gistfile1.sh

#!/bin/bash

# Based on the "Shell image viewer" by Andreas Schönfelder:
# https://bbs.archlinux.org/viewtopic.php?id=106089

# Defaults.
doublexres="yes"
colors="$(tput colors)"
size="$(($(tput cols) - 2))x$(($(tput lines) - 2))"

# Process arguments.
while getopts nc:s:b: name
do
	case $name in
		n)
			# Don't double the resolution in X direction?
			doublexres="no"
			;;
		c)
			# Number of colors?
			colors="$OPTARG";
			;;
		s)
			# Desired size?
			size="$OPTARG"
			;;
		b)
			# Background for transparent images?
			transparentbg=(-background "$OPTARG")
			;;
	esac
done
shift $((OPTIND - 1))

if [[ "$colors" != 8 ]] && [[ "$colors" != 256 ]]
then
	echo "'$colors' colors not supported." >&2
	exit 1
fi

if [[ "$doublexres" == "yes" ]]
then
	# First, we assume that there's twice as much space in Y direction.
	wid="${size%x*}"
	hei="${size#*x}"
	hei=$((hei * 2))
	size="${wid}x${hei}"

	# Afterwards, squash the image. This results in twice the pixels in
	# X direction.
	squash=(-scale "100%x50%")
else
	squash=()
fi

# Remaining arguments are filenames.
while (( $# > 0 ))
do
	# Convert the image into a PPM image which can easily be read by
	# awk.
	convert \
		"${transparentbg[@]}" \
		-compress none \
		-depth 8 \
		-resize "$size" \
		"${squash[@]}" \
		"$1" "ppm:-" |
		gawk '
	BEGIN {
		# Define colors and width of one pixel.
		blank = " "
		reset = "'"$(tput sgr0)"'"
		black = "'"$(tput setab 0)"'"
		blue = "'"$(tput setab 4)"'"
		green = "'"$(tput setab 2)"'"
		cyan = "'"$(tput setab 6)"'"
		red = "'"$(tput setab 1)"'"
		magenta = "'"$(tput setab 5)"'"
		yellow = "'"$(tput setab 3)"'"
		white = "'"$(tput setab 7)"'"

		if ('"$colors"' == 8)
			divideby = 128
		else
			divideby = 51

		# Read first three non-comment and non-empty lines and get the
		# image width.
		while (i < 3)
		{
			if (getline <= 0)
				break
			if (!($0 ~ /^#/ || $0 == ""))
				i++
			if (i == 2)
				wid = $1
		}
	}

	# Read the rest of the image.
	{
		for (i = 1; i <= NF; i += 3)
		{
			r = int($i / divideby)
			g = int($(i + 1) / divideby)
			b = int($(i + 2) / divideby)

			if ('"$colors"' == 8)
			{
				if (r == 0 && g == 0 && b == 0)
					printf "%s%s%s", black, blank, reset
				if (r == 0 && g == 0 && b == 1)
					printf "%s%s%s", blue, blank, reset
				if (r == 0 && g == 1 && b == 0)
					printf "%s%s%s", green, blank, reset
				if (r == 0 && g == 1 && b == 1)
					printf "%s%s%s", cyan, blank, reset
				if (r == 1 && g == 0 && b == 0)
					printf "%s%s%s", red, blank, reset
				if (r == 1 && g == 0 && b == 1)
					printf "%s%s%s", magenta, blank, reset
				if (r == 1 && g == 1 && b == 0)
					printf "%s%s%s", yellow, blank, reset
				if (r == 1 && g == 1 && b == 1)
					printf "%s%s%s", white, blank, reset
			}
			else
			{
				if (r == g && g == b)
				{
					# Try to use the full grayscale ramp. Using "r", "g"
					# and "b" automatically uses some fuzziness because
					# theyve already been divided by 51. But now, we
					# reset "g" to the real grayscale value. Full white
					# and full black are not part of the ramp.
					g = $i
					if (g == 255)
						printf "\033[48;5;231m%s%s", blank, reset
					else if (g == 0)
						printf "\033[48;5;16m%s%s", blank, reset
					else
					{
						printf "\033[48;5;%dm%s%s",
							(int(($i / 255) * 24) + 232), blank, reset
					}
				}
				else
				{
					# Find the nearest neighbor in XTerms 6x6x6 color
					# cube.
					printf "\033[48;5;%dm%s%s",
						(16 + r * 36 + g * 6 + b), blank, reset
				}
			}

			pixel++
			if (pixel == wid)
			{
				printf "\n"
				pixel = 0
			}
		}
	}'

	# One empty line between two images.
	shift
	if (( $# > 0 ))
	then
		echo
	fi
done