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Rendering

Status: Convention. This page collects rendering behavior that has no wire protocol of its own but changes how the same bytes look.

Bold, faint, italic

SGR 1 historically meant "bright" on eight-color hardware; the eight bright colors 90–97 were later given their own codes. Emulators now offer a choice: render bold with a heavier font, brighten the color, or both. xterm's boldColors, VTE's "allow bold" and bright-bold settings, kitty's bold_is_bright, and Windows Terminal's intenseTextStyle are the switches. An application that wants bright must use 90–97; one that wants a bold face must use SGR 1 and accept that some terminals will also brighten.

SGR 2 (faint) is usually rendered by dimming the foreground; SGR 3 (italic) requires an italic face and is ignored by terminals without one. Underline styles are on the SGR page.

Shaping and ligatures

Most emulators shape one cell at a time. Each cell holds a code point (or a cluster), the glyph is looked up in the font, and the result is placed at the cell origin. This is why ligatures, contextual forms in Arabic, and conjuncts in Indic scripts historically did not appear.

Emulators that run a shaper (HarfBuzz) over a run of cells and then split the result back into cells: kitty, WezTerm (harfbuzz_features), foot (with tweak.grapheme-shaping and tweak.font-shaping), Contour, Ghostty (?), Alacritty No. The shaped run is still constrained to the cell grid; a ligature is drawn across its original cells but does not change cursor movement, so selecting or deleting inside it behaves as if the characters were separate.

Font fallback

A single font rarely covers Latin, CJK, symbols, and emoji. Emulators build a fallback chain from configuration or fontconfig and pick the first font that has the glyph. Two consequences matter to applications:

  • width was already decided from tables, so a fallback glyph that is naturally wider or narrower is scaled or clipped to fit;
  • a glyph from a fallback font with different metrics can sit visibly higher or lower than neighbors.

Synthesized glyphs

Box drawing (U+2500–U+257F), block elements (U+2580–U+259F), Braille (U+2800–U+28FF), Powerline, and standardized legacy-computing symbols are drawn by the emulator rather than the font in kitty, WezTerm, foot, Ghostty, Alacritty (builtin_box_drawing), Windows Terminal, and VTE, so that lines connect exactly. xterm draws line-drawing characters itself when the font lacks them. Nerd Font private-use glyphs beyond the Powerline range come from the font.

TDN tracks the implementation technique as text-procedural-glyphs. Each terminal record names its exact procedural ranges, because terminals divide DEC scan lines, geometric pieces, Symbols for Legacy Computing, its Unicode 16 supplement, and private-use glyphs differently.

Bidirectional text

Status: Disputed.

ECMA TR/53 and the terminal-wg bidi specification define explicit modes:

CSI ? 2500 h    bidi: terminal performs implicit reordering (default on/off differs)
CSI ? 2501 h    bidi: box mirroring / explicit direction, per spec

The terminal-wg draft uses these numbers; adoption is limited. mlterm and Konsole implement bidi reordering by their own rules; VTE implements the terminal-wg draft (0.58+)1; foot, kitty, WezTerm, Ghostty, Alacritty, xterm do not reorder. Applications targeting Arabic or Hebrew users must assume either logical-order display or visual reordering with no way to tell except by asking the user, unless the emulator answers DECRQM for 2500.

See emoji artwork and fallback for separate monochrome, presentation, fitting and sequence capability probes.

Color emoji

Color emoji come from bitmap (CBDT, sbix) or vector (COLR) fonts. Emulators that render them: kitty, WezTerm, foot, Ghostty, Alacritty (via crossfont), Windows Terminal, iTerm2, Apple Terminal, VTE (through Pango), Konsole. xterm renders monochrome only. Color glyphs ignore the foreground SGR color.

Probe

printf '\033[1mbold\033[0m \033[91mbright red\033[0m \033[1;31mbold red\033[0m\n'
printf '\033[2mfaint\033[0m \033[3mitalic\033[0m\n'
printf 'fi ffl -> => != \n'          # ligature check
printf '┌─┬─┐\n│ │ │\n└─┴─┘\n'      # box drawing joins
printf 'שלום עולם\n'                 # bidi: logical or visual order?
printf '😀🎉\n'                       # color emoji

Sources

Background-relative faint text

Feature ID: resource-faint-is-relative. The xterm faintIsRelative resource selects background-relative mixing for faint text. This is independent of recognizing SGR 2 or rendering faint text with the default foreground scaling.

Compatibility

Feature support by terminal. Imported claims have not been reverified; unknown is not unsupported.
Feature / stable ID Alacritty Apple Terminal ConPTY (conhost) Contour DomTerm foot Ghostty iTerm2 kitty Konsole mintty mlterm PuTTY Revenant RLogin st tmux rxvt-unicode VTE wayst WezTerm Windows Terminal xterm xterm.js
Bold-is-bright option
text-bold-is-bright		 SupportedImported · unverified SupportedImported · unverified Unknown Unknown Unknown SupportedImported · unverified SupportedImported · unverified SupportedImported · unverified SupportedImported · unverified Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown SupportedImported · unverified Unknown SupportedImported · unverified SupportedImported · unverified SupportedImported · unverified PartialImported · unverified
Italic face
text-italic-face		 SupportedImported · unverified SupportedImported · unverified Unknown SupportedImported · unverified Unknown SupportedImported · unverified SupportedImported · unverified SupportedImported · unverified SupportedImported · unverified SupportedImported · unverified Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown SupportedImported · unverified Unknown SupportedImported · unverified SupportedImported · unverified SupportedImported · unverified SupportedImported · unverified
Ligatures / shaping
text-shaping		 Unknown SupportedImported · unverified Unknown SupportedImported · unverified Unknown SupportedImported · unverified Unknown SupportedImported · unverified SupportedImported · unverified Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown SupportedImported · unverified SupportedImported · unverified Unknown PartialImported · unverified
Synthesized box drawing
text-box-drawing		 SupportedImported · unverified Unknown Unknown Unknown Unknown SupportedImported · unverified SupportedImported · unverified Unknown SupportedImported · unverified Unknown Unknown Unknown Unknown Supported Unknown Unknown Unknown Unknown SupportedImported · unverified Unknown SupportedImported · unverified SupportedImported · unverified SupportedImported · unverified SupportedImported · unverified
Bidi reordering
text-bidi		 Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown SupportedImported · unverified Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown SupportedImported · unverified Unknown Unknown Unknown Unknown Unknown
Color emoji rendering
text-color-emoji		 SupportedImported · unverified SupportedImported · unverified Unknown SupportedImported · unverified Unknown SupportedImported · unverified SupportedImported · unverified SupportedImported · unverified SupportedImported · unverified SupportedImported · unverified Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown SupportedImported · unverified Unknown SupportedImported · unverified SupportedImported · unverified Unknown SupportedImported · unverified
Procedural glyph rendering
text-procedural-glyphs		 Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Supported Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown
Background-relative faint text
resource-faint-is-relative		 Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown

  1. VTE bidi documentation. ↩