--------------------------------------------------------------- HOWTO-bidi.txt: Editing Bidirectional Documents With Yudit --------------------------------------------------------------- From version 2.7 Yudit should show bidirectional text just as any other Unicode application that implement Unicode Bidirectional algorithm. 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 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. --------------------------------------------------------------- Usage --------------------------------------------------------------- 1. 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. 2. 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. 3. 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. a) Directional Override Text between RLO (Right to Left Override) … PDF (Pop Directional Format) LRO (Left to Right Override) … PDF (Pop Directional Format) Will have 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. 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 b) Directional Embedding Text between RLE (Right to Left Embedding) … PDF (Pop Directional Format) LRE (Left to Right Embedding) … PDF (Pop Directional Format) are embedded. 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 markers for a) and b), 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 the 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). 4. How to do Explicit Direction Override? To override implicit directionality of characters press Override Direction to change direction. Then simply continue typing. You can get out of this by the cursor (Yield Direction) button. You can clearly distinguish the embedded text. I said “‮NO WAY!‬”. 5, How to do simple Explicit Embedding Similarly embedding a Right-Left text in a Left-Right document needs (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!” 6. I already have a text that I need to embed/un-embed. How to do this? Before embedding/un-embedding select the text. Selection can be made for instance with arrow keys. After selection with the keys keep pressing and press for Direction Override or for Embedding Override. You can bring back the text to no embedding level with (Yield Embedding). 7. 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. 8. 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. 9. 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" 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 in this document. 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. 10. 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. --------------------------------------------------------------- Technical Details --------------------------------------------------------------- The current Yudit Bidi implementation is a reversible algorithm when resolving explicit levels. This is text embedded within LRE-PDF, RLE-PDF, LRO-PDF and RLO-PDF pairs. The algorithm can re-create the text from the view. This also means that superfluous embedding tags will be dropped when saving alien (non-Yudit) texts. These tags will be dropped from portions of the document that were at least once viewed. I will not give you an exhaustive list of such cases. 1. While alien Unicode stream text1text2text3 will be saved the same way, text1text2 will be saved as text1text2 as they are equivalent, and the latter is shorter. 2. Empty pairs of or will be deleted from the text, as they have no effect. 3. Spurious with no matching embedding marks will be deleted from the document. 4. To keep the text editable, LRM and RLM zero with marks are displayed in the editing window, but they will not appear when printing or when used in non-editable places, like labels. --------------------------------------------------------------- ‪ Surprise Effects ‬ --------------------------------------------------------------- --------------------------------------------------------------- The Problem Of Not Having Arabic RLM --------------------------------------------------------------- According to Unicode algorithm (Unicode Standard Annex #9) W2: search backward from each instance of a European number until the first strong type (R, L, AL, or sor) is found. If an AL is found, change the type of the European number to Arabic number. Probably nobody was thinking that sor can never be AL at the beginning of the line - this proves it: X10: The remaining rules are applied to each run of characters at the same level. For each run, determine the start-of-level-run (sor) and end-of-level-run (eor) type, either L or R. This depends on the higher of the two levels on either side of the boundary (at the start or end of the paragraph, the level of the 'other' run is the base embedding level). If the higher level is odd, the type is R, otherwise it is L. I think this is ridiculous. In Arabic context you will get: Logical: Visual: -10% TEST ARABIC TSET CIBARA -10% ARABIC -10% TEST TSET %10- CIBARA So what is the solution? The standard says that Higher-Level Protocols can: Override the number handling to use information provided by a broader context. For example, information from other paragraphs in a document could be used to conclude that the document was fundamentally Arabic, and that EN should generally be converted to AN. In Yudit I decided not to do this hack. The reason is this: When text using a higher-level protocol is to be converted to Unicode plain text, formatting codes should be inserted to ensure that the order matches that of the higher-level protocol... No, with Yudit I don't want to save -10% TEST ARABIC as -10%ARABIC, unless it is requested by the user. Please use explicit directionality markers in this case. --------------------------------------------------------------- The Problem Of Characters That Have Global Effects --------------------------------------------------------------- What are these characters? Segment Separator - its effect is well defined, but surprising. Boundary Neutral - the location of which is not defined it can pop up at any place. So let's see what we get for at least the one, that is defined: Segment Separator - like Tab. I tried to use RLE in my translation, so that I can see what I will see in this Label as a label Text: msgstr "‫سلام Gáspár محمد‬" Well as you see, I can not. If you set Yudit Editor's Document Text Alignment to the right, you will see what the label will show. Something ‪totally‬ different. Unfortunately the Unicode Algorithm requires me to. UAX #9 L1: “On each line, reset the embedding level of the following characters to the paragraph embedding level:" 1. Segment Separators.” Well this means that regardless of having this tab embedded in our text I have to reset it to this English document's embedding level. If you use gettext, please use '\t' instead of Tab. --------------------------------------------------------------- The Problem Of Having Only One Set Of + - / * . % Characters --------------------------------------------------------------- You might find it surprising, that programs conforming to Unicode Standard Annex #9 I must render the followings this way (I substituted HEBREW with ‫עברית‬ and ARABIC with ‫العربية‬, and I also inserted a Right to Left embedding so that you see what is going on): Surprise #1: Input : HEBREW ~~~23%%% HEBREW abc Output : ‫עברית ~~~23%%% עברית abc‬ Input : ARABIC ~~~23%%% ARABIC abc Output : ‫العربية ~~~23%%% العربية abc‬ Surprise #2: Input: HEBREW 1*5 1-5 1/5 1+5 Output: ‫עברית 1*5 1-5 1/5 1+5‬ Input: ARABIC 1*5 1-5 1/5 1+5 Output: ‫العربية 1*5 1-5 1/5 1+5‬ I have checked this with java reference code from Unicode Consortium http://www.unicode.org/unicode/reports/tr9/BidiReferenceJava/ so what you see here in Yudit is correct. Did you expect this? I feel like there is a fundamental flaw in the official Unicode Bidirectional algorithm that can not be solved unless there are separate character pairs for + - / * % Without that all you can do is embed your mathematical equations with explicit direction overrides. --------------------------------------------------------------- The Problem Of Ir-reversibility --------------------------------------------------------------- The Unicode Bidirectional Algorithm is irreversible. In other words, the logical text can be reordered into visual order, but there is no way to guess what the logically ordered text is, just by looking at the visual text. This is a serious problem for digital signatures. If you want to sign a document, what you sign is the bit-stream, but what you see is the text. As there is no algorithm provided you can not possibly imagine, what you sign if you are just looking at the text. --------------------------------------------------------------- The Problem Of Stateful Encoding --------------------------------------------------------------- Unicode always made a laugh at other stateful encodings like iso-2022-x. In fact the stateliness they introduced with the explicit bidirectional marks is even worse, and it would make binary editing of Unicode Text files with proper undo operation next to impossible. --------------------------------------------------------------- Remarks --------------------------------------------------------------- I tested Yudit and found that it is, probably, 100% Compliant to the full Unicode Bidirectional UAX #9 algorithm. However ‪I do not think that that UAX #9 algorithm is good.‬ Moreover, I think that that algorithm should be replaced with one that makes more sense. My clean-room implementation of the implicit algorithm mostly lies in stoolkit/SBiDi.h stoolkit/SBiDi.cpp, You can use it in your GNU programs. If Unicode Consortium ever change their mind it would be very easy to replace that file. So how much is: Input: HEBREW 10-2*5 Output: ‫עברי 10-2*5‬ Input: ARABIC 10-2*5 Output: ‫العربية 10-2*5‬ It is your choice. They both have 0 values, literally. Related documents: http://www.yudit.org/bidi/ Document version 1.7 2006-05-21 Document version 1.6 2002-11-19 Gaspar Sinai