Notcurses

My library for building complex, vibrant textual user interfaces (TUIs) on modern terminal emulators. It does not use Ncurses (though it does make use of libtinfo from that package), nor is it an X/Open Curses source-compatible replacement. It is written in C, with C++-safe headers.

Source and issue-tracking live at Github.

Contact sheet from Notcurses 0.4.0 demo

Features

• Optional use of "alternate screen" where available (smcup/rmcup terminfo capabilities)
• All APIs use 24-bit 8bpc RGB color natively
  • Color is quantized down for indexed palette terminals
• Transparency/semi-transparency plus dynamic high-contrast text
  • Lower planes can affect color of higher translucent ones
• Full support for Unicode, including wide glyphs and bidirectional text
  • Composed keys (number pad, etc.) are mapped into Private Supplementary Area B
• Image/video support via ffmpeg
• Subregion fade in/out
• Linear interpolation for coloring geometric objects

Rendering

The vast majority of functions draw to ncplane objects. A partial order (currently a total order) always exists over the planes. There is always at least one plane, the "standard plane". This plane cannot be resized, deleted, moved relative to the visible area, or reparented. Whenever notcurses updates its idea of the visible area's dimensions, it will resize the standard plane (references to the standard plane are not invalidated). Planes may otherwise be any size (invisible regions will not be rendered, and count only against memory), and can be moved to any position relative to the visible area. All planes, including the standard plane, can be freely moved along the z axis.

Every plane, and the notcurses object as a whole, maintains a per-row damage map, one boolean entry per line of the plane. These damage maps are likely to be replaced with O(1) damage detection.

When notcurses_render() is called, the visual area is constructed from the top left to the bottom right, row by row and column by column.

Each row is considered undamaged until proven otherwise. At each column, the character to be emitted (and its styling) is computed. This will require consulting some number of planes (see Transparency for more information). A damage value is computed as the boolean union over all relevant damage map entries. Relevant entries are those from the toplevel damage map, and the damage maps of all planes which contributed to the output glyph or styling.

Transparency/Contrasting

It is not obvious what "transparency" or "alpha blending" means in a character context. I have assigned my own meanings. Each of the foreground and background channel of an ncplane's cell have two bits dedicated to alpha. Channels are always initialized to 0, and thus the default alpha setting is CELL_ALPHA_OPAQUE. It is important to note that glyph selection is independent of alpha. The first glyph found while descending the planes intersecting a cell will be the glyph used. Only presentation of the glyph is modified by alpha.

Value	Macro	Foreground	Background
00	CELL_ALPHA_OPAQUE	Use the fg color unchanged.	Use the bg color unchanged.
01	CELL_ALPHA_BLEND	Blend the fg color down.	Blend the bg color down.
10	CELL_ALPHA_TRANSPARENT	Select the next fg color.	Select the next bg color.
11	CELL_ALPHA_HIGHCONTRAST	Complement bg color computed through this plane.	Forbidden.

The value 11 is currently forbidden for a bg alpha setting, but might be used in the future. To "blend the color down" means to average the colors encountered until hitting an opaque channel, or running out of planes. To "select the next color" means to ignore this color, and instead take the color as computed by lower planes.

High-contrast text is not strictly defined. notcurses will attempt to make the glyph as readable as possible, given the background color computed at the plane.

Note that a cell with the zero glyph will not have its channels considered. The containing plane's default channels will instead be factored into any color/transparency calculations (if a default glyph has been defined for the plane). A cell containing a zero glyph on a plane with a default zero glyph cannot impact the rendered scene; any associated channels will be ignored.

If loaded multimedia supports transparency (e.g. PNGs), transparent pixels will be considered as if CELL_ALPHA_TRANSPARENT was used.

Linear interpolation

notcurses can color lines via linear interpolation between the two endpoints. This is done with ncplane_hline_interp() and ncplane_vline_interp(). If provided two endpoints of the same color, the line will be that single color.

Releases

Release	Date	Key features
0.9.1	2019-12-21	Visual-adaptive planes, true ncvisual transparency, char32_t input
0.9.0	2019-12-18	Recognize COLORTERM, damage map, quantized colors, _yx API extensions, alignment
0.4.0	2019-12-05	Cell API, input, resize handling
0.3.0	2019-12-02	Video support, transparent planes, fades
0.2.0	2019-12-02	Panelreels, image support
0.1.0	2019-11-30	Ncplanes, basic output

See Also

• outcurses, my effects and widgets library for Ncurses