Editing Bidirectional Documents With Yudit

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From version 2.7 Yudit should show bidirectional text just as any other Unicode application that implement Unicode Bidirectional Algorithm UAX#9

Paragraphs with initial directionality LR, like English text will be aligned to the left while texts with RL initial directionality will be aligned to the right.

As Unicode Standard allows higher level protocols to impose a Document Embedding, Yudit can enforce an LR or RL embedding on the whole document if the user sets it with the text embedding button. This will force left or right alignment on the whole text.

What is implicit bidirectional behavior?

All characters in a Unicode belong to one of the many bidirectional classes. Depending on these character properties all characters in the documents must be reordered into a visual order dictated by a rather convoluted algorithm in UAX#9. Under implicit bidirectional behavior I mean the behavior that purely relies on the characters bidirectional class property.

How to invoke implicit bidi?

You don’t need to do anything, just type:
He said “سلام!‏”

Please note that I cheated here: I added a RLM (Right Left Mark) U+200F at the end. I wanted to make the text more digestible in this English document. This mark is visible in the editor window but it will not appear when printing, or, when used in labels.

What is explicit embedding and override?

In addition to the inherent bidirectional properties of the characters, Unicode allows text between certain markers to render Left to Right or believe that the embedding context is Left or Right.

These markers can be nested. The PDF (POP directional format) marker restores the last embedding state.

  1. Directional Override

    RLO (Right to Left Override)Embedded TextPDF (Pop Directional Format)
    LRO (Left to Right Override)Embedded TextPDF (Pop Directional Format)

    This encloses a text with an LR or RL explicit directionality, regardless of their bidirectional property. However, this directional property is (unfortunately) not used when the initial directionality is determined, so your text might not be aligned as you expect.

    According to UAX#9 P2:
    In each paragraph, find the first character of type L, AL, or R. Because paragraph separators delimit text in this algorithm, this will generally be the first strong character after a paragraph separator or at the very beginning of the text. Note that the characters of type LRE, LRO, RLE, RLO are ignored in this rule. This is because typically they are used to indicate that the embedded text is the opposite direction than the paragraph level

  2. Directional Embedding

    RLE (Right to Left Embedding)Embedded TextPDF (Pop Directional Format)
    LRE (Left to Right Embedding)Embedded TextPDF (Pop Directional Format)

    This encloses an embedded text. Embeddings supposed to give some protection for the embedding context. The text in the embedding is (in most cases) rendered as if the initial, embedding of the text would be RL or LR. Please note that there are some characters that make this mission impossible: in fact it is not really possible to make use of RLE or LRE if you use those characters. (Should they be forbidden? Read on).

In Yudit you do not need to care about LRE,RLE,RLO,LRO,PDF markers, they are totally hidden. Your embedded text will have a brighter or darker background, this way you can tell the embedding range.

Unicode allows for 3 levels of support for bidirectional algorithm:

1. No bidirectional formatting. This implies that the system does not visually interpret characters from right-to-left scripts.

2. Implicit bi-directionality. The implicit bidirectional algorithm and the directional marks RLM and LRM are supported.

3. Full bi-directionality. The implicit bidirectional algorithm, the implicit directional marks, and the explicit directional embedding codes are supported: RLM, LRM, LRE, RLE, LRO, RLO, PDF.

Yudit has now full bidirectional support (3).

How to do explicit direction override?

To override implicit directionality of characters press Override Direction <Ctrl><D> to change direction. Then simply continue typing. You can get out of this by the cursor <Ctrl><Y> (Yield Direction) button. You can clearly distinguish the embedded text.
I said “‮NO WAY!‬”.

How to do simple explicit embedding?

Similarly embedding a Right-Left text in a Left-Right document needs <Ctrl><E> (Embedding Override). This is good, for instance if you want to say:
He said: “‫سلام!‬
Without the Right-to-Left embedding this would look pretty bad in this English document:
He said “سلام!”

I already have a text that I need to embed/un-embed. How to do that?

Before embedding/un-embedding select the text. Selection can be made for instance with <Alt> arrow keys. After selection with the keys keep pressing <Alt> and press <D> for Direction Override or <Alt><E> for Embedding Override. You can bring back the text to no embedding level with <Alt><Y> (Yield Embedding).

What is document text embedding?

Yudit can enforce an initial embedding level to the whole document. When Yudit is started the initial embedding is reset to none. The text is also saved without initial embedding enforcement tags. When no initial embedding is enforced, your text can show up aligned to the left or to the right, depending on the natural paragraph embedding level.

I want to embed LR text but my embedding arrow is RL

The direction of the embedding arrows on the tool-bar always point to the opposite direction of the current embedding; the context where the cursor is. This is to make the operation faster and make less errors. It is usually not desired to embed a text in an LR document as LR. However, you can do this with this trick: If you want to embed LR text in the document with LR embedding change the Document Text Embedding to the RL. Now you can make the LR embedding.

Important Notes

In po file translations you might want to consider embedding your RL text with explicit RLO so that you will see what you will get on that label: Without explicit embedding:
msgstr "سلام Gáspár, محمد"
With explicit embedding, you will see what the label will eventually show:
msgstr "‫سلام Gáspár, محمد‬"
Please note that most applications do not support explicit embedding, so deal with them sparingly. Moreover, explicit embedding does not save you from the effects of Unicode Bidirectional algorithm. You have this text:
msgstr "‫سلام Gáspár محمد‬"
I put the whole thing into RL embedding marks, because I want to see them this way, in my RL text label. It works. But what if I replace the leftmost space with a tab?
msgstr "‫سلام Gáspár محمد‬"
For this html document I have to use the pre tag to see it:
     msgstr "‫سلام Gáspár	محمد‬"
Now try to put this in a label. (Try pressing the Document Text Embedding button in Yudit for the same effect). Now you see what you will see in that label. Well, to tell the truth nothing saves you from these effects of Unicode Bidirectional algorithm. If you want to see why this happens please read Surprise Effects on this server.

Fortunately, if you use gettext you will be able to use '\t' character for TAB. So when translating po file please always use '\t', like this:
msgstr "‫سلام Gáspár\tمحمد‬"
But in short: do not use segment separators in your po translation text as is. In case of a non-computer, non-gettext text you are on your own.

Comparing With Other Applications

I tried to compare Yudit bidi to other applications but, the applications had problems even with this simple text:
Hello ‫العربية 14محمد‬ ‮RLTXT‬ nothing
I may try it again at a later time.


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Gaspar Sinai
Last updated: 2002-11-20