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Add support for character-level and glyph-level mirroring of RTL operators (#277)
* Clarify layout of large operators algorithm * Add support for RTL mirroring of operators
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‎spec.html‎

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@@ -1738,13 +1738,23 @@ <h4>Layout of operators</h4>
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<a data-xref-type="css-property">color</a>
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of the <code>&lt;mo&gt;</code> element.
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</p>
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<p>
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Let <code>dir</code> be the element's computed
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<a data-xref-type="css-property">direction</a>.
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</p>
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<p>Operators are laid out as follows:</p>
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<ol class="algorithm">
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<li>
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If the content of the <code>&lt;mo&gt;</code> element is not
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made
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of a single character <code>c</code> then fall back to the
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layout algorithm of <a href="#layout-of-mtext"></a>.
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If it is not possible to <a>get a glyph</a> corresponding to
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<code>c</code> given directionality <code>dir</code>, then
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fall back to the layout algorithm of <a href="#layout-of-mtext"></a>.
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Otherwise, let <code>g</code> be the result of running
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<a>get a glyph</a> corresponding to <code>c</code>
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given directionality <code>dir</code>.
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</li>
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<li>
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If the operator has the [=embellished operator/stretchy=] property:
@@ -1754,10 +1764,8 @@ <h4>Layout of operators</h4>
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<ol>
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<li>
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If it is not possible to <a>shape a stretchy glyph</a>
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corresponding to <code>c</code> in the inline direction
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with the
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<a>first available font</a>
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then fall back to the
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<code>g</code> in the inline direction with the
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<a>first available font</a> then fall back to the
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layout algorithm of <a href="#layout-of-mtext"></a>.
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</li>
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<li>
@@ -1777,13 +1785,13 @@ <h4>Layout of operators</h4>
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<li>
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The <a>inline size</a> and (ink) block metrics of the math content
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are given by algorithm to
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<a>shape a stretchy glyph</a> to <a>inline dimension</a>
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<code>T<sub>inline</sub></code>.
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<a>shape a stretchy glyph</a> <code>g</code> to
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<a>inline dimension</a> <code>T<sub>inline</sub></code>.
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</li>
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<li>
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The painting of the operator is performed by the
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algorithm
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to <a>shape a stretchy glyph</a>
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to <a>shape a stretchy glyph</a> <code>g</code>
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stretched to <a>inline dimension</a>
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<code>T<sub>inline</sub></code> and
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at position determined by the previous box metrics.
@@ -1796,10 +1804,8 @@ <h4>Layout of operators</h4>
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<ol>
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<li>
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If it is not possible to <a>shape a stretchy glyph</a>
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corresponding to <code>c</code> in the block direction
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with the
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<a>first available font</a>
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then fall back to the
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<code>g</code> in the block direction with the
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<a>first available font</a> then fall back to the
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layout algorithm of <a href="#layout-of-mtext"></a>.
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</li>
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<li>
@@ -1862,7 +1868,7 @@ <h4>Layout of operators</h4>
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Let <code>minsize</code> and <code>maxsize</code>
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be the [=embellished operator/minsize=] and [=embellished operator/maxsize=] properties on the
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operator. Percentage values are interpreted relative
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to the height of the glyph for <code>c</code>.
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to the height of <code>g</code>.
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Let <code>T</code> =
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<code>T<sub>ascent</sub></code> +
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<code>T<sub>descent</sub></code> be the target size.
@@ -1920,7 +1926,7 @@ <h4>Layout of operators</h4>
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<a>line-descent</a>
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of the math content
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are obtained by the algorithm to
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<a>shape a stretchy glyph</a>
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<a>shape a stretchy glyph</a> <code>g</code>
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to <a>block dimension</a>
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<code>T<sub>ascent</sub></code> +
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<code>T<sub>descent</sub></code>.
@@ -1937,7 +1943,7 @@ <h4>Layout of operators</h4>
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</li>
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<li>
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The painting of the operator is performed by the
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algorithm to <a>shape a stretchy glyph</a>
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algorithm to <a>shape a stretchy glyph</a> <code>g</code>
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stretched to <a>block dimension</a>
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<code>T<sub>ascent</sub></code> +
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<code>T<sub>descent</sub></code>
@@ -1961,25 +1967,63 @@ <h4>Layout of operators</h4>
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then:
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<ol>
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<li>
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<p>
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Use the
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<code>MathVariants</code>
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table to try and find a glyph of height at least
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<a>DisplayOperatorMinHeight</a>.
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If none is found, fall back to the
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largest non-base glyph. If none is found, fall back to
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the layout algorithm of <a href="#layout-of-mtext"></a>.
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</p>
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If it is not possible to <a>shape a stretchy glyph</a>
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<code>g</code> in the block direction with the
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<a>first available font</a> then fall back to the
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layout algorithm of <a href="#layout-of-mtext"></a>.
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<div class="note">
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Here we treat a non-[=embellished operator/stretchy=] [=embellished operator/largeop=]
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glyph as stretchy with target dimension <a>DisplayOperatorMinHeight</a>.
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</div>
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</li>
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<li>
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The <a>min-content inline size</a>,
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<a>max-content inline size</a>,
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<a>inline size</a> and block metrics of the math content
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are given by the
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glyph found.
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The <a>min-content inline size</a> and
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<a>max-content inline size</a> of the math content
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are set to the <a>preferred inline size of a glyph
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stretched along the block axis</a>.
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</li>
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<li>
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The <a>inline size</a>,
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<a>ink line-ascent</a>,
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<a>ink line-descent</a>,
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<a>line-ascent</a> and
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<a>line-descent</a>
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of the math content
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are obtained by the algorithm to
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<a>shape a stretchy glyph</a> <code>g</code>
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to <a>block dimension</a>
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<a>DisplayOperatorMinHeight</a>.
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The <a>inline size</a> of the math content is the width of
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the stretchy glyph. The stretchy glyph is shifted
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towards the <a>line-under</a> by a value Δ so that its
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center aligns with the center of the target when
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[=embellished operator/symmetric=]:
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the ink ascent of the math content is
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the ascent of the stretchy glyph − Δ
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and the ink descent of the math content is
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the descent of the stretchy glyph + Δ.
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<ul>
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<li>
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If the operator has the [=embellished operator/symmetric=] property,
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then
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Δ = [(ascent of stretchy glyph − descent of stretchy glyph) − 2 * <a>AxisHeight</a>] / 2.
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</li>
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<li>
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Otherwise,
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Δ = 0.
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</li>
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</ul>
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<div class="note">
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The point of Δ here is simply to vertically align the operator when [=embellished operator/symmetric=].
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</div>
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</li>
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<li>
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Paint the glyph.
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The painting of the operator is performed by the
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algorithm to <a>shape a stretchy glyph</a> <code>g</code>
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stretched to <a>block dimension</a>
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<a>DisplayOperatorMinHeight</a>
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and at position determined by the previous box metrics
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shifted by Δ towards the <a>line-over</a>.
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</li>
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</ol>
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<figure id="figure-sum-base-and-displastyle-sizes">
@@ -2838,8 +2882,11 @@ <h5>Radical symbol</h5>
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of that element.
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</p>
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<p>
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The <dfn>radical glyph</dfn> is the glyph obtained for the
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character U+221A SQUARE ROOT.
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Let <code>dir</code> be the computed <a data-xref-type="css-property">direction</a>
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of the <code>&lt;msqrt&gt;</code> or <code>&lt;mroot&gt;</code> element.
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The <dfn>radical glyph</dfn> is the glyph obtained as a result of running
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<a>get a glyph</a> corresponding to the U+221A SQUARE ROOT character
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given <code>dir</code>.
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</p>
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<p>
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The <dfn>radical gap</dfn> is given by
@@ -2856,8 +2903,8 @@ <h5>Radical symbol</h5>
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<p>
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The <dfn>box metrics of the radical glyph</dfn>
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and <dfn>painting of the surd</dfn> are given by the algorithm to
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<a>shape a stretchy glyph</a> to <a>block dimension</a> the
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target size for the radical glyph.
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<a>shape a stretchy glyph</a> to the
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target size for the radical glyph in the <a>block dimension</a>.
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</p>
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</section>
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<section>
@@ -5639,6 +5686,55 @@ <h4>Algorithms for glyph stretching</h4>
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such as
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the one suggested in <a href="#unicode-based-glyph-assemblies"></a>.
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</div>
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<p>
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The algorithm to <dfn>get a glyph</dfn> corresponding to a character <code>c</code> given a directionality <code>dir</code>
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is the following:
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</p>
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<ul class="algorithm">
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<li>
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Let <code>g</code> be the glyph corresponding to <code>c</code>
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in the <a>first available font</a>.
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If it is not possible to find such a glyph, then exit with failure.
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</li>
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<li>
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If <code>dir</code> is <code>rtl</code>:
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<ul>
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<li>
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If there exists an OpenType rtlm variant of
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<code>g</code> in the <a>first available font</a>,
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then return it and exit with success. [[OPEN-FONT-FORMAT]]
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</li>
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<li>
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Otherwise, if <code>c</code> has the Bidi_Mirrored property [[BIDI]]:
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<ul>
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<li>
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If <code>c</code> has a corresponding mirrored codepoint,
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<code>c'</code>, then return the glyph corresponding
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to <code>c'</code> and exit with success.
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If it is not possible to find such a glyph, then exit with failure.
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</li>
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<li>
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Otherwise, exit with failure.
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</li>
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</ul>
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<div class="note">
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These failure cases are for when a character should be mirrored
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according to its Bidi_Mirrored property, but no corresponding codepoint
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or glyph exists.
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</div>
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</li>
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<li>
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Otherwise, return <code>g</code> and exit with success.
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</li>
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</ul>
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</li>
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<li class="assert">
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Assert: <code>dir</code> is <code>ltr</code>.
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</li>
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<li>
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Return <code>g</code> and exit with success.
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</li>
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</ul>
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</section>
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</section>
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</section>

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