git://git.nikiroo.be / nikiroo-utils.git / blob
TODO: Crib text from the first message of https://gitlab.freedesktop.org/terminal-wg/specifications/issues/12 as to why people want images in their terminals.
The same mechanism that can put raster-based images on the screen is easily generalizable to other media types such as vector-based images, animations, and embedded GUI widgets. This document is thus a “multimedia” proposal, not just “simple images”.
This proposal has been informed from the following prior work:
DEC VT300 series sixel graphics standard: https://vt100.net/docs/vt3xx-gp/chapter14.html
iTerm2 image protocol: https://iterm2.com/documentation-images.html
Kitty image protocol:
Jexer Terminal User Interface: https://gitlab.com/klamonte/jexer
The core (“must-have”) design goals are:
Be easy to implement in existing terminals and applications:
Sacrifice “10%” of potential function to eliminate “90%” of implementation pain. “Less is more.”
Be a strict superset of the existing iTerm2 and DEC sixel image solutions. One should be able to take an existing terminal or application that emits/consumes iTerm2 or sixel sequences, and only change the control sequence introducer/termination to achieve the same effect as a terminal/application that conforms with this standard.
Have no ambiguity. If two terminal or application developers can read this document and reach different conclusions on what should be on the screen, then an error exists in this document that must be fixed.
Every feature should be straightforward to validate via automated unit testing.
Every conformant terminal should produce the same output (pixels on screen) given the same input (terminal font, terminal sequences).
Every option must have a defined default value.
Erroneous sequences must have defined expected results.
Every operation must act atomically: either everything worked (image is on screen, cursor has moved, etc.) or nothing did.
Be straightforward to implement in non-“physical” terminals, including:
Future versions of terminal control libraries such as ncurses and termbox.
Terminal multiplexers that support “headless” terminals (no physical screen) and “multi-head” terminals (many different physical screens).
Be platform-agnostic, and easy to implement on (at the least): POSIX, Windows, and web.
Support graceful fallback:
Terminal emulators and physical terminals that do not support this standard should remain usable with no undefined screen artifacts, even when the application blindly emits these sequences to those terminals.
This standard must able to be versioned for future enhancements.
An application must be able to detect that its terminal supports this standard, and at what version.
Support secure programming practices:
Applications must not be able to obtain unauthorized data from terminal memory, such as: images emitted by other applications still present in the terminal’s scrollback buffer, terminal or system memory limits.
Applications must not be able to compromise the terminal through denial-of-service such as: excessive memory usage, unterminated control sequences. Similarly, terminals must not be able to compromise application through their responses to application queries.
Applications must not be able to manipulate the terminal into performing an insecure operation such as: reading arbitrary shared memory regions, reading arbitrary files on disk, deleting arbitrary files on disk, etc. Similarly, terminals must not be able to manipulate applications into performing insecure operations.
This standard must be implementable when the terminal has a fixed maximum memory, such as a kernel-level device driver.
The secondary (“nice-to-have”) design goals are:
Minimal redundant network traffic for on-screen data that is repeated: either on screen in multiple places, or in the same place but refreshed multiple times.
Asynchronous notification from terminal to application that the screen has been changed by outside or user action. Examples: font change, session detach/attach, user changed image preferences.
The following items are out of scope for this standard:
Bidirectional output. Applications are expected to generate Tiles and place them on screen where they need. The cursor response to image sequences are defined as left-to-right, consistent with ECMA-48 / ANSI X3.64 sequences. An independent BIDI standard is free to apply whatever solution will work for ECMA-48 sequences to the sequences described in this document.
Capabilities. This standard defines a limited number of terminal reports. These are not intended to be used as a general-purpose capabilities model.
Terminal - The hardware, or a program that simulates hardware, comprising a keyboard, screen, and mouse.
Application - A program that utilizes the terminal for its input/output with the user.
Multiplexer - A special case of an application that simulates one or more “inner” terminals for other applications to use, and composes these inner terminals into a combined screen to emit to one or more “outer” terminals that obtain input/output from the user. Multiplexers are thus both applications and terminals.
X - The column coordinate of a cell. This standard is 0-based: the left-most column of the screen is numbered 0.
Y - The row coordinate of a cell. This standard is 0-based: the top-most row of the screen is numbered 0.
Z - The layer that text or multimedia is placed on. This proposal uses a right-hand coordinate system with (X, Y, Z) = (0, 0, 0) defined as the top-left corner on the default layer: positive Z projects “away” from the user and “into” or “behind” the screen. Rendering the Cells on the screen must produce the same result as painter’s algorithm (see Rendering section below).
Cell - A fixed-width-and-height rectangle on the screen. The cells of the screen are arranged in a grid of X columns and Y rows. A Cell has dimensions of cellWidth and cellHeight, which can be measured in either pixels or points. Every Cell has a coordinate of (X, Y, Z).
Tile - One or more contiguous Cells with data to be displayed. The data can be text or image data, but not both. A Tile has width of 1, 2, or more, and a coordinate of (X, Y, Z) that is the same as its left-most (first) Cell’s (X, Y, Z). In practice, Tiles are typically one Cell wide for ASCII and Latin language glyphs, and two Cells wide for “fullwidth” glyphs as used in Asian langauges, emojis, and symbols. This standard does not preclude Tiles from encompassing entire grapheme clusters.
Layer - A screen-sized grid of Cells that have the same Z coordinate. Layers are drawn to the screen in descending Z order. Layers may have optional additional attributes such as transparency.
A terminal will display its Cells such that the screen will look as if it was rendered in the following pseudo-code manner:
for each layer Z, in descending order from maxZ to minZ:
for each row Y, in ascending order from minY to maxY:
for each column X, in ascending order from minX to maxX:
draw tile at (X, Y, Z)
advance X by tile width
next column
advance Y by 1
next row
decrease Z by 1
next layer
A terminal is free to optimize its rendering as it sees fit, so long as the final screen output looks equivalent to the above method.