18

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.

19

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="\x00\x00\xFF\x00" #red colored

function pixel()
{  xx=$1 ; yy=$2
   printf "$color" | dd bs=$bpp seek=$(($yy * $width + $xx)) \
                        of=$fbdev &>/dev/null
}
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 "\x00\xFF\xFF\x00" | 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

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  • There's a couple of dangerous things going on here: the first example appears to write random bytes to a floppy disk, for some reason. The follow up commands use dd which has often been called "Disk Destroy" for specific reasons ... don't go near these commands unless you know what you're doing ... – robert Mar 27 '15 at 8:31
  • 4
    @robert I think Omar meant /dev/fbX and the /dev/fd was just a typo. And yes, dd is dangerous but so is rm. That doesn't mean it shouldn't be used. It just means that it should be used with care. – terdon Mar 27 '15 at 11:35
  • ah /dev/fb0 makes more sense! Everybody knows what rm means, but dd is a little more obscure, still think it should carry a health warning. – robert Mar 27 '15 at 11:37
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    "yes, outside X-server, in tty, try command:" I do not understand if I got it right, so I tried just opening the terminal and writing "cat /dev/urandom > /dev/fd0" but I only got an error message : "cat: write error: no space left on device". I really do not know how to get out of the xserver. – Abdul Al Hazred Mar 27 '15 at 15:34
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    ... i called it tty, but it is virtual console, non-gui thing, terminal over all screen, that you reach with ctrl-alt-f1,2,3... or "sudo chvt 1" ... 'no space left' seems like it will be ok, just you are still in X-session. – Asain Kujovic Mar 27 '15 at 17:03
0

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.

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