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"rtlm" OpenType feature for RTL mirroring #67

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fred-wang opened this issue Sep 23, 2019 · 5 comments
Open

"rtlm" OpenType feature for RTL mirroring #67

fred-wang opened this issue Sep 23, 2019 · 5 comments
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i18n-tracker Group bringing to attention of Internationalization, or tracked by i18n but not needing response. level-2 opentype/tex

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@fred-wang
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This is described in https://mathml-refresh.github.io/mathml-core/#opentype-features :

If on the element the CSS property direction is rtl, then user agents must enable the rtlm OpenType feature on text nodes [OPEN-FONT-FORMAT] unless it contradicts what the page author has specified with the font-feature-settings CSS property.

and in https://mathml-refresh.github.io/mathml-core/#radical-symbol :

The radical glyph is the glyph obtained for the character U+221A SQUARE ROOT, applying the rtlm OpenType feature if the CSS property direction is rtl [OPEN-FONT-FORMAT].

I think we need more experiments / tests for these.
@fred-wang
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Consensus from 15 June 2020: remove the text above and move this to level 2.

In addition, I believe we need to perform glyph-level (rtlm) and character-level (Unicode) mirroring to obtain the glyph "corresponding to c" in https://mathml-refresh.github.io/mathml-core/#layout-of-operators ; before calling https://mathml-refresh.github.io/mathml-core/#algorithms-for-glyph-stretching

fred-wang referenced this issue Jun 16, 2020
@fred-wang
Remove mention of rtlm mirroring from the current text.
69c7776 
https://github.com/mathml-refresh/mathml/issues/148
@NSoiffer
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Since not part of level 1, I have removed the 'needs spec update' label.

@NSoiffer NSoiffer transferred this issue from w3c/mathml Jun 29, 2021
@fred-wang
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OK, I'm adding the level-2 label.

@fred-wang
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Additional note: Unicode has a Bidi_Mirrored property and some libraries like ICU allows to test that:

https://www.compart.com/en/unicode/mirrored
https://github.com/sillsdev/icu-dotnet/blob/164655115621f06333bfc3d2e4cd1ec2c984063e/source/icu.net/NativeMethods/NativeMethods.cs#L887

(so no need to store a property in the operator dictionary for that, as that was done in Firefox)

@xfq xfq added the i18n-tracker Group bringing to attention of Internationalization, or tracked by i18n but not needing response. label Jul 18, 2023
@w3cbot w3cbot mentioned this issue Jul 18, 2023
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This was referenced Apr 9, 2024
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@fred-wang
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fred-wang commented Aug 7, 2024 •
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Testcase demonstrating various cases of math mirroring:

<!DOCTYPE html>
<meta charset="utf-8">
<link
  rel="stylesheet"
  href="https://fred-wang.github.io/MathFonts/XITS/mathfonts.css" />
<style>
  table, th, td {
      border: 1px solid black;
  }
  td.example, math {
      font-size: 32pt;
      font-family: XITS Math;
  }
  .rtlm {
      font-feature-settings: "rtlm";
  }
  .overline {
      text-decoration: overline;
  }
</style>
<h1>RTL math mirroring (<a href="https://github.com/w3c/mathml-core/issues/67">issue #67</a>)</h1>
<p>The following table contains various math examples that involve mirroring of
  some operators.</p>
<p>
  The HTML ones should be supported by modern browsers and
  uses two techniques:
</p>
<ol>
<li>Character-level via Unicode. For example LEFT
  PARENTHESIS U+0028 becomes RIGHT PARENTHESIS U+0029</li>
<li>Glyph-level mirroring via the <a href="https://en.wikipedia.org/wiki/List_of_typographic_features#Features_depending_on_writing_direction">rtlm OpenType feature</a>. This is font-specific, in this case we rely on the
  <a href="https://github.com/fonts/xits-math?tab=readme-ov-file#the-xits-font-project">xits-math.otf font</a>. For example SQUARE ROOT U+221A has glyph name <code>uni221A</code> in that font, for which the <code>rtlm</code> mirrored form is the glyph called <code>uni221A.rtlm</code>.
</li>
</ol>
</p>

<p>The MathML ones contains various examples. For the one doing normal text layout, the HTML mirroring works. But when <a href="https://w3c.github.io/mathml-core/#dfn-shape-a-stretchy-glyph">shaping a stretchy glyph</a> or drawing a <a href="https://w3c.github.io/mathml-core/#layout-of-operators">large operator</a>, MathML Core does not define how to perform the mirroring.</p>

<p>Some browsers (e.g. Firefox) are able to perform glyph-level/character-level
  mirroring of MathML operators ; sometimes even mirroring via a
  "negative" scale transform.
  This latter approach allows to provide a fallback for math fonts that don't
  provide <code>rtlm</code> variants, but does not work for things like
  clockwise integrals and the like.</p>

<table>
  <tr>
    <th>Description</th>
    <th>Mirroring</th>
    <th>English (LTR)</th>
    <th>Arabic (RTL)</th>
  </tr>
  <tr>
    <td rowspan="2">HTML</td>
    <td>character-level via Unicode</td>
    <td class="example">(1,2]</td>
    <td class="example" dir="rtl">(١،٢]</td>
  </tr>
  <tr>
    <td>glyph-level mirroring via <code>rtlm</code></td>
    <td class="example">√<span class="overline">3÷4</span></td>
    <td class="example rtlm" dir="rtl">√<span class="overline">٣÷٤</span></td>
  </tr>
  <tr>
    <td>MathML <code>&lt;mo stretchy="false"&gt;</code></td>
    <td rowspan="2">character-level mirroring via Unicode</td>
    <td class="example">
      <math>
        <mrow>
          <mo stretchy="false">(</mo>
          <mn>1</mn>
          <mo>,</mo>
          <mn>2</mn>
          <mo stretchy="false">]</mo>
        </mrow>
      </math>
    </td>
    <td dir="rtl" class="example">
      <math dir="rtl">
        <mrow>
          <mo stretchy="false">(</mo>
          <mn>١</mn>
          <mo>،</mo>
          <mn>٢</mn>
          <mo stretchy="false">]</mo>
        </mrow>
      </math>
    </td>
  </tr>
  <tr>
    <td>MathML <code>&lt;mo stretchy="true"&gt;</code></td>
    <td class="example">
      <math>
        <mrow>
          <mo>{</mo>
          <mfrac>
            <mn>5</mn>
            <mn>6</mn>
          </mfrac>
          <mo>)</mo>
        </mrow>
      </math>
    </td>
    <td class="example" dir="rtl">
      <math dir="rtl">
        <mrow>
          <mo>{</mo>
          <mfrac>
            <mn>٥</mn>
            <mn>٦</mn>
          </mfrac>
          <mo>)</mo>
        </mrow>
      </math>
    </td>
  </tr>
  <tr>
    <td>MathML <code>&lt;mo largeop="true"&gt;</code></td>
    <td rowspan="2">glyph-level mirroring via <code>rtlm</code></td>
    <td class="example">
      <math displaystyle="true">
        <msub>
          <mo largeop="true">∲</mo>
          <mi>C</mi>
        </msub>
      </math>
    </td>
    <td class="example" dir="rtl">
      <math dir="rtl" displaystyle="true">
        <msub>
          <mo largeop="true">∲</mo>
          <mi>ج</mi>
        </msub>
      </math>
    </td>
  </tr>
  <tr>
    <td>MathML <code>msqrt</code></td>
    <td class="example">
      <math>
        <msqrt>
          <mfrac>
            <mn>3</mn>
            <mn>4</mn>
          </mfrac>
        </msqrt>
      </math>
    </td>
    <td class="example" dir="rtl">
      <math dir="rtl">
        <msqrt>
          <mfrac>
            <mn>٤</mn>
            <mn>٣</mn>
          </mfrac>
        </msqrt>
      </math>
    </td>
  </tr>
</table>

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