Standard

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Introduction

This document intends to trace the guidelines of a User Interface (in short: UI) for editing bidirectional (in short: Bidi) text. It is assumed that the user enters text in logical sequence, and that the Unicode Bidi Algorithm (in short: UBA) is used to reorder the text for presentation. We assume that the readers of this document have a working knowledge of the UBA. The UBA is described in Unicode Technical Report 9 (see http://www.unicode.org/unicode/reports/tr9).

When designing these guidelines, the following objectives were set, in order of decreasing priority:

1. Prevent actions unexpected by the user, particularly when the action is destructive (erases one or more characters).
2. Make the interface efficient.
3. Keep the interface easy to implement.

Scope

This standard deals with BiDi aspects of text editing. Every attempt has been made to make sure that it does not impose one editor model on implementors. Different implementations differ in parameters, such as whether it is allowed for the caret to reside outside the buffer space, behavior when moving past the buffer, and many others. It is not the intent of this standard to impose homogeneous behavior, but rather to resolve the BiDi aspects of text editing. Wherever possible, such differences are simply not mentioned in this standard.

Sometimes, these differences carry BiDi implications. For example, some implementations move the caret to the beginning of the line when an up arrow is pressed while the caret is on the first line of the text buffer. This standard does define what is the correct way to perform this caret move. This is not to say that the standard mandates, or even recommends, that the caret move in such a way. Only that, should it move, it should move in a certain way.

Notational Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

Text Editing vs. Word Processing

There are two broad categories for editing bidirectional text. One type is a text editor, and another is a word processor. The main difference, for our purposes, between the two is that a text editor produces pure text output, to be displayed strictly according to the Unicode Bidi algorithm, while a word processor is allowed to format the text in more liberal fashion. Also, a text editor is expected to output only and exactly what the user typed, while a word processor is allowed to shape the text using the UBA's control characters, or any other way.

It should be noted that, while the standard does not mandate that word processors use the UBA to format text, use of the UBA is assumed in the text of this standard. Implementors SHOULD use the UBA to order the text, but if they do not, implementors MUST carry out the standard so that the end result is as if implementors did implement the UBA. For example, when we state that a word processor, when copying text from an RTL paragraph into an LTR paragraph, must embed that text within an RLE/PDF sequence, the word processor MAY use any other mechanism other than an RLE/PDF pair, but still MUST make sure that the pasted text is treated as a RTL embedded sequence. Also, when exporting pure text representation of the output, placing an RLE/PDF pair around the text is the only way to ensure it will be displayed correctly by text editors and other output means.

Terms and Definitions

Abbreviations

Acronym	Meaning
Bidi	bidirectional
LTR	left-to-right
RTL	right-to-left
UBA	Unicode Bidi Algorithm
UI	User Interface

Definitions

Bidi Embedding Levels	The UBA assigns a level to each character in the logical buffer, including neutrals, which determines if it is part of LTR or RTL text, and eventually affects the presentation. Level 0 corresponds to base LTR text. Level 1 corresponds to base RTL text, or to RTL text embedded within level 0 LTR text. Level 2 corresponds to LTR text embedded within level 1 RTL text, itself possibly embedded within level 0 LTR text. And so on for higher levels. Even levels always correspond to LTR text, odd levels always correspond to RTL text.
Caret (aka Text Cursor)	Graphic representation of where actions like text entry or Delete are going to take effect. The caret is often displayed as a vertical bar. The text cursor is not to be confused with the mouse cursor. Throughout this document, the term "caret" refers to the text cursor, and the term "cursor" refers to the mouse cursor.
Cursor Level	For the needs of the UI, a Bidi level is assigned also to the cursor. This level reflects the Bidi level which is expected to be assigned to the next character entered (there are cases when the actual level of the entered character will be different). The level of the cursor is manipulated by UI functions, like changing the keyboard language. It may also be affected by all functions which change the position of the cursor.
Keyboard Language	Language of the next character that will be entered from the keyboard.
Logical Buffer	Buffer containing the text data in logical sequence (as opposed to visual sequence).
Paragraph Embedding Level	Bidi level of text belonging to the main language used in a paragraph. This is 0 if the main language is LTR, 1 if the main language is RTL. Note: there is a one-to-one correspondence between the paragraph embedding level and the "Base Direction", which is the direction of the main language of a paragraph.

Caret

The caret's role is to represent, to the user, where the next action is going to take place. Common implementations use two types of carets. One type, which in modern UIs is usually referred to as "insert mode", is represented as a vertical line drawn between characters. We shall refer to this form as a "line caret". The other, typically representative of overwrite mode, is displayed as an underline beneath the character, or as a block which highlights the character. We shall refer to it as "block caret".

Some archaic implementations use a block caret for both insert and overwrite mode. Modern implementations, however, use the line caret almost exclusively. This document assumes a line caret mode unless explicitly stated otherwise.

A line caret occupies zero space, and is always between two displayed characters (glyphs). We call this position the "caret location". Though the caret location is, logically, of zero width, display does need some actual width in order for the caret to display. See open issues for discussion.

Logical vs. Visual Operations

Throughout this document, there are references to a "visual caret" and a "logical caret". The distinction between the two is an important one. As written above, the caret is a visual indication of where actions take place. For Bidi text, it often happens that a single position in the logical buffer can be interpreted to refer to two (or more) visual positions. Typically, this is a result of whether the caret should be interpreted to be before the next character, or after the previous one. Likewise, a single visual position might, under some circumstances, be interpreted to refer to two positions in the logical buffer.

At the time of this writing, all editors implement a caret that acts as a visual aid to indicate the position in the logical buffer. The caret has a definite and well-defined position in the logical buffer, and its visual position is affected from a variety of considerations. We call such a caret a "logical caret".

This document also defines a new type of caret, called a "visual caret". A visual caret has a definite and well defined visual position. That visual position may, under some circumstances, translate to more than one positions in the logical buffer.

Throughout this document, almost any operation will affect the caret. After each operation the caret will either be a visual caret or a logical caret. In other words, the operation will either leave a well defined logical caret position or a visual caret position. An operation that sets the logical caret position is called a "logical operation", and the caret after the operation is a logical caret. An operation that sets the visual caret position is called a "visual operation", and the caret after it is a visual caret.

References like "before", "after", "first" and "last" rely on the order of characters in the logical buffer, and thus imply a logical caret. For example, the sentence "Pressing the END key MUST bring the caret to the last character of the line" means that the caret following pressing "END" is a logical caret. Likewise, references such as "left" and "right" rely on the visual arrangement of the characters on screen. The sentence "Pressing the LEFT key SHOULD move the caret one character to the left" means that the caret following pressing "LEFT" is a visual caret.

Caret Movement

This section covers operations explicitly designed to move the caret around. All operations under this section do not change the logical buffer.

Positioning Using a Pointing Device

The caret can be positioned using a pointing device. This is, typically, done by clicking with a mouse in or near the text area. Since a pointing device usually has a pixel accuracy, some rounding will be performed. Implementations SHOULD select the caret location that is nearest to the point where the pointing device asked to locate the caret. Implementations SHOULD NOT care whether the rounding is to the left or to the right.

This is a visual operation.

Home and End Keys

On most platforms, the "Home" key places the caret at the beginning of the line it is currently on, and "End" places it at the end of the same line. The end of the line means positioning the caret right after the last character in the logical buffer that is displayed on the current line. Likewise, the beginning of the line means positioning the caret right before the first character in the logical buffer that is displayed on the current line.

On some platforms, most notably Apple Macintosh, the same functionality is achieved using a modifier over the arrow keys (command-right and command-left). On such platforms, the meaning is the same as above, except the mapping between command-right/left to home/end depends on the paragraph direction. If the paragraph is left to right, command-left translates to the behavior discussed above under "Home", and command-right to "End". If the paragraph is right to left, command-left translates to "End" and command-right to "Home". See open issues

In either cases, this is a logical operation.

Start/End of Paragraph/Document

Some implementations have a key combination for moving the caret to the beginning/end of the current paragraph, or the beginning/end of the entire document.

Moving to the beginning of the paragraph means positioning the caret right before the first character of the paragraph. Likewise, moving to the beginning of the document means positioning the caret right before the first character of the document.

In a similar way, moving to the end of the paragraph/document means positioning the caret right after the last character of the paragraph/document.

This is a logical operation.

Up and Down Arrow Keys

Pressing the "up" or "down" keys MUST move the caret one line up or down, respectively. The horizontal caret location is determined by the same algorithm as in "Positioning Using a Pointing Device" above.

If the caret is already on the first/last line of the text buffer, some implementations choose to move it to the beginning/end of the text buffer. If this is the case, implementations MUST use the same as logic as in the "Start/End of Paragraph/Document" section above. If that is the case, the operation MUST NOT be visual.

In the normal case, this is a visual operation.

Index of Operations

This section lists all of the operations defined in the document above. Each operation lists the section in which it is defined, as well as whether it is a logical or visual operation (i.e. - whether the caret following performing it is a visual or logical caret).

L/V	Operation	Section	Description
L	Filler	name	Just a filler operation, until we have an actual operation to place here