fbtest.sh

#!/bin/ash

# This script will draw color gradient on the framebuffer
# http://seenaburns.com/2018/04/04/writing-to-the-framebuffer/
# fbset --show
# cat /sys/class/graphics/fb0/bits_per_pixel

fbdev=/dev/fb0
width=320
height=240
bpp=2
# rgb565
# color="\xe0\x07" green

pixel()
{
   local xx=$1 # [0, width-1]
   local yy=$2 # [0, height-1]
   local r=$3  # [0, 255]
   local g=$4  # [0, 255]
   local b=$5  # [0, 255]
   local r=$((r>>3))
   local g=$((g>>2))
   local b=$((b>>3))
   local r5g6b5=$(( (r<<11) | (g<<5) | (b) ))
   local hw=$(((r5g6b5>>8) & 0xFF))
   local lw=$((r5g6b5 & 0xFF))
   printf \\$(printf '%03o' $lw)\\$(printf '%03o' $hw) | \
      dd of=$fbdev bs=$bpp seek=$(($yy * $width + $xx)) &>/dev/null
}


for x in $(seq 0 1 $((width-1))); do
   for y in $(seq 0 1 $((height-1))); do
     r=$((255*x/width))
     g=$((255*y/height))
     b=0
     pixel $x $y $r $g $b
   done
done