A script which finds the minimum PNG dimensions for a given aspect ratio and pixel count, creates a colour-cycling PPM file, and then converts it to PNG.

ppm2png.sh

#!/usr/bin/env -S unshare --mount --map-root-user /bin/sh

# Licensed under the Zero-Clause BSD terms: https://opensource.org/license/0bsd
# Requires: imagemagick & [sisyphus](https://gist.github.com/Winterhuman/21d7b148db40ff041f397b07a7aafb83)
#
# Argument 1: Number of pixels to fill, required
# Argument 2: Height ratio, default 23
# Argument 3: Width ratio, default 5

set -Ceuf

pstat() {
	printf "%b\033[1m%b\033[0;39m" "${2:-\033[1m\033[34m::\033[0;39m }" "$1"
}

pquit() {
	pstat "$1" "\033[1m\033[31m!!\033[0;39m "
	exit 1
}


# Using '$RANDOM' is the only non-POSIX part of this script
_="${RANDOM:?$(pquit "The 'RANDOM' environment variable is not available!\n")}"


# Check if all arguments are valid
fill="${1:?$(pquit "No arguments given!\n")}"
if ! printf "%d" "$fill" >/dev/null 2>&1 || [ "$fill" -le 0 ]; then
	pquit "'$fill' is not a positive integer!\n"; fi

pstat "Fill: $fill\n"

hratio="${2:-23}"
wratio="${3:-5}"
if ! printf "%d" "$hratio" "$wratio" >/dev/null 2>&1 ||
	[ "$hratio" -le 0 ] || [ "$wratio" -le 0 ]; then
		pquit "Aspect ratio values must be positive integers!\n"; fi


# Calculate the optimal/minimal image dimensions for the given fill value
calc_w() {
	## '+ wratio - 1' yields a rounded up answer, instead of rounded down
	printf "%d\n" "$(( (("$1" * hratio) + wratio - 1 ) / wratio ))"
}

calc_h() {
	printf "%d\n" "$(( ( ("$1" * wratio) + hratio - 1 ) / hratio ))"
}

height=1
width="$(calc_w "$height")"
bad=0
## 'input + 1' is required to make this round up, since '$wratio' is in
## both the numerator and denominator (manual calculations verify this).
### '-1' is omitted from 'hratio + wratio' to handle '2:1' ratios.
good="$((
	( (fill + 1) * wratio + hratio + wratio ) /
	(hratio + wratio)
))"
good="$(( "$(calc_h "$good")" + 2 ))"

if [ "$good" -gt 1 ]; then
	while [ "$bad" -le "$(( good - 1 ))" ]; do
		if [ "$(( width * height ))" -le "$fill" ]; then
				bad="$(( height + 1 ))"
			else
				good="$height"
		fi

		height="$(( (bad + good) / 2 ))"
		width="$(calc_w "$height")"
	done
fi

height="$good"
width="$(calc_w "$height")"
blank="$(( ("$height" * "$width") - "$fill" ))"
groups="$(( "$fill" / 3 ))"
remainofgroups="$(( "$fill" % 3 ))"

pstat "Image dimensions: $width x $height\n"
pstat "Pixel fill: ${fill}px (${blank}px left)\n"


# Setup a private tmpfs for this script to use, which is what 'unshare' is for.
## 'nr_inodes' must be >= to 'max number of files + 1'
mount -t tmpfs -o nosuid,nodev,noexec,size=256M,nr_inodes=3 ppm2png /tmp ||
	pquit "Failed to overmount '/tmp'!\n"


# Write the P3 PPM header
ppm="/tmp/img.ppm"
printf "P3\n%d %d\n255\n" "$width" "$height" >"$ppm"


# Write the RGB IDs
## Many things worth noting here:
##
## 1. "A\0B\0C\n" is faster to write than "A\nB\nC\n" due to line buffering, so
##    avoiding them actually improves the speed even with `sed` afterwards.
## 2. `>>file` outside the while-loop is faster since it reuses the file's FD.
## 3. Using a background job for `printf` **drastically** slows things down,
##    because `$RANDOM` is evaluated before the background job starts.
count="$groups"
while [ "$count" -gt 0 ]; do
	printf "%d\0%d\0%d\n" \
		"$(( RANDOM % 6 + 1 ))" \
		"$(( RANDOM % 6 + 8 ))" \
		"$(( RANDOM % 6 + 15 ))"

	count="$(( count - 1 ))"
done >>"$ppm"

sed -i "s/\o0/\n/g" "$ppm" || pquit "Failed to replace null bytes in '$ppm'!\n"


# Write the remaining partial ID groups (1-2 or 1, as opposed to a full 1-2-3)
if [ "$remainofgroups" -eq 2 ]; then
		printf "%d\n%d\n" \
				"$(( "$RANDOM" % 6 + 1 ))" \
				"$(( "$RANDOM" % 6 + 8 ))" \
			>>"$ppm"
	elif [ "$remainofgroups" -eq 1 ]; then
		printf "%d\n" "$(( "$RANDOM" % 6 + 1 ))" >>"$ppm"
fi


# Replace the numeric IDs with the actual RGB colours.
sed -i  -e "s/^1$/0 255 170/" \
	-e "s/^2$/0 223 148/" \
	-e "s/^3$/0 191 127/" \
	-e "s/^4$/0 159 106/" \
	-e "s/^5$/0 127 85/" \
	-e "s/^6$/0 96 64/" \
	-e "s/^7$/0 64 42/" \
	-e "s/^8$/255 187 0/" \
	-e "s/^9$/223 164 0/" \
	-e "s/^10$/191 140 0/" \
	-e "s/^11$/159 117 0/" \
	-e "s/^12$/127 93 0/" \
	-e "s/^13$/96 70 0/" \
	-e "s/^14$/64 47 0/" \
	-e "s/^15$/255 76 136/" \
	-e "s/^16$/223 66 119/" \
	-e "s/^17$/191 57 102/" \
	-e "s/^18$/159 48 85/" \
	-e "s/^19$/127 38 68/" \
	-e "s/^20$/96 29 51/" \
	-e "s/^21$/64 19 34/" \
	"$ppm"


# Write the leftover blank lines
printf "%${blank}s" ' ' | sed "s/ /0 0 0\n/g" >>"$ppm"


# Convert PPM to PNG
tmp_img="/tmp/tmp.png"
magick "$ppm" -transparent "#000" "$tmp_img" ||
	pquit "Failed to create '$tmp_img'!\n"


# Move any existing output to a new '.old' path
mv_exist() {
	if [ -f "$1" ]; then
		mv "$1" "${1}.old" ||
			pquit "Failed to move existing '$1' image!\n"
	fi
}

mv_exist "header.png"
mv_exist "header.webp"


# Optimise PNG with 'sisyphus'
pstat "Optimising with Sisyphus...\n"

## '$tmp_img' exists outside Sisyphus's private tmpfs, meaning it can't see it.
## So, instead of copying '$tmp_img' somewhere, just give it a file descriptor.
exec 3<"$tmp_img"
sisyphus --all-oxi true /proc/self/fd/3 "header"