by SailfishOS during installation, I looked for information on what it does and learned from the manpage at https://github.com/sailfishos/mce/blob/91198feec76f1069ed7b38f519e9a0f7852ed358/man/mce.8#L89 (that isn’t actually shipped in SailfishOS itself) that the “GConf key” /system/osso/dsm/display/display_brightness controls the current brightness profile used, which let me to believe I just need to change that value to use a different brightness curve – while one can indeed change that file to load a different curve, one can also just do so in system settings.

Finally, I had to figure out how to generate dimmer curves since even the default 0 curve is still too bright. This involved playing around with the existing values in a Python shell and trying different binary operator on them to hopefully try and spot the pattern. Plotting the existing curves was quite helpful for this once I decided to finally do that, since you can see discontinuities, trends and repetitions much better than when just starting at the numbers.

How I generated the extra curve values

Based on analysis I wrote the following Python code to generate some additional dimmer curves, but latter realized that it was impossible to load them:

limprf = lambda max: f"1;2;4;6;11;19;34;61;109;195;350;{';'.join(str(round(max * 0.84**p)) for p in range(9, -1, -1))}"
lvlprf = lambda mid: f"{';'.join(str(max(floor(mid / 11 * i), 1)) for i in range(1, 11 + 1))};{';'.join(str(ceil(((100 - mid) / 10 * i) + mid)) for i in range(1, 10 + 1))}"
def gen_curves(n):
    for i in range(1, n + 1):
        print()
        print(f"LimitsProfile{20 + i}={limprf(2000 / (0.966 ** i))}")
        print(f"LevelsProfile{20 + i}={lvlprf(20 - (3 * i))}")
gen_curves(9)

Once they didn’t load, I begrudinginly dug through the available source code and found that the system is actually limited to 21 curves (0–20), so instead I generated some modified curves (as showcased in the main part of the article) that start lower and end at the same brightness level:

limprf = lambda max: f"1;2;4;6;11;19;34;61;109;195;350;{';'.join(str(round(max * 0.84**p)) for p in range(9, -1, -1))}"
lvlprf = lambda mid: f"{';'.join(str(max(floor(mid / 11 * i), 1)) for i in range(1, 11 + 1))};{';'.join(str(ceil(((100 - mid) / 10 * i) + mid)) for i in range(1, 10 + 1))}"
for i in range(0, 20 + 1):
    print()
    print(f"LimitsProfile{i}={limprf(2000 * (0.96 ** (i - 5)))}")
    print(f"LevelsProfile{i}={lvlprf(20 + (4 * (i - 5)))}")

In any case, these just extrapolates the same trends found in the pre-existing curves, so I cannot really explain why exactly those values, but apparently they work. Oh well. 🤷🏽