RE: Help Regarding RtoL alnguage rendering

From: Philippe Verdy (
Date: Tue Mar 20 2007 - 06:06:06 CST

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    It looks like your glyph metrics for diacritics were computed for
    left-to-right rendering, so that diacritics must not be reordered.

    What RtoL languages are reordering are complete grapheme clusters. A base
    character with its diacritics must remain as a single group, and you just
    need to reorder the complete clusters without changing their individual
    internal encoding order.


    But really, this way of rendering Arabic diacritics (or Hebrew) will produce
    poor output, given that Arabic base letters have very variable width, and
    the exact form and position of diacritics are normally changing depending on
    the base character. So true Arabic fonts are actually using composition
    rules where pairs of glyph ids are replaced by a single other glyph id whose
    drawing code is fitted to produce the expected design (i.e. with correct
    centering of Arabic points). You cant do that simply with a simple fixed
    value for the left side bearing


    In fact this is not a problem specific to Arabic or Hebrew, but this also
    happens with the Latin script, compare how accents are positioned over I or
    over O (The problem seems simpler with Latin, only because the most common
    pairs of letter+accent are also encoded separately as compatibility
    characters with either canonical or ompatibility equivalence, but this is
    not true for all possible combinations of a Latin letter + diacritic.


    Note also that Arabic base letters also have contextual forms that you MUST
    support (not rendering the contextual forms will result into a rendered text
    that will be difficult or nearly impossible to read for most Arabic readers,
    or would be thought really offensive or at least as unacceptable
    orthography, as if you were sending a Latin-written text using only capital
    letters, it will look like if one was reading old Roman sculture texts in
    Latin language, i.e. a foreign language or an offensive way to denature the
    Arabic script; remember that Arabic writers proud of their script and have a
    long history of excellent typography and artistic calligraphy); one simple
    way to support them is, in your renderer, to replace the default letter
    codes by their compatibility letter forms for each case (most Arabic letters
    that have multiple contextual forms are also encoded with compatibility
    codes corresponding to their position).




    De : [] De la
    part de Lokesh Joshi
    Envoy : mardi 20 mars 2007 11:37
    Objet : Help Regarding RtoL alnguage rendering


    Hi All,

    We are developing a fast rendering engine for games for RtoL languages.
     Need help from from anyone :) regardng the way we shld render the arabic
    and in general form RtoL text. Here i am trying to first explain the common
    terms i will use to describe the problem.

    PenX: This i mean the horizontal coordinate position from where i am before
    rendering the character
    LSB: Left Side Bearing: Can be -ve or +ve and depicts how much delta i shld
    move before rendering the character from PenX
    AW: AdvanceWidth: Shows how much in pixels i shld move after rendering the
    current character.
    Width: Actual width of the glyph
    Height : Actual height of the glyph

    Now suppose we have a RtoL input whose memory order is AB, and then its
    reordered for visual order as BA

    There are two cases where i have the confusion, both are bit related but for
    clarity i wish to explain them in two.

    1. Please assume that A is a consonant: U+0627 and B is a diacritic as:
    A has follwoing metrics
    Width: 6
    AW: 8
    LSB: 0

    B has the following metrics, since its a diacritic, it need to come over A.
    WIdth: 2
    AW: 0
    LSB: -2

    Now if this has been latin, i wud have assumed the following way

    Set PenX =0, render the Glyph A at position at a distance equal to LSB of
    A, and then move PenX by AW of A,
    Then do the same for B and in such a way B will be rendered at a distance of
    2 towards left from the right hand corner of A, and thats i believe is
    perfect for Latin.

    For Arabic, suppose i reorder first and then render from left to right only,
    then first i will render B, which will be 2 pixels away from the the current
    PenX which will be ZERO, then advance by AW of B which is again ZERO, and
    then A and then advance the pen X. This is wrong and will make the diacritic
    not above A and infact at a distance of 2 pixels away towards left hand side
    from the left corner of A, this is i am sure wrong and not designed by the
    type designer.

    The way we work now is, in cases of diacritic, we change the LSB = AW of
    next Character + current LSB. So in this case LSB of B will change to 8 +
    (-2) = 6. So we render B away from current penX at right hand side witha
    distance of 6 pixels and then move by nothing since AW is zero and then
    Render A. and this was B comes over A at exactly 2 pixel away from right
    hand corner. But this also now seems to be wrong because we alter the LSB
    etc which is incorrect.

    So my fist query is what shld be do ?

    2. This query is related to the first. Suppose we shld render starting from
    right to left in the visual order. The Visual order is BA, and i move to
    the the right most point of the area,, and render A at a pont distant from
    the curent PenX = LSB + AW of A towards left, render the A, then move
    towards left by AW of A, then render B the same way. But then this method
    will also endup in same effect, B away towards left equal LSB of B. This is

    Another way is to see the character AW first. If its zero, then its a
    diacritic and we shld not advance the penX and keep penX where it is and
    then render the B after A. And if in case B is not a diacritic then first
    advance the PenX by AW of A towards left and then render B. Is this
    assumption correct ?

    Pls suggest if i am able to explain all this correctly, i need the help in
    this regard asap to have the generic implementation for RtoL languages.


    {The harder I work, the luckier I get ! (Failure is never an option)} 

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