📄 ltsh.htm

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<TITLE>The 'LTSH' Table</TITLE>
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<TD><H1>Table Formats</H1></TD></TR>

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<H2>LTSH - Linear Threshold</H2>
<P>
There are noticeable improvements to fonts
on the screen when instructions are carefully applied to the sidebearings.
The gain in readability is offset by the necessity for the OS
to grid fit the glyphs in order to find the actual advance width
for the glyphs (since instructions may be moving the sidebearing
points). TrueType already has one mechanism to side step the speed
issues: the 'hdmx' table, where precomputed advance widths may
be saved for selected ppem sizes. The 'LTSH' table (Linear ThreSHold)
is a second, complementary method.
<P>
The LTSH table defines the point at which it is reasonable to
assume linearly scaled advance widths on a glyph-by-glyph basis.
This table should <CITE>not</CITE> be included unless bit 4 of the &quot;flags&quot;
field in the 'head' table is set. The criteria for linear scaling
is:
<P>
a. (ppem size is <FONT FACE="Symbol">&#179;</FONT><TT> </TT>50)
AND (difference between the rounded linear width and the rounded
instructed width <FONT FACE="Symbol">&#163;</FONT>
2% of the rounded linear width)
<P>
or b. Linear width == Instructed width
<P>
The LTSH table records the ppem for each glyph at which the scaling
becomes linear again, despite instructions effecting the advance
width. It is a requirement that, at and above the recorded threshold
size, the glyph remain linear in its scaling (i.e., not legal
to set threshold at 55 ppem if glyph becomes non-linear again
at 90 ppem). The format for the table is:

<TABLE WIDTH=480 BGCOLOR="#F0F0F0">
<THEAD>
<TR><TH BGCOLOR="#C0C0C0">Type</TH><TH BGCOLOR="#C0C0C0">Name</TH>
<TH BGCOLOR="#C0C0C0">Description</TH></TR></THEAD>
<TBODY>
<TR><TD VALIGN=TOP>USHORT</TD><TD VALIGN=TOP>version</TD>
<TD VALIGN=TOP>Version number (starts at 0).</TD></TR>
<TR><TD VALIGN=TOP>USHORT</TD><TD VALIGN=TOP>numGlyphs</TD>
<TD VALIGN=TOP>Number of glyphs (from &quot;numGlyphs&quot; in 'maxp' table).</TD></TR>
<TR><TD VALIGN=TOP>BYTE</TD><TD VALIGN=TOP>yPels[numGlyphs]</TD>
<TD VALIGN=TOP>The vertical pel height at which the glyph can be assumed to scale linearly. On a per glyph basis.</TD></TR>
</TABLE>&nbsp;&nbsp;<P>
<P>
Note that glyphs which do not have instructions
on their sidebearings should have yPels = 1; i.e., always scales
linearly.

&nbsp;<br>&nbsp;<br>
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Microsoft Typography Web Site <A HREF="/truetype/otspec/CPYRIGHT.htm">&copy; 1996 Microsoft Corporation</A>
<BR>
Comments to the Microsoft Typography group: <A HREF="mailto:ttwsite@microsoft.com">ttwsite@microsoft.com</A>
<BR>
<A HREF="/truetype/default.htm">Home</a> | <a href="/truetype/creators.htm">Information for Developers</a>
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Last updated 05 September 1996
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