Notcurses: blingful TUIs and character graphics

• What it is: a library facilitating complex TUIs on modern terminal emulators, supporting vivid colors, multimedia, threads, and Unicode to the maximum degree possible. Things can be done with Notcurses that simply can't be done with NCURSES.

• What it is not: a source-compatible X/Open Curses implementation, nor a replacement for NCURSES on existing systems.

birthed screaming into this world by nick black (nickblack@linux.com). C++ wrappers by marek habersack (grendel@twistedcode.net). the Notcurses API is stable as of version 2.0.

for more information, see dankwiki and the man pages. there's also a reference in this repo. in addition, there is Doxygen output. there is a mailing list which can be reached via notcurses@googlegroups.com. i wrote a coherent guidebook, which is available for free download, or paperback purchase.

i've not yet added many documented examples, but there are many small C/C++ programs available in src/poc/ and src/pocpp/, respectively. notcurses-demo covers most of the functionality of Notcurses, and can be found in src/demo/.

If you're running Notcurses applications in a Docker, please consult "Environment notes" below. I track some capabilities of terminal emulators, and also maintain a list of other TUI libraries.

• Introduction
• Requirements
  • Building
• Included tools
• Differences from NCURSES
  • Features missing relative to NCURSES
  • Adapting NCURSES programs
• Environment notes
  • TrueColor detection
  • Fonts
  • FAQs
• Supplemental material
  • Useful links
  • History
  • Thanks

Introduction

Notcurses abandons the X/Open Curses API bundled as part of the Single UNIX Specification. For some necessary background, consult Thomas E. Dickey's superb and authoritative NCURSES FAQ. As such, Notcurses is not a drop-in Curses replacement.

Wherever possible, Notcurses makes use of the Terminfo library shipped with NCURSES, benefiting greatly from its portability and thoroughness.

Notcurses opens up advanced functionality for the interactive user on workstations, phones, laptops, and tablets, at the expense of e.g. some industrial and retail terminals.

Why use this non-standard library?

• Thread safety, and efficient use in parallel programs, has been a design consideration from the beginning.

• A svelter design than that codified by X/Open:

  • Exported identifiers are prefixed to avoid common namespace collisions.
  • The library object exports a minimal set of symbols. Where reasonable, static inline header-only code is used. This facilitates compiler optimizations, and reduces loader time.
  • Notcurses can be built without its multimedia functionality, requiring a significantly lesser set of dependencies.

• All APIs natively support the Universal Character Set (Unicode). The cell API is based around Unicode's Extended Grapheme Cluster concept.

• Visual features including images, fonts, video, high-contrast text, sprites, and transparent regions. All APIs natively support 24-bit color, quantized down as necessary for the terminal.

• It's Apache2-licensed in its entirety, as opposed to the drama in several acts that is the NCURSES license (the latter is summarized as "a restatement of MIT-X11").

Much of the above can be had with NCURSES, but they're not what NCURSES was designed for. On the other hand, if you're targeting industrial or critical applications, or wish to benefit from its time-tested reliability and portability, you should by all means use that fine library.

Requirements

Minimum versions generally indicate the oldest version I've tested with; it may well be possible to use still older versions. Let me know of any successes!

• (build) A C11 and a C++17 compiler
• (build) CMake 3.14.0+
• (build+runtime) From NCURSES: terminfo 6.1+
• (build+runtime) GNU libunistring 0.9.10+
• (build+runtime) GNU Readline 8.0+
• (OPTIONAL) (build+runtime) From QR-Code-generator: libqrcodegen 1.5.0+
• (OPTIONAL) (build+runtime) From FFmpeg: libswscale 5.0+, libavformat 57.0+, libavutil 56.0+
• (OPTIONAL) (build+runtime) OpenImageIO 2.15.0+
• (OPTIONAL) (testing) Doctest 2.3.5+
• (OPTIONAL) (documentation) pandoc 1.19.2+
• (OPTIONAL) (python bindings): Python 3.7+, CFFI 1.13.2+, pypandoc 1.5+
• (OPTIONAL) (rust bindings): rust 1.47.0+, bindgen 0.55.1+, pkg-config 0.3.18+, cty 0.2.1+
• (runtime) Linux 5.3+, FreeBSD 11+, or DragonFly BSD 5.9+

Here's more information on building and installation.

Included tools

Seven binaries are installed as part of Notcurses:

• notcurses-demo: some demonstration code
• ncls: an ls that displays multimedia in the terminal
• ncneofetch: a neofetch ripoff
• ncplayer: renders visual media (images/videos)
• notcurses-input: decode and print keypresses
• notcurses-tester: unit testing
• notcurses-tetris: a tetris clone

To run notcurses-demo from a checkout, provide the tests/ directory via the -p argument. Demos requiring data files will otherwise abort. The base delay used in notcurses-demo can be changed with -d, accepting a floating-point multiplier. Values less than 1 will speed up the demo, while values greater than 1 will slow it down.

notcurses-tester expects ../tests/ to exist, and be populated with the necessary data files. It can be run by itself, or via make test.

Differences from NCURSES

The biggest difference, of course, is that Notcurses is not an implementation of X/Open (aka XSI) Curses, nor part of SUS4-2018.

The detailed differences between Notcurses and NCURSES probably can't be fully enumerated, and if they could, no one would want to read them. With that said, some design decisions might surprise NCURSES programmers:

• There is no distinct PANEL type. The z-buffer is a fundamental property, and all drawable surfaces are ordered along the z axis. There is no equivalent to update_panels().
• Scrolling is disabled by default, and cannot be globally enabled.
• The Curses cchar_t has a fixed-size array of wchar_t. The Notcurses cell instead supports a UTF-8 encoded extended grapheme cluster of arbitrary length. The only supported encodings are ASCII via ANSI_X3.4-1968 and Unicode via UTF-8.
• The cursor is disabled by default, when supported (civis capability).
• Echoing of input is disabled by default, and cbreak mode is used by default.
• Colors are usually specified as 24 bits in 3 components (RGB). If necessary, these will be quantized for the actual terminal. There are no "color pairs", but indexed palettes are supported.
• There is no distinct "pad" concept (these are NCURSES WINDOWs created with the newpad() function). All drawable surfaces can exceed the display size.
• Multiple threads can freely call into Notcurses, so long as they're not accessing the same data. In particular, it is always safe to concurrently mutate different ncplanes in different threads.
• NCURSES has thread-ignorant and thread-semi-safe versions, trace-enabled and traceless versions, and versions with and without support for wide characters. Notcurses is one library: no tracing, UTF-8, thread safety.
• There is no ESCDELAY concept; Notcurses expects that all bytes of a keyboard escape sequence arrive at the same time. This improves latency and simplifies the API.
• It is an error in NCURSES to print to the bottommost, rightmost coordinate of the screen when scrolling is disabled (because the cursor cannot be advanced). Failure to advance the cursor does not result in an error in Notcurses (but attempting to print at the cursor when it has been advanced off the plane does).

Features missing relative to NCURSES

This isn't "features currently missing", but rather "features I do not intend to implement".

• There is no support for soft labels (slk_init(), etc.).
• There is no concept of subwindows which share memory with their parents.
• There is no tracing functionality ala trace(3NCURSES). Superior external tracing solutions exist, such as bpftrace.

Adapting NCURSES programs

Do you really want to do such a thing? NCURSES and the Curses API it implements are far more portable and better-tested than Notcurses is ever likely to be. Will your program really benefit from Notcurses's advanced features? If not, it's probably best left as it is.

Otherwise, most NCURSES concepts have clear partners in Notcurses. Any functions making implicit use of stdscr ought be replaced with their explicit equivalents. stdscr ought then be replaced with the result of notcurses_stdplane() (the standard plane). PANELs become ncplanes; the Panels API is otherwise pretty close. Anything writing a bare character will become a simple cell; multibyte or wide characters become complex cells. Color no longer uses "color pairs". You can easily enough hack together a simple table mapping your colors to RGB values, and color pairs to foreground and background indices into said table. That'll work for the duration of a porting effort, certainly.

I have adapted two large (~5k lines of C UI code each) programs from NCURSES to Notcurses, and found it a fairly painless process. It was helpful to introduce a shim layer, e.g. compat_mvwprintw for NCURSES's mvwprintw:

static int
compat_mvwprintw(struct ncplane* nc, int y, int x, const char* fmt, ...){
  va_list va;
  va_start(va, fmt);
  if(ncplane_vprintf_yx(nc, y, x, fmt, va) < 0){
    va_end(va);
    return ERR;
  }
  va_end(va);
  return OK;
}

These are pretty obvious, implementation-wise.

Environment notes

• If your TERM variable is wrong, or that terminfo definition is out-of-date, you're going to have a very bad time. Use only TERM values appropriate for your terminal. If this variable is undefined, or Notcurses can't load the specified Terminfo entry, it will refuse to start, and you will not be going to space today.

• Ensure your LANG environment variable is set to a UTF8-encoded locale, and that this locale has been generated. This usually means "[language]_[Countrycode].UTF-8", i.e. en_US.UTF-8. The first part (en_US) ought exist as a directory or symlink in /usr/share/locales. This usually requires editing /etc/locale.gen and running locale-gen. On Debian systems, this can be accomplished with dpkg-reconfigure locales, and enabling the desired locale. The default locale is stored somewhere like /etc/default/locale.

• If your terminal has an option about default interpretation of "ambiguous-width characters" (this is actually a technical term from Unicode), ensure it is set to Wide, not narrow (if that doesn't work, ensure it is set to Narrow, heh).

• The unit tests assume dimensions of at least 80x24. They might work in a smaller terminal. They might not. Don't file bugs on it.

TrueColor detection

Notcurses primarily loads control sequences from terminfo(5), using the database entry specified by the TERM environment variable. 24-bit "TrueColor" color support (or at least the ability to specify 3 8-bit channels as arguments to setaf and setbf) is indicated by the rgb terminfo capability. Many terminals with RGB support do not advertise the rgb capability. If you believe your terminal to support 24-bit TrueColor, this can be indicated by exporting the COLORTERM environment variable as truecolor or 24bit. Note that some terminals accept a 24-bit specification, but map it down to fewer colors.

Fonts

Fonts end up being a whole thing, little of which is pleasant. I'll write this up someday FIXME.

It is worth knowing that several terminals draw the block characters directly, rather than loading them from a font.

FAQs

If things break or seem otherwise lackluster, please consult the Environment Notes section! You need to have a correct TERM and LANG definition, and probably want COLORTERM.

• Q: The demo fails in the middle of intro. A: Check that your TERM value is correct for your terminal. intro does a palette fade, which is prone to breaking under incorrect TERM values. If you're not using xterm, your TERM should not be xterm!

• Q: Can I have Notcurses without this huge multimedia stack? A: Yes! Build with -DUSE_MULTIMEDIA=none.

• Q: In xterm, Alt doesn't work as expected. A: Check out the eightBitInput resource of xterm. Add XTerm*eightBitInput: false to your $HOME/.Xresources, and run xrdb -a $HOME/.Xresources.

• Q: Notcurses looks like absolute crap in screen. A: screen doesn't support RGB colors (at least as of 4.08.00); if you have COLORTERM defined, you'll have a bad time. If you have a screen that was compiled with --enable-colors256, try exporting TERM=screen-256color as opposed to TERM=screen.

• Q: Notcurses looks like absolute crap in mosh. A: Yeah it sure does. I'm not yet sure what's up.

• Q: Why didn't you just use Sixel? A: Many terminal emulators don't support Sixel. Sixel doesn't work well with mouse selection. With that said, I do intend to support Sixel soon, as a backend, when available, for certain types of drawing (see issue #200).

• Q: I'm not seeing NCKEY_RESIZE until I press some other key. A: You've almost certainly failed to mask SIGWINCH in some thread, and that thread is receiving the signal instead of the thread which called notcurses_getc_blocking(). As a result, the poll() is not interrupted. Call pthread_sigmask() before spawning any threads.

• Q: One of the demos claimed to spend more than 100% of its runtime rendering. Do you know how to count? A: Runtime is wall clock time. A multithreaded demo can spend more than the wall-clock time rendering if multiple threads run concurrently.

• Q: Using the C++ wrapper, how can I ensure that the NotCurses destructor is run when I return from main()? A: As noted in the C++ FAQ, wrap it in an artificial scope (this assumes your NotCurses is scoped to main()).

• Q: How do I hide a plane I want to make visible later? A: In order of least to most performant: move it offscreen using ncplane_move_yx(), move it underneath an opaque plane with ncplane_move_below(), or move it off-pile with ncplane_reparent().

• Q: Why isn't there an ncplane_box_yx()? Do you hate orthogonality, you dullard? A: ncplane_box() and friends already have far too many arguments, you monster.

• Q: Why doesn't Notcurses support 10- or 16-bit color? A: Notcurses supports 24 bits of color, spread across three eight-bit channels. You presumably mean 10-bit-per-channel color. Notcurses will support it when a terminal supports it.

• Q: The name is dumb. A: That's not a question?

• Q: I'm not finding qrcodegen on BSD, despite having installed graphics/qr-code-generator. A: Try cmake -DCMAKE_REQUIRED_INCLUDES=/usr/local/include. This is passed by bsd.port.mk.

• Q: Do you support musl? A: I try to! You'll need at least 1.20.

• Q: I only seem to blit in ASCII, and/or can't emit Unicode beyond ASCII in general. A: Your LANG environment variable is underdefined or incorrectly defined, or the necessary locale is not present on your machine (it is also possible that you explicitly supplied NCOPTION_INHIBIT_SETLOCALE, but never called setlocale(3), in which case don't do that).

• Q: I pretty much always need an ncplane when using a cell. Why doesn't the latter hold a pointer to the former? A: Besides the massive redundancy this would entail, cell needs to remain as small as possible, and you almost always have the ncplane handy if you've got a reference to a valid cell anyway.

• Q: I ran notcurses-demo with a single demo, but my summary numbers don't match that demo's numbers, you charlatan. A: notcurses-demo renders several frames beyond the actual demos.

• Q: When my program exits, I don't have a cursor, or text is invisible, or colors are weird, ad nauseam. A: Ensure you're calling notcurses_stop()/ncdirect_stop() on all exit paths, including fatal signals.

• Q: How can I use Direct Mode in conjunction with libreadline? A: Pass NCDIRECT_OPTION_CBREAK to ncdirect_init(), and ensure you do not pass NCDIRECT_OPTION_NO_READLINE. If you'd like, set rl_readline_name and rl_attempted_completion_function prior to calling ncdirect_init(). With that said, consider using a Notcurses ncreader.

• Q: Will there ever be Java wrappers? A: I should hope not.

• Q: Given that the glyph channel is initialized as transparent for a plane, shouldn't the foreground and background be initialized as transparent, also? A: Probably (they are instead initialized to default opaque). This would change some of the most longstanding behavior of Notcurses, though, so it isn't happening.

• Q: Why does my right-to-left text appear left-to-right? A: Notcurses doesn't honor the BiDi state machine, and in fact forces left-to-right with BiDi codes. Likewise, ultra-wide glyphs will have interesting effects. ﷽!

Supplemental material

Useful links

• BiDi in Terminal Emulators
• The Xterm FAQ
  • XTerm Control Sequences
• The NCURSES FAQ
• ECMA-35 Character Code Structure and Extension Techniques (ISO/IEC 2022)
• ECMA-43 8-bit Coded Character Set Structure and Rules
• ECMA-48 Control Functions for Coded Character Sets (ISO/IEC 6429)
• Unicode 12.1 Full Emoji List
• Unicode Standard Annex #29 Text Segmentation
• Unicode Standard Annex #15 Normalization Forms
• The TTY demystified
• Dark Corners of Unicode
• UTF-8 Decoder Capability and Stress Test
• Emoji: how do you get from U+1F355 to 🍕?
• Glyph Hell: An introduction to glyphs, as used and defined in the FreeType engine

Useful man pages

• Linux: console_codes(4)
• Linux: termios(3)
• Linux: ioctl_tty(2)
• Linux: ioctl_console(2)
• Portable: terminfo(5)
• Portable: user_caps(5)

History

• 2021-02-06: FOSDEM 2021 presentation.
• 2021-01-11: Notcurses is accepted into DragonFly BSD.
• 2020-12-13: Notcurses 2.1.0 "rubberband man".
• 2020-11-23: Invited presentation at DebMiniConf #2.
• 2020-10-12: Notcurses 2.0.0 "stankonia".
  • A stable API! This API will be supported going forward. Hype video!
• 2020-08-30: Notcurses 1.7.0 "don't pull the bang out".
• 2020-07-04: Notcurses 1.6.0 "aquemini".
• 2020-07-03: Notcurses is accepted into Alpine Edge.
• 2020-06-08: Notcurses 1.5.0 "ghetto bird".
• 2020-05-13: Notcurses is accepted into Fedora Core.
• 2020-05-10: Notcurses 1.4.0 "the saga continues".
• 2020-05-09: Notcurses is accepted into FreeBSD.
• 2020-04-19: Notcurses is accepted into Debian.
• 2020-04-12: Notcurses 1.3.0 "hypnotize".
• 2020-04-08: The Notcurses book is published.
• 2020-03-23: Notcurses is featured on Linux World News.
• 2020-02-17: Notcurses 1.2.0 "check the résumé, my record's impeccable".
• 2019-01-19: Notcurses 1.1.0 "all the hustlas they love it just to see one of us make it". Much better video support, pulsing planes, palette256.
  • The new hype video gets a lot of attention.
• 2019-01-04: Notcurses 1.0.0 "track team, crack fiend, dying to geek" is released, six days ahead of schedule. 147 issues closed. 702 commits.
• 2019-12-18: Notcurses 0.9.0 "You dig in! You dig out! You get out!", and also the first contributor besides myself (@grendello). Last major pre-GA release.
• 2019-12-05: Notcurses 0.4.0 "TRAP MUSIC ALL NIGHT LONG", the first generally usable Notcurses.
  • I prepare a demo, and release it on YouTube.
• November 2019: I begin work on Outcurses. Outcurses is a collection of routines atop NCURSES, including ncreels. I study the history of NCURSES, primarily using Thomas E. Dickey's FAQ and the mailing list archives.
  • 2019-11-14: I file Outcurses issue #56 regarding use of TrueColor in outcurses. This is partially inspired by Lexi Summer Hale's essay everything you ever wanted to know about terminals. I get into contact with Thomas E. Dickey and confirm that what I'm hoping to do doesn't really fit in with the codified Curses API.
  • 2019-11-16: I make the first commit to Notcurses.
• September 2019: I extracted fade routines from Growlight and Omphalos, and offered them to NCURSES as extensions. They are not accepted, which is understandable. I mention that I intend to extract ncreels, and offer to include them in the CDK (Curses Development Kit). Growlight issue #43 is created regarding this extraction. A few minor patches go into NCURSES.
• 2011, 2013: I develop Growlight and Omphalos, complicated TUIs making extensive use of NCURSES.

Thanks

• Notcurses could never be what it is without decades of tireless, likely thankless work by Thomas E. Dickey on NCURSES. His FAQ is a model of engineering history. He exemplifies documentation excellence and conservative, thoughtful stewardship. The free software community owes Mr. Dickey a great debt.
• Robert Edmonds provided tremendous assistance Debianizing the package, and David Cantrell did likewise for Fedora. Both are hella engineers.
• Justine Tunney, one of my first friends at Google NYC, was always present with support, and pointed out the useful memstream functionality of POSIX, eliminating the need for me to cons up something similar.
• I one night read the entirety of Lexi Summer Hale's essays, and began implementing her vision the next morning.
• NES art was lifted from The Spriters Resource and NES Sprite, the kind of sites that make the Internet great. It probably violates any number of copyrights. C'est la vie.
• Mark Ferrari, master of the pixel, for no good reason allowed me to reproduce his incredible and groundbreaking color-cycling artwork. Thanks Mark!
• The world map image was made by Vecteezy, and is used according to the terms of their License.
• Finally, the demoscene and general l33t scene of the 90s and early twenty-first century endlessly inspired a young hax0r. There is great joy in computing; no one will drive us from this paradise Turing has created!

“Our fine arts were developed, their types and uses were established, in times very different from the present, by men whose power of action upon things was insignificant in comparison with ours. But the amazing growth of our techniques, the adaptability and precision they have attained, the ideas and habits they are creating, make it a certainty that profound changes are impending in the ancient craft of the Beautiful.” —Paul Valéry

Grotesque vanity and meaningless metrics