jexer/docs/images.md

Terminal Emulator Multimedia Standard - Proposed Design

Version: 1

Purpose

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:

1. A method to transfer multimedia data for immediate display within the screen cell grid ("Direct Multimedia").

2. A method to transfer multimedia data to a terminal-managed cache, and later display that data within the screen cell grid ("Cached Multimedia").

3. 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.

Acknowledgements

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

Design Goals - Core

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.

  • All features must be available even if the only means of communication between the application and terminal is control sequences (e.g. no shared disk, no shared memory, no shared DOM, etc.).

• 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.

Design Goals - Secondary

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.

Out Of Scope

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 Cache Management. This standard defines a means for applications and terminals to communicate around cached multimedia items, but terminals are free to implement whatever cache management strategies they deem fit.

• Reliable Transport. This standard defines a two-way command/response protocol that may get out of order on unreliable channels such as 3-wire RS232. Applictions that require reliable transport on unreliable links may choose to use one of the many successful standards available for this purpose.

Definitions

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.

All Features - Detection

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

Direct Multimedia - Summary

Non-text data (multimedia) can be sent to the terminal for immediate display in a rectangular (single-layer) region of text Cells. Multimedia data is transmitted to the terminal using one of two wire formats described later in this document.

Setting a Cell to multimedia is a destructive operation: the Cell's original text is lost. Multimedia pixels will not overlap rendered text in the same Cell. To achieve pixels overlaid on text, the layers feature can be used.

Setting any part of a multi-Cell Tile to multimedia also "breaks up" the Tile into a range of single Cells. In other words, multimedia can only be carried by a Cell, not a Tile.

The pixels of a multimedia Cell are assigned to the Cell's foreground; multimedia Cells have no background. If a terminal supports the layers feature, setting a multimedia Cell's foreground transparency to true/enabled causes that Cell to not be displayed at all; setting its background transparency to either true/enabled or false/disabled has no visible effect.

The pixels of multimedia Cells can come from two sources:

1. The application can generate pixels and send them to the terminal for display at the current cursor position.

2. The application can specify a source for the multimedia and the terminal will generate the pixels for display at the current cursor position.

Direct Multimedia - Required Support For Existing Sequences

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

Direct Multimedia - New Sequences

A terminal with direct multimedia feature must support the following new sequences:

Sequence	Command	Description
OSC 1 3 3 8 ; s i x e l : {data} BEL	SIXEL	Display sixel at (x, y)
OSC 1 3 3 8 ; s i x e l : {data} ST	SIXEL	Display sixel at (x, y)
OSC 1 3 3 8 ; F i l e = {args} : {data} BEL	DMDISPLAY	Display media at (x, y)
OSC 1 3 3 8 ; F i l e = {args} : {data} ST	DMDISPLAY	Display media at (x, y)
CSI ? 3 0 0 0 h	DECSET 3000	Enable SCRCHANGE notification
CSI ? 3 0 0 0 l	DECRST 3000	Disable SCRCHANGE notification
OSC 1 3 3 9 ; Pe ; {args} ST	DMRESP	Terminal response to DMDISPLAY
CSI ? 3 0 0 1 h	DECSET 3001	Enable DMDISPLAY responses
CSI ? 3 0 0 1 l	DECRST 3001	Disable DMDISPLAY responses

If SCRCHANGE is set/enabled, then the terminal will send the "CSI 6 ; cellHeight ; cellWidth t" when the font size has changed, and "CSI 8 ; rows ; columns t" when the number of rows/columns on the screen has changed.

For the SIXEL command:

• The {data} is a sixel sequence as described in the VT330/340 Programmer Reference Manual, Chapter 14, available online at: http://vt100.net/docs/vt3xx-gp/chapter14.html . The {data} is the "P1 ; P2 ; P3 ; q s..s" portion of the Device Control String, i.e. a complete sixel sequence minus the leading DCS and trailing ST.

• The sixel image is processed as shown below. Note that this behavior is equivalent to Sixel Scrolling mode enabled.

  • The sixel active position starts at the upper-left corner of the text cursor position.

  • The screen is scrolled up if the image overflows into the bottom text row.

  • Pixels that would be drawn to the right of the visible region on screen are discarded.

  • The cursor's final position is on the same column as the starting cursor position, and on the row immediately below the image.

For the DMDISPLAY command:

• The {args} is a set of key-value pairs (each pair separated by semicolon (';')), followed by a colon (':'), followed by a base-64 encoded string ({data}).

• A key can be any alpha-numeric ASCII string ('0' - '9', 'A' - 'Z', 'a' - 'z').

• A value is any printable ASCII string not containing whitespace, colon, or semicolon ('!' - '9', '<' - '~').

• Any alpha-numeric key may be specified. A key that is not supported by the terminal is ignored without error.

• The multimedia pixels are processed as shown below.

  • The pixel are drawn starting at the upper-left corner of the text cursor position.

  • If scroll is specified as 1 (enabled), then:

    a. The screen is scrolled up if the image overflows into the bottom text row.

    b. The cursor's final position is on the same column as the starting cursor position, and on the row immediately below the image.

  • If scroll is omitted or specified as 0 (disabled), then:

    a. The screen is never scrolled.

    b. Pixels that would be drawn below the visible region on screen are discarded.

    c. The cursor's final position is at the same column and row as the starting cursor position, i.e. the cursor does not move at all.

  • Pixels that would be drawn to the right of the visible region on screen are discarded.

The keys for the key-value pairs that must be supported by the terminal are listed below:

Key	Default Value	Description
type	"image/rgb"	mime-type describing data field
url	""	If set, a location containing the media data
width	1	Number of Cells or pixels wide to display in
height	1	Number of Cells or pixels high to display in
scale	"none"	Scale/zoom option, see below
align	"nw"	Align image to edge option, see below
sourceX	0	Media source X position to display
sourceY	0	Media source Y position to display
sourceWidth	"auto"	Media width in pixels to display
sourceHeight	"auto"	Media height in pixels to display
scroll	1	If 1, scroll the display if needed

A terminal may support additional keys. If a key is specified but not supported by the terminal, then it is ignored without error.

The "type" value is a mime-type string describing the format of the base64-encoded binary data. The terminal must support at mimunum these mime-types:

Type String	Description
"image/rgb"	Big-endian-encoded 24-bit red, green, blue values
"image/rgba"	Big-endian-encoded 32-bit red, green, blue, alpha values
"image/png"	PNG file data as described by (reference to PNG format)

A terminal may support additional types. An application can detect terminal support for a format by: enabling terminal responses (DECSET 3001), sending a DMDISPLAY command, and examining the terminal's response sequence for success or error.

The "url" value is a RFC-XXXX defined Universal Resource Located, encoded in RFC-XXXX form as a printable ASCII string not containing: whitespace, colon (':'), semicolon (';'), or equals ('=').

A terminal is not required to support any URLs.

The "width" and "height" values can take the following forms:

Value	Meaning
N (a positive integer)	Number of Cells
Npx (positive integer + "px")	Number of pixels
N% (positive integer + "%")	Percent of screen width or height
"auto"	Number of pixels as defined by the multimedia data

The "scale" value can take the following values:

Value	Meaning
"none"	No scaling along either axis.
"scale"	Stretch image, preserving aspect ratio, to maximum size in the target area without cropping
"stretch"	Stretch along both axes, distorting aspect ratio, to fill the target area
"crop"	Stretch along both axes, preserving aspect ration, to completely fill the target area, cropping pixels that will not fit

The "align" value can take the following values:

Value	Meaning
"nw"	Media is placed at the top-left corner (northwest)
"n"	Media is placed on the top and centered horizontally (north)
"ne"	Media is placed at the top-right corner (northest)
"w"	Media is placed on the left and centered vertically (west)
"c"	Media is centered in the target area (center)
"e"	Media is placed on the right and centered vertically (east)
"sw"	Media is placed on the bottom-left corner (southwest)
"s"	Media is placed on the bottom and centered horizontally (south)
"se"	Media is placed on the bottom-right corner (southeast)

"sourceX", "sourceY", "sourceWidth", and "sourceHeight" define the rectangle of pixels from the media that will be displayed on the screen. The ranges for these values is shown below:

Key	Minimum Value	Maximum Value	Default Value
sourceX	0	Media's full width - 1	0
sourceY	0	Media's full height - 1	0
sourceWidth	1	Media's full width - sourceX	"auto"
sourceHeight	1	Media's full height - sourceY	"auto"

If any of these values are specified and outside the range, no image is displayed, and the cursor does not move. "sourceWidth" and "sourceHeight" can be "auto", which means use the maximum available width/height (given sourceX/sourceY) from the media's inherent dimensions.

Direct Multimedia - Terminal Responses / Error Handling

If DMDISPLAY reponses are enabled, then a terminal will respond to the DMDISPLAY display with DMRESP. DMRESP responses must be sent in the same sequential order as the DMDISPLAY commands they are responses to: the terminal may not re-order responses.

No provision is made for reliable delivery. On unreliable links (example: 3-wire RS232), the DMDISPLAY and DMRESP command/response sequence may get out of order.

The format of DMRESP is:

• Pe - a non-negative integer error code.

• The {args} is a set of key-value pairs (each pair separated by semicolon (';')).

• A key can be any alpha-numeric ASCII string ('0' - '9', 'A' - 'Z', 'a' - 'z').

• A value is any printable ASCII string not containing whitespace, colon, or semicolon ('!' - '9', '<' - '~').

The Pe error codes are defined as:

Value	Meaning	{args} containts
0	No error occurred, i.e. success	nothing
1	Unsupported "type"	"type" value that was incorrect
2	Invalid value - no media displayed	"key" that was incorrect
3	Unsupported key - media displayed	"key" that unsupported
4	Insufficient memory	nothing
5	Other error - no media displayed	nothing
6	Other - media displayed	nothing
7	Conflicting keys - no media displayed	nothing
8	RESERVED FOR FUTURE USE	RESERVED FOR FUTURE USE

Additional Pe error codes may be returned; any Pe value except 0, 3, and 6 must mean that the media was not displayed, and the cursor was not moved.

If both "type" and "url" are set, no media is diaplyed, the cursor is not moved, and the DMRESP error code is 7.

Direct Multimedia - Examples

Cached Multimedia - Summary

Non-text data (multimedia) can be sent to the terminal for later display in a rectangular (single-layer) region of text Cells. Multimedia data is transmitted to the terminal using the CMCACHE command described below, and displayed on screen using the CMDISPLAY command. A single CMCACHE command can support many CMDISPLAY commands.

Upon display, setting a Cell to multimedia is a destructive operation: the Cell's original text is lost. Multimedia pixels will not overlap rendered text in the same Cell. To achieve pixels overlaid on text, the layers feature can be used.

Setting any part of a multi-Cell Tile to multimedia also "breaks up" the Tile into a range of single Cells. In other words, multimedia can only be carried by a Cell, not a Tile.

The pixels of a multimedia Cell are assigned to the Cell's foreground; multimedia Cells have no background. If a terminal supports the layers feature, setting a multimedia Cell's foreground transparency to true/enabled causes that Cell to not be displayed at all; setting its background transparency to either true/enabled or false/disabled has no visible effect.

The pixels of multimedia Cells can come from two sources:

1. The application can generate pixels and send them to the terminal for display at the current cursor position.

2. The application can specify a source for the multimedia and the terminal will generate the pixels for display at the current cursor position.

Cached Multimedia - Cache/Memory Management

The terminal manages a cache of multimedia data on behalf of one or more applications. Applications request media be stored in the cache, and if successful the terminal provides an identification number that applications must use to request display from the cache to the screen.

The amount of memory and retention/eviction strategy for the cache is wholly managed by the terminal, with the following restrictions:

• The terminal may not remove items from the cache that have any portion being actively displayed on the primary or alternate screens.

• The terminal must respond to every CMCACHE command with a new unique ID.

The scrollback buffer is permitted, and recommended, to contain only a few (or zero) multimedia images. Terminals should consider retaining only the last 2-5 screens' worth of pixel data in the scrollback buffer.

Cached Multimedia - Required Support For Existing Sequences

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

Cached Multimedia - New Sequences

A terminal with cached multimedia feature must support the following new sequences:

Sequence	Command	Description
CSI ? 3 0 0 0 h	DECSET 3000	Enable SCRCHANGE notification
CSI ? 3 0 0 0 l	DECRST 3000	Disable SCRCHANGE notification
OSC 1 3 4 0 ; F i l e = {args} : {data} BEL	CMCACHE	Display media at (x, y)
OSC 1 3 4 1 ; Pi ; {args} ST	CMDISPLAY	Display media at (x, y)
OSC 1 3 4 2 ; Pi ; Pe ; {args} ST	CMCRESP	Terminal response to CMCACHE
OSC 1 3 4 3 ; Pi ; Pe ; {args} ST	CMDRESP	Terminal response to CMDISPLAY

If SCRCHANGE is set/enabled, then the terminal will send the "CSI 6 ; cellHeight ; cellWidth t" when the font size has changed, and "CSI 8 ; rows ; columns t" when the number of rows/columns on the screen changes.

Cached Multimedia - CMCACHE

For the CMCACHE command:

• The {args} is a set of key-value pairs (each pair separated by semicolon (';')), followed by a colon (':'), followed by a base-64 encoded string ({data}).

• A key can be any alpha-numeric ASCII string ('0' - '9', 'A' - 'Z', 'a' - 'z').

• A value is any printable ASCII string not containing whitespace, colon, or semicolon ('!' - '9', '<' - '~').

The keys for the key-value pairs that must be supported by the terminal are listed below:

Key	Default Value	Description
type	"image/rgb"	mime-type describing data field
url	""	If set, a location containing the media data

The "type" value is a mime-type string describing the format of the base64-encoded binary data. The terminal must support at mimunum these mime-types:

Type String	Description
"image/rgb"	Big-endian-encoded 24-bit red, green, blue values
"image/rgba"	Big-endian-encoded 32-bit red, green, blue, alpha values
"image/png"	PNG file data as described by (reference to PNG format)

A terminal may support additional types. An application can detect terminal support for a format by: sending a CMCACHE command, and examining the terminal's CMCRESP sequence for success or error.

The "url" value is a RFC-XXXX defined Universal Resource Located, encoded in RFC-XXXX form as a printable ASCII string not containing: whitespace, colon (':'), semicolon (';'), or equals ('=').

A terminal is not required to support any URLs.

Cached Multimedia - CMDISPLAY

For the CMDISPLAY command:

• Pi - a non-negative integer media ID that was returned by a CMCRESP response to a previous CMCACHE command.

• The {args} is a set of key-value pairs (each pair separated by semicolon (';')), followed by a colon (':'), followed by a base-64 encoded string.

• A key can be any alpha-numeric ASCII string ('0' - '9', 'A' - 'Z', 'a' - 'z').

• A value is any printable ASCII string not containing whitespace, colon, or semicolon ('!' - '9', '<' - '~').

• Any alpha-numeric key may be specified. A key that is not supported by the terminal is ignored without error.

• The multimedia pixels are processed as shown below.

  • The pixel are drawn starting at the upper-left corner of the text cursor position.

  • If scroll is specified as 1 (enabled), then:

    a. The screen is scrolled up if the image overflows into the bottom text row.

    b. The cursor's final position is on the same column as the starting cursor position, and on the row immediately below the image.

  • If scroll is omitted or specified as 0 (disabled), then:

    a. The screen is never scrolled.

    b. Pixels that would be drawn below the visible region on screen are discarded.

    c. The cursor's final position is at the same column and row as the starting cursor position, i.e. the cursor does not move at all.

  • Pixels that would be drawn to the right of the visible region on screen are discarded.

The keys for the key-value pairs that must be supported by the terminal are listed below:

Key	Default Value	Description
width	1	Number of Cells or pixels wide to display in
height	1	Number of Cells or pixels high to display in
scale	"none"	Scale/zoom option, see below
align	"nw"	Align image to edge option, see below
sourceX	0	Media source X position to display
sourceY	0	Media source Y position to display
sourceWidth	"auto"	Media width in pixels to display
sourceHeight	"auto"	Media height in pixels to display
scroll	1	If 1, scroll the display if needed

A terminal may support additional keys. If a key is specified but not supported by the terminal, then it is ignored without error.

The "width" and "height" values can take the following forms:

Value	Meaning
N (a positive integer)	Number of Cells
Npx (positive integer + "px")	Number of pixels
N% (positive integer + "%")	Percent of screen width or height
"auto"	Number of pixels as defined by the multimedia data

The "scale" value can take the following values:

Value	Meaning
"none"	No scaling along either axis.
"scale"	Stretch image, preserving aspect ratio, to maximum size in the target area without cropping
"stretch"	Stretch along both axes, distorting aspect ratio, to fill the target area
"crop"	Stretch along both axes, preserving aspect ration, to completely fill the target area, cropping pixels that will not fit

The "align" value can take the following values:

Value	Meaning
"nw"	Media is placed at the top-left corner (northwest)
"n"	Media is placed on the top and centered horizontally (north)
"ne"	Media is placed at the top-right corner (northest)
"w"	Media is placed on the left and centered vertically (west)
"c"	Media is centered in the target area (center)
"e"	Media is placed on the right and centered vertically (east)
"sw"	Media is placed on the bottom-left corner (southwest)
"s"	Media is placed on the bottom and centered horizontally (south)
"se"	Media is placed on the bottom-right corner (southeast)

"sourceX", "sourceY", "sourceWidth", and "sourceHeight" define the rectangle of pixels from the media that will be displayed on the screen. The ranges for these values is shown below:

Key	Minimum Value	Maximum Value	Default Value
sourceX	0	Media's full width - 1	0
sourceY	0	Media's full height - 1	0
sourceWidth	1	Media's full width - sourceX	"auto"
sourceHeight	1	Media's full height - sourceY	"auto"

If any of these values are specified and outside the range, no image is displayed, and the cursor does not move. "sourceWidth" and "sourceHeight" can be "auto", which means use the maximum available width/height (given sourceX/sourceY) from the media's inherent dimensions.

Cached Multimedia - Error Handling

A terminal will always respond to the CMCACHE command with CMCRESP, and to the CMDISPLAY command with CMDRESP. Responses must be sent in the same sequential order as the CMCACHE/CMDISPLAY commands they are responses to: the terminal may not re-order responses.

No provision is made for reliable delivery. On unreliable links (example: 3-wire RS232), the command/response sequence may get out of order.

Cached Multimedia - Error Handling - CMCRESP

The format of CMCRESP is:

• Pi - a non-negative integer media ID. The terminal will generate a new ID for every image successfully loaded into the cache. The application must use this ID for CMDISPLAY commands.

• Pe - a non-negative integer error code.

• The {args} is a set of key-value pairs (each pair separated by semicolon (';')).

• A key can be any alpha-numeric ASCII string ('0' - '9', 'A' - 'Z', 'a' - 'z').

• A value is any printable ASCII string not containing whitespace, colon, or semicolon ('!' - '9', '<' - '~').

The Pe error codes are defined as:

Value	Meaning	{args} containts
0	No error occurred, i.e. success	nothing
1	Unsupported "type"	"type" value that was incorrect
2	Invalid value - no media stored	"key" that was incorrect
3	Unsupported key - media stored	"key" that unsupported
4	Insufficient memory - no media stored	nothing
5	Other error - no media stored	nothing
6	Other - media stored	nothing
7	Conflicting keys - no media stored	nothing
8	RESERVED FOR FUTURE USE	RESERVED FOR FUTURE USE

Additional Pe error codes may be returned; any Pe value except 0, 3, and 6 must mean that the media was not stored in the cache.

If both "type" and "url" are set, no media is diaplyed, the cursor is not moved, and the CMCRESP error code is 7.

Cached Multimedia - Error Handling - CMDRESP

The format of CMDRESP is:

• Pi - a non-negative integer media ID.

• Pe - a non-negative integer error code.

• The {args} is a set of key-value pairs (each pair separated by semicolon (';')).

• A key can be any alpha-numeric ASCII string ('0' - '9', 'A' - 'Z', 'a' - 'z').

• A value is any printable ASCII string not containing whitespace, colon, or semicolon ('!' - '9', '<' - '~').

The Pe error codes are defined as:

Value	Meaning	{args} containts
0	No error occurred, i.e. success	nothing
1	RESERVED FOR FUTURE USE	RESERVED FOR FUTURE USE
2	Invalid value - no media displayed	"key" that was incorrect
3	Unsupported key - media displayed	"key" that unsupported
4	Insufficient memory - no media displayed	nothing
5	Other error - no media displayed	nothing
6	Other - media displayed	nothing
7	RESERVED FOR FUTURE USE	RESERVED FOR FUTURE USE
8	Media was evicted - no media displayed	nothing

Additional Pe error codes may be returned; any Pe value except 0, 3, and 6 must mean that the media was not displayed.

Cached Multimedia - Examples

Layers - Summary

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 can act on more than one Cell are defined such to act on all layers simultaneously by default; most of these operations can also be set to act only on the current layer.

Layers - Number of Layers

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.

Layers - Terminal State

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

Layers - Required Support For Existing Sequences

A terminal with layers feature must support the standard VT100/VT102 sequences defined in their respective manuals.

Layers - New Sequences

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 2 2 5 ; 1 ; Pa t	SLA	Set layer alpha
CSI ? 3 0 0 2 h	DECSET 3002	Enable Manupulate Single Layer (MSL)
CSI ? 3 0 0 2 l	DECRST 3002	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	24	1	varies
RSZCUBE	3 / w	80	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
2 3 0	Set text foreground color to transparent
2 3 9	Set text foreground color to solid (opaque)
2 4 0	Set text background color to transparent
2 4 9	Set text background color to solid (opaque)

Layers - Error Handling

No additional error reporting is provided for layer feature.

Layers - Rendering

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.

Layers - Integration With Existing Sequences

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 (Manipulate Single Layer) 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

(( TODO: add many more to the above table... ))

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 - Use With Multiplexers

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.

Layers - Examples

References

• xterm control sequences:

• ECMA-48: