The text sizing protocol - kitty
Description: Classically, because the terminal is a grid of equally sized characters, only a single text size was supported in terminals, with one minor exception, some characters were allowed to be rendered in...

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# The text sizing protocol

Added in version 0.40.0.

Classically, because the terminal is a grid of equally sized characters, only a single text size was supported in terminals, with one minor exception, some characters were allowed to be rendered in two cells, to accommodate East Asian square aspect ratio characters and Emoji. Here, by single text size we mean the font size of all text on the screen is the same.

This protocol allows text to be displayed in the terminal in different sizes both larger and smaller than the base text. It also solves the long standing problem of robustly determining the width (in cells) a character should have. Applications can interleave text of different sizes on the screen allowing for typographic niceties like headlines, superscripts, etc.

Note that this protocol is fully backwards compatible, terminals that implement it will continue to work just the same with applications that do not use it. Because of this, it is not fully flexible in the font sizes it allows, as it still has to work with the character cell grid based fundamental nature of the terminal. Public discussion of this protocol is here.

## Quickstart

Using this protocol to display different sized text is very simple, let’s illustrate with a few examples to give us a flavor:

    printf "\e]66;s=2;Double sized text\a\n\n"
    printf "\e]66;s=3;Triple sized text\a\n\n\n"
    printf "\e]66;n=1:d=2;Half sized text\a\n"

Note that the last example, of half sized text, has half height characters, but they still each take one cell, this can be fixed with a little more work:

    printf "\e]66;n=1:d=2:w=1;Ha\a\e]66;n=1:d=2:w=1;lf\a\n"

The `w=1` mechanism allows the program to tell the terminal what width the text should take. This not only fixes using smaller text but also solves the long standing terminal ecosystem bugs caused by the client program not knowing how many cells the terminal will render some text in.

## The escape code

There is a single escape code used by this protocol. It is sent by client programs to the terminal emulator to tell it to render the specified text at the specified size. It is an `OSC` code of the form:

    <OSC> 66 ; metadata ; text <terminator>

Here, `OSC` is the bytes `ESC ] (0x1b 0x5b)`. The `metadata` is a colon separated list of `key=value` pairs. The final part of the escape code is the text which is simply plain text encoded as Escape code safe UTF-8, the text must be no longer than `4096` bytes. Longer strings than that must be broken up into multiple escape codes. Spaces in this definition are for clarity only and should be ignored. The `terminator` is either the byte `BEL (0x7)` or the bytes `ESC ST (0x1b 0x5c)`.

There are only a handful of metadata keys, defined in the table below:

The text sizing metadata keys
Key | Value | Default | Description
--- | --- | --- | ---
s | Integer from 1 to 7 | 1 | The overall scale, the text will be rendered in a block of `s * w` by `s` cells
w | Integer from 0 to 7 | 0 | The width, in cells, in which the text should be rendered. When zero, the terminal should calculate the width as it would for normal text, splitting it up into scaled cells.
n | Integer from 0 to 15 | 0 | The numerator for the fractional scale.
d | Integer from 0 to 15 | 0 | The denominator for the fractional scale. Must be `> n` when non-zero.
v | Integer from 0 to 2 | 0 | The vertical alignment to use for fractionally scaled text. `0` - top, `1` - bottom, `2` - centered
h | Integer from 0 to 2 | 0 | The horizontal alignment to use for fractionally scaled text. `0` - left, `1` - right, `2` - centered

## How it works

This protocol works by allowing the client program to tell the terminal to render text in multiple cells. The terminal can then adjust the actual font size used to render the specified text as appropriate for the specified space.

The space to render is controlled by four metadata keys, `s (scale)`, `w (width)`, `n (numerator)` and `d (denominator)`. The most important are the `s` and `w` keys. The text will be rendered in a block of `s * w` by `s` cells. A special case is `w=0` (the default), which means the terminal splits up the text into cells as it would normally without this protocol, but now each cell is an `s by s` block of cells instead. So, for example, if the text is `abc` and `s=2` the terminal would normally split it into three cells:

    │a│b│c│

But, because `s=2` it instead gets split as:

    │a░│b░│c░│
    │░░│░░│░░│

The terminal multiplies the font size by `s` when rendering these characters and thus ends up rendering text at twice the base size.

When `w` is a non-zero value, it specifies the width in scaled cells of the following text. Note that all the text in that escape code must be rendered in `s * w` cells. When both `s` and `w` are present, the resulting multicell contains all the text in the escape code rendered in a grid of `(s * w, s)` cells, i.e. the multicell is `s*w` cells wide and `s` cells high.

If the text does not fit, the terminal is free to do whatever it feels is best, including truncating the text or downsizing the font size when rendering it. It is up to client applications to use the `w` key wisely and not try to render too much text in too few cells. When sending a string of text with non zero `w` to the terminal emulator, the way to do it is to split up the text into chunks that fit in `w` cells and send one escape code per chunk. So for the string: `cool-🐈` the actual escape codes would be (ignoring the header and trailers):

    w=1;c w=1;o w=1;o w=1;l w=1;- w=2:🐈

Note, in particular, how the last character, the cat emoji, `🐈` has `w=2`. In practice client applications can assume that terminal emulators get the width of all ASCII characters correct and use the `w=0` form for efficient transmission, so that the above becomes:

    cool- w=2:🐈

The use of non-zero `w` should mainly be restricted to non-ASCII characters and when using fractional scaling, as described below.

Note

Text sizes specified by scale are relative to the base font size, thus if the base font size is changed, these sizes are changed as well. So if the terminal emulator is using a base font size of `11pt`, then `s=2` will be rendered in approximately `22pt` (approx. because the terminal may need to slightly adjust font size to ensure it fits as not all fonts scale sizes linearly). If the user changes the base font size of the terminal emulator to `12pt` then the scaled font size becomes `~24pt` and so on.

### Fractional scaling

Using the main scale parameter (`s`) gives us only 7 font sizes. Fortunately, this protocol allows specifying fractional scaling, fractional scaling is applied on top of the main scale specified by `s`. It allows niceties like:

- Normal sized text but with half a line of blank space above and half a line below (`s=2:n=1:d=2:v=2`)

- Superscripts (`n=1:d=2`)

- Subscripts (`n=1:d=2:v=1`)

- …

The fractional scale does not affect the number of cells the text occupies, instead, it just adjusts the rendered font size within those cells. The fraction is specified using an integer numerator and denominator (`n` and `d`). In addition, by using the `v` key one can vertically align the fractionally scaled text at top, bottom or middle. Similarly, the `h` key does horizontal alignment — left, right or centered.

When using fractional scaling one often wants to fit more than a single character per cell. To accommodate that, there is the `w` key. This specifies the number of cells in which to render the text. For example, for a superscript one would typically split the string into pairs of characters and use the following for each pair:

    OSC 66 ; n=1:d=2:w=1 ; ab <terminator>
    ... repeat for each pair of characters

## Fixing the character width issue for the terminal ecosystem

Terminals create user interfaces using text displayed in a cell grid. For terminal software that creates sophisticated user interfaces it is particularly important that the client program running in the terminal and the terminal itself agree on how many cells a particular string should be rendered in. If the two disagree, then the entire user interface can be broken, leading to catastrophic failures.

Fundamentally, this is a co-ordination problem. Both the client program and the terminal have to somehow share the same database of character properties and the same algorithm for computing string lengths in cells based on that shared database. Sadly, there is no such shared database in reality. The closest we have is the Unicode standard. Unfortunately, the Unicode standard has a new version almost every year and actually changes the width assigned to some characters in different versions. Furthermore, to actually get the “correct” width for a string using that standard one has to do grapheme segmentation, which is a complex algorithm, specified below. Expecting all terminals and all terminal programs to have both up-to-date character databases and a bug free implementation of this algorithm is not realistic.

So instead, this protocol solves the issue robustly by removing the co-ordination problem and putting only one actor in charge of determining string width. The client becomes responsible for doing whatever level of grapheme segmentation it is comfortable with using whatever Unicode database is at its disposal and then it can transmit the segmented string to the terminal with the appropriate `w` values so that the terminal renders the text in the exact number of cells the client expects.

Note

It is possible for a terminal to implement only the width part of this spec and ignore the scale part. This escape code works with only the w key as well, as a means of specifying how many cells each piece of text occupies. In such cases `s` defaults to 1. See the section on Detecting if the terminal supports this protocol on how client applications can query for terminal emulator support.

## Wrapping and overwriting behavior

If the multicell block (`s * w by s` cells) is larger than the screen size in either dimension, the terminal must discard the character. Note that in particular this means that resizing a terminal screen so that it is too small to fit a multicell character can cause the character to be lost.

When drawing a multicell character, if wrapping is enabled (DECAWM is set) and the character’s width (`s * w`) does not fit on the current line, the cursor is moved to the start of the next line and the character is drawn there. If wrapping is disabled and the character’s width does not fit on the current line, the cursor is moved back as far as needed to fit `s * w` cells and then the character is drawn, following the overwriting rules described below.

When drawing text either normal text or text specified via this escape code, and this text would overwrite an existing multicell character, the following rules must be followed, in decreasing order of precedence:

1. If the text is a combining character it is added to the existing multicell character

2. If the text will overwrite the top-left cell of the multicell character, the entire multicell character must be erased

3. If the text will overwrite any cell in the topmost row of the multicell character, the entire multicell character must be replaced by spaces (this rule is present for backwards compatibility with how overwriting works for wide characters)

4. If the text will overwrite cells from a row after the first row, then cursor should be moved past the cells of the multicell character on that row and only then the text should be written. Note that this behavior is independent of the value of DECAWM. This is done for simplicity of implementation.

The skipping behavior of the last rule can be complex requiring the terminal to skip over lots of cells, but it is needed to allow wrapping in the presence of multicell characters that extend over more than a single line.

## Detecting if the terminal supports this protocol

To detect support for this protocol use the CPR (Cursor Position Report) escape code. Send a `CPR` followed by `\e]66;w=2; \a` which will draw a space character in two cells, followed by another `CPR`. Then send `\e]66;s=2; \a` which will draw a space in a `2 by 2` block of cells, followed by another `CPR`.

Then wait for the three responses from the terminal to the three CPR queries. If the cursor position in the three responses is the same, the terminal does not support this protocol at all, if the second response has a different cursor position then the width part is supported and if the third response has yet another position, the scale part is supported.

## Interaction with other terminal controls

This protocol does not change the character grid based nature of the terminal. Most terminal controls assume one character per cell so it is important to specify how these controls interact with the multicell characters created by this protocol.

### Cursor movement

Cursor movement is unaffected by multicell characters, all cursor movement commands move the cursor position by single cell increments, as has always been the case for terminals. This means that the cursor can be placed at any individual single cell inside a larger multicell character.

When a multicell character is created using this protocol, the cursor moves s * w cells to the right, in the same row it was in.

Terminals should display a large cursor covering the entire multicell block when the actual cursor position is on any cell within the block. Block cursors cover all the cells of the multicell character, bar cursors appear in all the cells in the first column of the character and so on.

### Editing controls

There are many controls used to edit existing screen content such as inserting characters, deleting characters and lines, etc. These were all originally specified for the one character per cell paradigm. Here we specify their interactions with multicell characters.

Insert characters (`CSI @` aka `ICH`)

  When inserting `n` characters at cursor position `x, y` all characters after `x` on line `y` are supposed to be right shifted. This means that any multi-line character that intersects with the cells on line `y` at `x` and beyond must be erased. Any single line multicell character that is split by the cells at `x` and `x + n - 1` must also be erased.

Delete characters (`CSI P` aka `DCH`)

  When deleting `n` characters at cursor position `x, y` all characters after `x` on line `y` are supposed to be left shifted. This means that any multi-line character that intersects with the cells on line `y` at `x` and beyond must be erased. Any single line multicell character that is split by the cells at `x` and `x + n - 1` must also be erased.

Erase characters (`CSI X` aka `ECH`)

  When erasing `n` characters at cursor position `x, y` the `n` cells starting at `x` are supposed to be cleared. This means that any multicell character that intersects with the `n` cells starting at `x` must be erased.

Erase display (`CSI J` aka `ED`)

  Any multicell character intersecting with the erased region of the screen must be erased. When using mode `22` the contents of the screen are first copied into the history, including all multicell characters.

Erase in line (`CSI K` aka `EL`)

  Works just like erase characters above. Any multicell character intersecting with the erased cells in the line is erased.

Insert lines (`CSI L` aka `IL`)

  When inserting `n` lines at cursor position `y` any multi-line characters that are split at the line `y` must be erased. A split happens when the second or subsequent row of the multi-line character is on the line `y`. The insertion causes `n` lines to be removed from the bottom of the screen, any multi-line characters are split at the bottom of the screen must be erased. A split is when any row of the multi-line character except the last row is on the last line of the screen after the insertion of `n` lines.

Delete lines (`CSI M` aka `DL`)

  When deleting `n` lines at cursor position `y` any multicell character that intersects the deleted lines must be erased.

## The algorithm for splitting text into cells

Note

kitty comes with a utility to test terminal compliance with this algorithm. Install kitty and run: `kitten __width_test__` in any terminal to test it. This uses tests published by the Unicode consortium, GraphemeBreakTest.txt.

Warning

This algorithm is under public discussion in #8533. Until that issue is closed, it is subject to change based on feedback from the community. Additionally, in the future if the Unicode standard changes in ways that affect this algorithm, it will be updated. Currently the algorithm is based on Unicode version 16.

Here, we specify how a terminal must split up text into cells, where a cell is a width one unit in the character grid the terminal displays.

The basis for the algorithm is the Grapheme segmentation algorithm from the Unicode standard. However, that algorithm alone is insufficient to fully specify text handling for terminals. The full algorithm is specified below. When a terminal receives a Unicode character:

1. First check if the character is an ASCII control code, and handle it appropriately. ASCII control codes are the characters less than 32 and the character 127 (DEL). The NUL character (0) must be discarded.

2. Next, check if the character is invalid, and if it is, discard it and finish processing. Invalid characters are characters with Unicode category `Cc or Cs` and 66 additional characters: `[0xfdd0, 0xfdef]`, `[0xfffe, 0x10ffff-1, 0x10000]` and `[0xffff, 0x10ffff, 0x10000]`.

3. Next, check if there is a previous cell before the current cursor position. This means either the cursor is at x > 0 in which case the previous cell is at x-1 on the same line, or the previous cell is the last cell of the previous line, provided there is no line break between the previous and current lines.

4. Next, calculate the width in cells of the received character, which can be 0, 1, or 2 depending on the character properties in the Unicode standard.

5. If there is no previous cell and the character width is zero, the character is discarded and processing of the character is finished.

6. If there is a previous cell, the Grapheme segmentation algorithm UAX29-C1-1 is used to determine if there is a grapheme boundary between the previous cell and the current character.

7. If there is no boundary the current character is added to the previous cell and processing of the character is finished. See the Unicode variation selectors section below for handling of Unicode Variation selectors.

8. If there is a boundary, but the width of the current character is zero it is added to the previous cell and processing is finished.

9. The character is added to the current cell and the cursor is moved forward (right) by either 1 or 2 cells depending on the width of the character.

It remains to specify how to calculate the width in cells of a Unicode character. To do this, characters are divided into various classes, as described by the rules below, in order of decreasing priority:

Note

Notation: `[start, stop, step]` means the integers from `start` to `stop` in increments of `step`. When the step is not specified, it defaults to one.

1. Regional indicators: 26 characters starting at `0x1F1E6`. These all have width 2

2. Doublewidth: Parse EastAsianWidth.txt from the Unicode standard. All characters marked `W` or `F` have width two. All characters in the following ranges have width two unless they are marked as `A` in `EastAsianWidth.txt`: `[0x3400, 0x4DBF], [0x4E00, 0x9FFF], [0xF900, 0xFAFF], [0x20000, 0x2FFFD], [0x30000, 0x3FFFD]`

3. Wide emoji: Parse emoji-sequences.txt from the Unicode standard. All `Basic_Emoji` have width two unless they are followed by `FE0F` in the file. The leading copdepoints in all `RGI_Emoji_Modifier_Sequence` and `RGI_Emoji_Tag_Sequence` have width two. All codepoints in `RGI_Emoji_Flag_Sequence` have width two.

4. Marks: These are all zero width characters. They are characters with Unicode categories whose first letter is `M` or `S`. Additionally, characters with Unicode category: `Cf`. Finally, they include all modifier codepoints from `RGI_Emoji_Modifier_Sequence` in the Wide emoji rule above.

5. All remaining codepoints have a width of one cell.

### Unicode variation selectors

There are two codepoints (`U+FE0E` and `U+FE0F`) that can actually alter the width of the previous codepoint. When adding a codepoint to the previous cell these have to be handled specially.

`U+FE0E` - Variation Selector 15

  When the previous cell has width two and the last character in the previous cell is one of the `Basic_Emoji` codepoints from the Wide emoji rule above that is not followed by `FEOF` then the width of the previous cell is decreased to one.

`U+FE0F` - Variation Selector 16

  When the previous cell has width one and the last character in the previous cell is one of the `Basic_Emoji` codepoints from the Wide emoji rule above that is followed by `FEOF` then the width of the previous cell is increased to two.

Note that the rule for `U+FE0E` is particularly problematic for terminals as it means that the width of a string cannot be determined without knowing the width of the screen it will be rendered on. This is because when there is only one cell left on the current line and a wide emoji is received it wraps onto the next line. If subsequently a `U+FE0E` is received, the emoji becomes one cell wide but it is not moved back to the previous line.

To avoid this issue, it is recommended applications detect when `U+FE0E` is present and in such cases use the width part of the text sizing protocol to control rendering.