True Color, Image Protocols and Modern Terminal Features

Modern terminal emulators have outpaced the assumptions baked into most shell configs. True color (24-bit RGB), sixel graphics, the Kitty graphics protocol, and inline image support are all real and usable today — but only if your environment variables, terminal settings, and applications are aligned. This guide maps the full picture: what each capability is, how to detect it, and how to configure it correctly.

Understanding the Key Environment Variables

Two variables gate most color behavior. Getting them wrong causes either washed-out 8-color output or garbled escape sequences.

TERM

TERM tells applications which terminfo entry to load. It describes capabilities in a structured database rather than encoding them directly. Common values:

• xterm-256color — 256-color palette; the safe default for most emulators
• tmux-256color — use inside a tmux session
• kitty — set automatically by Kitty; unlocks Kitty-specific extensions
• foot — set by Foot; enables its specific terminfo entries
• alacritty — Alacritty's own entry (available since its 0.12 release)

Never hard-code TERM=xterm. It caps you at 8 colors regardless of what the emulator supports. Conversely, do not set TERM manually inside an emulator that sets it itself — you will break capability detection.

COLORTERM

COLORTERM is the informal but widely-adopted signal for 24-bit (true color) support. Applications like Neovim, bat, delta, and fzf check it before emitting 24-bit RGB escape sequences.

• truecolor — the standard value; set by Kitty, Foot, WezTerm, iTerm2, GNOME Terminal ≥ 3.36, Alacritty, and most modern emulators
• 24bit — an older synonym; some apps accept either

Check what your current session exports:

echo "TERM=$TERM"
echo "COLORTERM=$COLORTERM"

If COLORTERM is unset and your emulator does support true color, set it in your shell profile:

# ~/.bashrc or ~/.zshrc — only if your emulator supports true color
export COLORTERM=truecolor

Quick True Color Smoke Test

This one-liner prints a gradient. If you see smooth color transitions, true color is working. If you see banding, it is not.

awk 'BEGIN{
  for(c=0;c<256;c++){
    printf "\033[48;2;%d;%d;%dm ", c, 100, 200-c;
  }
  printf "\033[0m\n"
}'

Sixel Graphics

Sixel is a decades-old DEC protocol that encodes bitmap images as character streams. It has seen a major revival because several modern terminals now support it, making inline images available without proprietary extensions.

Which Terminals Support Sixel

• foot — enabled by default
• WezTerm — enabled by default
• mlterm — long-standing sixel leader
• xterm — requires -ti 340 flag or decTerminalID: 340 in .Xresources
• Contour — enabled by default
• iTerm2 (macOS) — partial; its own protocol (below) is preferred
• Kitty — does not support sixel; uses its own superior protocol
• GNOME Terminal / VTE-based terminals — no sixel support as of 2024

Detecting Sixel Support at Runtime

The terminal's DA1 (Device Attributes) response includes a 4 parameter if sixel is supported. Query it with:

printf '\033[c'; sleep 0.1; read -s -t 1 -d 'c' response </dev/tty; echo "$response"

Look for ;4; or ;4 at the end in the output. The exact string varies by terminal.

Displaying Sixel Images

Install img2sixel from the libsixel package:

# Debian/Ubuntu
sudo apt install libsixel-bin

# Fedora
sudo dnf install libsixel

# Arch
sudo pacman -S libsixel

img2sixel photo.jpg

For a quick test with Python's timg:

pip install timg
timg image.png

The Kitty Graphics Protocol

Kitty's graphics protocol (APC-based, \033_G...\033\\) transmits image data in chunked base64 payloads with a separate display command. It supports full RGBA, animation, virtual image placement, and Unicode placeholder rendering — capabilities sixel does not have.

Which Terminals Support the Kitty Protocol

• Kitty — reference implementation
• WezTerm — full support
• Konsole (KDE) ≥ 22.08 — partial support
• foot — as of 1.17, partial support
• Ghostty — full support

Checking Support Programmatically

Send a zero-size query chunk and check for an APC response:

printf '\033_Gi=1,s=1,v=1,a=q,t=d,f=24;AAAA\033\\'; sleep 0.1
read -s -t 1 response </dev/tty
echo "$response"

An OK in the response confirms support. No response or an error means unsupported.

Displaying Images with the Kitty Protocol

The kitten icat helper is the simplest path inside Kitty itself:

kitten icat photo.jpg

For a protocol-agnostic tool that falls back gracefully, use timg or chafa:

# Arch
sudo pacman -S chafa

# Debian/Ubuntu
sudo apt install chafa

# Fedora
sudo dnf install chafa

# chafa auto-detects the best protocol available
chafa photo.jpg

iTerm2 Inline Image Protocol

iTerm2 (macOS) uses an OSC 1337 escape sequence to transmit base64-encoded image data. It is simpler than Kitty's protocol but widely supported by macOS tooling.

Outside of macOS, WezTerm also supports this protocol. No mainstream Linux-native terminal does as a primary feature.

Displaying Images via iTerm2 Protocol

The official imgcat script wraps the encoding:

# Install via Homebrew on macOS
brew install iterm2-utilities
imgcat photo.jpg

Or manually from the iTerm2 repository:

curl -Lo imgcat https://iterm2.com/utilities/imgcat
chmod +x imgcat
./imgcat photo.jpg

Emulator Capability Reference

Terminal	True Color	Sixel	Kitty Protocol	iTerm2 Protocol
Kitty	Yes	No	Yes (native)	No
WezTerm	Yes	Yes	Yes	Yes
foot	Yes	Yes	Partial (1.17+)	No
Ghostty	Yes	No	Yes	No
Alacritty	Yes	No	No	No
GNOME Terminal	Yes	No	No	No
Konsole	Yes	Yes (limited)	Partial	No
xterm	Yes	Yes (opt-in)	No	No
iTerm2	Yes	Partial	No	Yes (native)
mlterm	Yes	Yes	No	No

Tmux and Multiplexer Considerations

Tmux wraps the outer terminal and sets its own TERM. This breaks graphics protocols by default because tmux does not pass APC or DCS sequences through.

• For true color inside tmux, add to ~/.tmux.conf:

set -g default-terminal "tmux-256color"
set -ga terminal-overrides ",*256col*:Tc"

• For sixel passthrough (tmux ≥ 3.3a):

set -g allow-passthrough on

• For Kitty graphics passthrough (tmux ≥ 3.3a with Kitty as the outer terminal):

set -g allow-passthrough all

Zellij handles graphics passthrough more transparently than tmux and is worth considering if inline images are a primary use case.

Verification

Run the following to get a concise capability summary of your current session:

printf 'TERM: %s\nCOLORTERM: %s\n' "$TERM" "$COLORTERM"

# Check terminfo for 24-bit color flag
tput colors

tput colors should return 256 or higher. It will not return 16777216 even with true color — true color is indicated by COLORTERM, not terminfo, due to historical limitations in the terminfo spec. The RGB terminfo capability was added in ncurses 6.1 and can be checked with:

tput -T "$TERM" RGB 2>/dev/null && echo "terminfo RGB: yes" || echo "terminfo RGB: no"

Troubleshooting

• Colors look wrong in SSH sessions: The remote shell inherits TERM from the SSH client, but COLORTERM is not forwarded by default. Add SendEnv COLORTERM to your local ~/.ssh/config and AcceptEnv COLORTERM to the remote /etc/ssh/sshd_config.
• Images not displaying in tmux: Confirm allow-passthrough on is set and you have tmux ≥ 3.3a. Check with tmux -V.
• Sixel works in xterm but not other apps: Some apps check the DA1 response; others rely solely on environment variables. Set TERM=xterm-256color and ensure xterm was launched with -ti 340.
• kitten icat fails outside Kitty: icat requires either native Kitty graphics protocol support or a fallback to sixel. Use chafa for a portable alternative.
• TERM set to a value with no terminfo entry: Run infocmp "$TERM". If it errors, install the missing entry or fall back to xterm-256color. Kitty ships its own terminfo; install it on remote servers with kitten ssh or manually copy ~/.terminfo.