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Multiple standards exist to incorporate image data in text-based terminals and terminal emulators. Few standards have wide adoption despite frequent user requests for these features and hardware support for several of the standards.
A group including developers of several widely-used terminal emulators has been working on defining the needs and limitations for a standard that can be implemented in current-gen terminal emulators. The discussion has been primarily captured here: https://gitlab.freedesktop.org/terminal-wg/specifications/issues/12
This document collects many of the reported desires and practical constraints of that discussion into a proposed standard that encompasses three independent new features:
A method to transfer multimedia data for immediate display within the screen cell grid (“Direct Multimedia”).
A method to transfer multimedia data to a terminal-managed cache, and later display that data within the screen cell grid (“Cached Multimedia”).
A method to assign cell data to different layers with options for both layer and cell transparency (“Layers”).
A terminal may implement any combination of these features independently of each other. If all features are supported, then all of the design goals outlined in this document can be met.
The same mechanisms that can put raster-based images on the screen are also readily generalizable to other media types such as vector-based images and animations. This document is thus a “multimedia” proposal rather than a “simple images” proposal.
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: https://sw.kovidgoyal.net/kitty/graphics-protocol.html
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 corrected.
Every feature must be straightforward to validate via automated unit testing.
Every conformant terminal must 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, terminal state has changed, etc.) or nothing did.
Integrate with existing ECMA-48 / ANSI X3.64 defined sequences:
Operations on Tiles/Cells containing text will have the same effect when applied to Tiles/Cells containing image data.
Existing sequences are given new parameters to cover needed features rather than entirely new sequences introduced.
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 listed below. These might not all be possible, but will kept in mind:
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 ability for a multiplexer to “pass-thru” the image drawing sequence to its “outer” terminal, with some support for limited clipping.
The following items are out of scope:
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-top-to-bottom, consistent with ECMA-48 / ANSI X3.64 sequences. An independent BIDI standard is free to apply whatever solution will work for ECMA-48 / ANSI X3.64 sequences to the sequences described in this document.
Capabilities. This standard defines a limited number of new terminal reports and responses. 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 1-based (like ECMA-48): the left-most column of the screen is numbered 1.
Y - The row coordinate of a cell. This standard is 1-based (like ECMA-48): the top-most row of the screen is numbered 1.
Z - The layer that text or multimedia is placed on. This proposal uses a right-hand coordinate system with (X, Y, Z) = (1, 1, 1) 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 “Layers - 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 pixels. Every Cell has a coordinate of (X, Y) (or (X, Y, Z) when the terminal supports the layers feature).
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. Note that ECMA-48 / ANSI X3.64 operations are performed against Tiles, not Cells: if a 2-Cell-wide Tile is deleted via backspace, then the cursor will decrement on screen by two columns.
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. Layer support is an orthogonal (independent) option to multimedia support. It is acceptable for terminals to support multimedia without layers and vice versa.
Applications can detect support for these features using Primary Device Attributes (DA) and DECID (ESC Z, or 0x9A).
Terminals that support this standard will repond with additional parameter(s): “224” for direct multimedia, “225” for cached multimedia, and “226” for layers. A recap of the parameters xterm supports is listed below, with these new feature responses included:
| VT220 (and higher) Response | Description |
|---|---|
| 1 | 132-columns |
| 2 | Printer |
| 3 | ReGIS graphics |
| 4 | Sixel graphics |
| 6 | Selective erase |
| 8 | User-defined keys |
| 9 | National Replacement Character sets |
| 1 5 | Technical characters |
| 1 6 | Locator port |
| 1 7 | Terminal state interrogation |
| 1 8 | User windows |
| 2 1 | Horizontal scrolling |
| 2 2 | ANSI color, e.g., VT525 |
| 2 8 | Rectangular editing |
| 2 9 | ANSI text locator (i.e., DEC Locator mode) |
| 2 2 4 | Direct Multimedia Version 1 |
| 2 2 5 | Cached Multimedia Version 1 |
| 2 2 6 | Layers |
A terminal with direct multimedia feature must support the following defined xterm sequences:
| Sequence | Description |
|---|---|
| CSI 16 t | Responds with CSI 6 ; cellHeight ; cellWidth t |
| CSI 18 t | Responds with CSI 8 ; rows ; columns t |
Pixel data that has scrolled off the displayed screen and into the scrollback buffer is required to be persistent even if the cache entry containing that image data has been evicted by the terminal or removed by the application.
A terminal with cached multimedia feature must support the following defined xterm sequences:
| Sequence | Description |
|---|---|
| CSI 16 t | Responds with CSI 6 ; cellHeight ; cellWidth t |
| CSI 18 t | Responds with CSI 8 ; rows ; columns t |
Layers introduce the concept of a layer “Z” coordinate to the existing rows (“Y”) by columns (“X”) grid. Put another way, the two-dimensional grid of columns-by-rows becomes a three-dimensional cube of columns-by-rows-by-layers. For this document, the column, row, and layer coordinates are referred to as X, Y, and Z. This cartesian coordinate system is right-handed, with the Z axis pointing “away” from the user “into” the screen.
An application treats the Z coordinate exactly as it does X and Y (rows and columns) coordinates:
If it attemps to set Z to a value less than 1, then Z is set to 1.
If it attempts to set Z to a value greater than the number of layers, then Z is set to the number of layers.
New sequences are provided to set and query Z, Y, X, to set and query the screen cube size, and control visibility of Cells in-front-of other Cells.
Operations that act on more than one Cell are defined such to act on all layers simultaneously by default.
A terminal is required to provide between 1 and a finite number of layers.
The number of layers may be different between the primary and alternate screens.
An application may request that the terminal allocate additional layers. The terminal is free to honor or ignore such requests as it sees fit.
The scrollback buffer is permitted, and recommended, to contain only a “flattened” single layer.
The terminal maintains a complex state at all times. This state includes variables such as cursor position, foreground/background color, attributes to apply to the next displayed character, and so on. The layers feature adds more variables to the state, and these variables are required to be stored with DECSC (ESC 7) and restored with DECRC (ESC 8). The new variables are listed below:
| Mnemonic | Description | Default value |
|---|---|---|
| Z | Cursor position Z | 1 |
| MSL | Manipulate single layer | off / disabled |
| TFT | Text foreground transparent | false |
| TBT | Text background transparent | false |
A terminal with layers feature must support the standard VT100/VT102 sequences defined in their respective manuals.
A terminal with layer feature must support the following new sequences:
| Sequence | Command | Description |
|---|---|---|
| CSI ? z ; y ; x H | CUPZ | Move cursor to (x, y, z) |
| CSI ? z ; y ; x H | SLA | Set layer alpha |
| CSI ? 3 0 0 1 h | DECSET 3001 | Enable Manupulate Single Layer (MSL) |
| CSI ? 3 0 0 1 l | DECRST 3001 | Disable Manupulate Single Layer (MSL) |
| CSI ? l ; h ; w t | RSZCUBE | Resize cube to (layers, height, width) |
Default parameters and ranges are listed below:
| Command | Position / Variable | Default Value | Minumum | Maximum |
|---|---|---|---|---|
| CUPZ | 1 / z | 1 | 1 | # layers |
| CUPZ | 2 / y | 1 | 1 | # rows |
| CUPZ | 3 / x | 1 | 1 | # columns |
| SLA | 1 / alpha | 255 | 0 | 255 |
| RSZCUBE | 1 / l | 1 | 1 | varies |
| RSZCUBE | 2 / h | 80 | 1 | varies |
| RSZCUBE | 3 / w | 24 | 1 | varies |
The terminal must also support the following new queries:
| Query | Response | Description |
|---|---|---|
| CSI ? 1 0 0 n | CSI ? z ; y ; x n | Report cursor Z, Y, X position |
| CSI ? 1 8 t | CSI ? 8 ; l ; h ; w t | Report the text area cube layers, height, width |
The terminal must support the following new Set Graphics Rendition (SGR) character attributes commands:
| SGR Parameter | Description |
|---|---|
| 230 | Set text foreground color to transparent |
| 239 | Set text foreground color to solid (opaque) |
| 240 | Set text background color to transparent |
| 249 | Set text background color to solid (opaque) |
No additional error reporting is provided for layer feature.
A terminal with layer feature will display its Cells such that the screen will appear as if it was rendered in the manner of the pseudo-code below:
``` 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:
if tile at (X, Y, Z) background color is solid:
draw rectangle of background color with layer alpha
if tile at (X, Y, Z) foreground color is solid:
if tile at (X, Y, Z) is glyph:
draw glyph with foreground color with layer alpha
else
draw pixel data of tile as red/green/blue/alpha pixels with
layer alpha
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.
Sequences that insert characters/lines, delete characters/lines, or modify larger regions are changed to act upon multiple layers as defined below. By default, MSL (Modify All Layers) is off/unset, and Z is 1, so if the application never changes MSL or Z then these sequences will produce the same visible output as a terminal without layer support.
A terminal is not required to support all of these sequences; however, for those sequences it does support, if it supports the layers feature then the sequences must behave as shown below:
| Sequence | Command | Additional behavior |
|---|---|---|
| BS (0x08) | Backspace | Only current layer affected if MSL=on |
| DEL (0x7F) | Delete | Only current layer affected if MSL=on |
| IND (0x84) | Index | Only current layer affected if MSL=on |
| RI (0x8D | Reverse Index | Only current layer affected if MSL=on |
| ESC # 3 | DECDHL | Cells on all layers always affected |
| ESC # 4 | DECDHL | Cells on all layers always affected |
| ESC # 5 | DECSWL | Cells on all layers always affected |
| ESC # 6 | DECDWL | Cells on all layers always affected |
| ESC # 8 | DECALN | All layers > 1 cleared; Z, MSL, TFT, TBT reset to default |
| ESC 7 | DECSC | Also store Z, MSL, TFT, TBT |
| ESC 8 | DECRC | Also restore Z, MSL, TFT, TBT |
| ESC c | RIS | All layers > 1 cleared; Z, MSL, TFT, TBT reset to default |
| CSI @ | ICH | Only current layer affected if MSL=on |
| CSI J | ED | Only current layer affected if MSL=on |
| CSI K | EL | Only current layer affected if MSL=on |
| CSI ? K | DECSEL | Only current layer affected if MSL=on |
| CSI L | IL | Only current layer affected if MSL=on |
| CSI M | DL | Only current layer affected if MSL=on |
| CSI X | ECH | Only current layer affected if MSL=on |
| CSI M | DL | Only current layer affected if MSL=on |
| CSI P | DCH | Only current layer affected if MSL=on |
| CSI R | DECSTBM | Cells on all layers always affected |
| CSI $ t | DECARA | Only current layer affected if MSL=on |
| CSI $ v | DECCRA | Only current layer affected if MSL=on |
| CSI x | DECSACE | Cells on all layers always affected |
| CSI $ x | DECFRA | Only current layer affected if MSL=on |
| CSI $ z | DECERA | Only current layer affected if MSL=on |
The VT52 sub-mode commands:
| Sequence | Command | Additional behavior |
|---|---|---|
| ESC J | ED | Only current layer affected if MSL=on |
| ESC K | EL | Only current layer affected if MSL=on |
Layers are inteded to provide a means for multiplexers to pass on the job of multimedia support to the “outer” or host terminal. The proposed mechanics of that is outlined in the pseudo-code below:
``` for each inner terminal in descending order from maxZ to minZ:
emit CUPZ(inner terminal Z, inner terminal Y, inner terminal X)
draw inner terminal text with standard VT100/VT102/xterm sequences
for each multimedia sequence emitted by the inner terminal: emit CUP(inner terminal Y, inner terminal X) emit multimedia sequences to outer terminal next multimedia sequence
decrease Z by 1 next inner terminal ```
The method above may not be effective for complex multi-terminal screen layouts, but is hoped to work well for many simple cases.
xterm control sequences:
ECMA-48: