'xrandr' command (solved)

[jtp]

Set up: Mint 19 Cinnamon 32-bit
Problem: xrandr usage
Tried to change gamma with the following:

$ xrandr --output LVDS-1 --gamma 0.3:0.3:0.3
xrandr: Need crtc to set gamma on.

Q: What is 'crtc' and how to know it?

[thx-1138]

...are you certain your monitor is LVDS-1?
xrandr --listactivemonitors

[jtp]

You were right, the monitor was VGA-1:

0: +*VGA-1 1280/376x1024/301+0+0 VGA-1
1: +HDMI-1 1366/609x768/347+0+0 HDMI-1
sk@Petrovich-X200CA:~$ xrandr --output VGA-1 --gamma 0.3:0.3:0.3

xrandr has worked, i.e. changed the gamma but only for a second, after which the gamma has returned to the previous value I suppose because the contrast dropped again.

NB: The goal of the exercise is to get higher contrast which I need to compensate for my poor eyesight. NNB: I have set up "high contrast" mode in the Accessibility menu but this is not enough.

[thx-1138]

...do you by any chance use / run Redshift? Asking because from what i read, it appears to interfere with xrandr / xgamma...

...could you try with xcalib and / or xbacklight? apt install xcalib xbacklight
Sample commands...

xcalib -gammacor 2.2 -a

xbacklight -set 30

Note also that xbacklight appears to be somewhat broken...
=====================================

...If nothing of the above does the trick, then i'd try writing directly to ACPI's backlight control:
https://wiki.archlinux.org/index.php/backlight#ACPI

Additional example from my own system, say in order to compare with Arch wiki's example above:

ls /sys/class/backlight/
intel_backlight

ls /sys/class/backlight/intel_backlight
actual_brightness bl_power brightness device max_brightness power subsystem type uevent

cat /sys/class/backlight/intel_backlight/max_brightness
208

echo 60 | sudo tee /sys/class/backlight/intel_backlight/brightness

[jtp]

TY for the detailed answer.
1. Yes, I have pressed the 'Redshift' button without knowing what it actually does.
2. I've the used the 'xbacklight -set 30' and 'xcalib' as prescribed with the result of an Error:

:~$ xcalib -gammacor 2.2 -a
Error - unsupported ramp size 0

Please advise further steps if any.
Can the Redshift be undone?

[jtp]

After I've stopped Redshift the problem was resolved.
TY again for the tip.