POSIX sh script to create AVIF for target SSIM value using binary search, requires `imagemagick` and `cavif`.

avif.sh

#!/bin/sh

# Licensed under the Zero-Clause BSD terms: https://opensource.org/license/0bsd
# Requires `imagemagick` and `cavif`.

## Arguments
# 1: /path/to/input
# 2: /path/to/output (optional, default output: 'input_no_ext'-'quality'.avif)
# 3: Target SSIM value (default value: 96%)
#	Since the SSIM score is output as a 6 decimal place number (e.g. 0.960000),
#	the value given will be converted into the closest allowed value as shown:
#		96 = 96%	(all values can be prefixed with '0.')
#		855 = 85.5%
#		123456 = 12.3456%
#		5 = 50%
#		1000000 (100%) = 10%

pquit() { printf "\033[1m\033[31m%b\033[0;39m" "$1"; exit 1; }
pstat() { printf "\033[1m\033[34m%b\033[0;39m\033[1m%b\033[0;39m\n" "$1" "$2"; }
clean() { if ! rm -r "$tmp"; then pquit "Failed to delete '$tmp'!\n"; fi }
trap clean EXIT
tmp="$(mktemp -d)"

if [ -z "$1" ]; then pquit "No input image given!\n"; fi
if [ ! -f "$1" ]; then pquit "'$1' does not exist, or isn't a file!\n"; fi

input="$1"
pstat "Input: " "$input"

# `cavif` only supports JPG & PNG for its input, so convert all other image
# formats to PNG.
tmp_input="${tmp}/input.png"
mime="$(file -ib "$input")"

if [ "${mime%;*}" = "image/png" ] || [ "${mime%;*}" = "image/jpeg" ]; then
		if ! cp "$input" "$tmp_input"; then
			pquit "Failed to copy '$input' to '$tmp_input'!\n"; fi
	else
		if ! convert "$input" "$tmp_input"; then
			pquit "Failed to convert '$input' to PNG!\n"; fi
fi

fancy_percent() {
	fancy_percent_digit="$(printf "%d" "$1" | cut -c -2)"
	fancy_percent_decimal="$(printf "%s" "$1" | cut -c 3-)"
	printf "%s.%s%%" "${fancy_percent_digit}" "${fancy_percent_decimal}"
}

ideal="${3:-0.96}"
ideal="$(printf "%d0000" "${ideal#*.}" | cut -c -6)"
pstat "Target: " "~$(fancy_percent "$ideal")"

# Only complain about an existing output if it was explicitly given.
if [ -n "$2" ]; then
		final_output="$2"
		pstat "Output: " "$final_output\n"
		if [ -e "$final_output" ]; then
			pquit "'$final_output' already exists!\n"; fi
	else
		printf "\n"
fi

bad_quality="1"
quality="50"
break_quality="99"
upper_quality="$break_quality"

# Use binary search to find the lowest quality image which meets or exceeds the
# target SSIM score.
while [ "$bad_quality" -le "$(( "$upper_quality" - 1 ))" ]; do
	trial="${tmp}/${quality}-trial.avif"
	pstat "Quality: " "$quality"

	if ! cavif --quiet --overwrite --threads "$(nproc)" --speed 1 --quality "$quality" "$tmp_input" --output "$trial"; then
		pquit "Failed to create '$trial' from '$tmp_input'!\n"; fi

	# The order you pass the images in for `compare` matters; the trial must
	# come first.
	score="$(compare -metric SSIM "$trial" "$tmp_input" null: 2>&1)"
	score_int="$(printf "%d0000" "${score#*.}" | cut -c -6)"
	score_int_fancy="$(fancy_percent "$score_int")"

	if [ "$score_int" -lt "$ideal" ]; then
			pstat "\033[31mBad score: " "${score_int_fancy}\n"
			if ! rm "$trial"; then
				pquit "Failed to remove '$trial'!\n"; fi

			bad_quality="$(( "$quality" + 1 ))"
		else
			pstat "\033[32mGood score: " "${score_int_fancy}\n"
			good_trial="$trial"
			upper_quality="$quality"
	fi

	if [ "$quality" -ge "$break_quality" ]; then break; fi

	# This has to go at the end since otherwise '$good_trial' will be reset
	# to the upcoming '$quality' value, and not the last good one.
	quality="$(( ("$bad_quality" + "$upper_quality") / 2 ))"
done

final_output="${2:-${input%.*}-${quality}.avif}"
if [ -z "$2" ]; then
	pstat "Output: " "$final_output"; fi

if ! mv "$good_trial" "$final_output"; then
	# Give user enough time to move the good trial themselves if desired.
	printf "\033[1m\033[31mFailed to move '%s' to '%s'! Closing in 30 seconds.\033[0;39m\n" "$good_trial" "$final_output"
	sleep 30
fi