mjhjmtu

I am not sure if it is the only possible way, but
I read that in order to put a single pixel onto the screen at a location of your choice one has to write something into a place called framebuffer.
So I became curious, if it is possible to enter into this place and write something into it in order to display a single pixel somewhere on the screen.

yes, outside X-server, in tty, try command:

cat /dev/urandom >/dev/fb0

if colourfull pixels fills the screen, then your setup is ok, and you can try playing with this small script:

#!/usr/bin/env bash

fbdev=/dev/fb0 ; width=1280 ; bpp=4
color="x00x00xFFx00" #red colored

function pixel()
 xx=$1 ; yy=$2
 printf "$color" 
x=0 ; y=0 ; clear
for i in 1..500; do
 pixel $((x++)) $((y++))
done

where function 'pixel' should be an answer... write a pixel to screen by changing byte values (blue-green-red-alpha) on x-y offset of device /dev/fbX which is frame buffer for the video-card.

or try one liner pixel draw (yellow on x:y=200:100, if width is 1024):

printf "x00xFFxFFx00" | dd bs=4 seek=$((100 * 1024 + 200)) >/dev/fb0

UPDATE: this code works even inside X-server, if we just configure X to use frame buffer. by specifying fb0 inside /usr/share/X11/xorg.conf.d/99-fbdev.conf

I just posted this this morning, still investigating why it only works on Raspberry Pis. https://www.raspberrypi.org/forums/viewtopic.php?f=72&t=213964&p=1428891#p1428891

Open /dev/fb0, mmap it so you get a pointer, and it's much faster. Doesn't use X at all but it will happily ignore X, it's just something on the screen.

Oh, from a command line, sort of, you can write to /dev/fb0. But whatever you write at offset 0 will be in the upper left corner so it will immediately scroll off the screen. You could do a for loop in Bash and write a couple thousand times. Or use /dev/urandom. Destroying what's in the screenbuffer, especially while you're in X, is no big deal. As soon as you drag a window over the area X causes an expose event and repaints it. You don't need to kill the power to recover.