<!DOCTYPE html>
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
</head>
<body>
<p>A note on conditions as defined in
WG03-varc-and-other-updates-03.pdf (up for review tomorrow
morning).</p>
<p>When I sketched out the VARC condition idea I had them arranged
in if/else if/.../else sequences. From my reading of the document
they're currently in "if" form -- a condition set can be attached
to a component and if it is it will only be used if the condition
set evaluates to true.</p>
<p>There's nothing wrong with that semantic; it may be preferable
because it's simpler. However, the most common case with this
system is that you want one shape to render in some region of
design space (defined by the condition set) and another shape to
render outside of that. And negating a condition <i>set</i> can
be expensive and/or tricky. So I would recommend that one of these
two things change:<br>
</p>
<ul>
<li>The if/else if/.../else semantic is restored</li>
<li>Some way of negating a condition set is added to the
specification<br>
</li>
</ul>
<p>We talked about the possibility of condition set negation in a
previous meeting but didn't come to any conclusions. It's tricky
because the table has no versioning. If we wanted to hack
negations into the current format the way to do that might be to
add a new condition format (possibly 5 if newfeatvar_spec.pdf is
accepted) that looks something like:<br>
</p>
<blockquote>
<p>ConditionTableFormat5</p>
</blockquote>
<blockquote>
<p><b>Type Name
Description</b></p>
<p>uint16 Format
Format (set to 5)</p>
</blockquote>
<blockquote>
<p>When present in a condition set this condition indicates the
set should apply when<br>
at least one of the other conditions is false and not otherwise.
That is, it makes the<br>
condition set apply when it would normally not and vice-versa.
There should be at <br>
most one format 5 condition in a condition set and it should be
the first.<br>
</p>
</blockquote>
<p>If this is preferred it could be used to replace/simplify the
trueLookupIndexListOffset/falseLookupIndexListOffset pair in the
LookupCondition record of newfeatvar_spec.pdf to just
lookupIndexListOffset (because one could just follow the record<br>
with the "positive" condition set with another record with the
negated one when needed).</p>
<p>Skef<br>
</p>
</body>
</html>