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Clarify layout of large operators algorithm #276

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Clarify layout of large operators algorithm #276

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70 changes: 55 additions & 15 deletions spec.html
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Expand Up @@ -1961,25 +1961,65 @@ <h4>Layout of operators</h4>
then:
<ol>
<li>
<p>
Use the
<code>MathVariants</code>
table to try and find a glyph of height at least
<a>DisplayOperatorMinHeight</a>.
If none is found, fall back to the
largest non-base glyph. If none is found, fall back to
the layout algorithm of <a href="#layout-of-mtext"></a>.
</p>
If it is not possible to <a>shape a stretchy glyph</a>
corresponding to <code>c</code> in the block direction
with the
<a>first available font</a>
then fall back to the
layout algorithm of <a href="#layout-of-mtext"></a>.
<div class="note">
Here we treat a non-[=embellished operator/stretchy=] [=embellished operator/largeop=]
glyph as stretchy with target dimension <a>DisplayOperatorMinHeight</a>.
</div>
</li>
<li>
The <a>min-content inline size</a>,
<a>max-content inline size</a>,
<a>inline size</a> and block metrics of the math content
are given by the
glyph found.
The <a>min-content inline size</a> and
<a>max-content inline size</a> of the math content
are set to the <a>preferred inline size of a glyph
stretched along the block axis</a>.
</li>
<li>
Paint the glyph.
The <a>inline size</a>,
<a>ink line-ascent</a>,
<a>ink line-descent</a>,
<a>line-ascent</a> and
<a>line-descent</a>
of the math content
are obtained by the algorithm to
<a>shape a stretchy glyph</a>
to <a>block dimension</a>
<a>DisplayOperatorMinHeight</a>.
The <a>inline size</a> of the math content is the width of
the stretchy glyph. The stretchy glyph is shifted
towards the <a>line-under</a> by a value Δ so that its
center aligns with the center of the target when
[=embellished operator/symmetric=]:
the ink ascent of the math content is
the ascent of the stretchy glyph − Δ
and the ink descent of the math content is
the descent of the stretchy glyph + Δ.
<ul>
<li>
If the operator has the [=embellished operator/symmetric=] property,
then
Δ = [(ascent of stretchy glyph − descent of stretchy glyph) − 2 * <a>AxisHeight</a>] / 2.
</li>
<li>
Otherwise,
Δ = 0.
</li>
</ul>
<div class="note">
The point of Δ here is simply to vertically align the operator when [=embellished operator/symmetric=].
</div>
</li>
<li>
The painting of the operator is performed by the
algorithm to <a>shape a stretchy glyph</a>
stretched to <a>block dimension</a>
<a>DisplayOperatorMinHeight</a>
and at position determined by the previous box metrics
shifted by Δ towards the <a>line-under</a>.
</li>
</ol>
<figure id="figure-sum-base-and-displastyle-sizes">
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