Commit f72b00746c64e13625cf8371f031411fbd0d6161
2019-02-02T15:45:31
[sfnt] Use typo metrics if OS/2 fsSelection USE_TYPO_METRICS bit is set. If the OS/2 table exists and fsSelection bit 7 (USE_TYPO_METRICS) is set, use the sTypo* set of values to compute the FT_Face's ascender, descender and height. Otherwise, fall back to old behavior. * src/sfnt/sfobjs.c (sfnt_load_face): Implement.
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diff --git a/ChangeLog b/ChangeLog
index 6f08b20..aac45a4 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,13 @@
+2019-02-02 Nikolaus Waxweiler <madigens@gmail.com>
+
+ [sfnt] Use typo metrics if OS/2 fsSelection USE_TYPO_METRICS bit is set.
+
+ If the OS/2 table exists and fsSelection bit 7 (USE_TYPO_METRICS) is set,
+ use the sTypo* set of values to compute the FT_Face's ascender, descender
+ and height. Otherwise, fall back to old behavior.
+
+ * src/sfnt/sfobjs.c (sfnt_load_face): Implement.
+
2019-01-18 John Tytgat <John.Tytgat@esko.com>
[sfnt] Handle TT fonts having two PostScript font names (#55471).
@@ -420,7 +430,7 @@
* src/base/ftobjs.c (ft_glyphslot_preset_bimap): Another tweak.
This one should be clearer. When the rounded monochrome bbox collapses
- we add a pixel that covers most if not all original cbox.
+ we add a pixel that covers most if not all original cbox.
2018-09-21 Alexei Podtelezhnikov <apodtele@gmail.com>
diff --git a/src/sfnt/sfobjs.c b/src/sfnt/sfobjs.c
index 7705c08..126e174 100644
--- a/src/sfnt/sfobjs.c
+++ b/src/sfnt/sfobjs.c
@@ -1641,59 +1641,73 @@
root->units_per_EM = face->header.Units_Per_EM;
- /* XXX: Computing the ascender/descender/height is very different */
- /* from what the specification tells you. Apparently, we */
- /* must be careful because */
- /* */
- /* - not all fonts have an OS/2 table; in this case, we take */
- /* the values in the horizontal header. However, these */
- /* values very often are not reliable. */
- /* */
- /* - otherwise, the correct typographic values are in the */
- /* sTypoAscender, sTypoDescender & sTypoLineGap fields. */
- /* */
- /* However, certain fonts have these fields set to 0. */
- /* Rather, they have usWinAscent & usWinDescent correctly */
- /* set (but with different values). */
- /* */
- /* As an example, Arial Narrow is implemented through four */
- /* files ARIALN.TTF, ARIALNI.TTF, ARIALNB.TTF & ARIALNBI.TTF */
- /* */
- /* Strangely, all fonts have the same values in their */
- /* sTypoXXX fields, except ARIALNB which sets them to 0. */
- /* */
- /* On the other hand, they all have different */
- /* usWinAscent/Descent values -- as a conclusion, the OS/2 */
- /* table cannot be used to compute the text height reliably! */
- /* */
-
- /* The ascender and descender are taken from the `hhea' table. */
- /* If zero, they are taken from the `OS/2' table. */
-
- root->ascender = face->horizontal.Ascender;
- root->descender = face->horizontal.Descender;
-
- root->height = root->ascender - root->descender +
- face->horizontal.Line_Gap;
-
- if ( !( root->ascender || root->descender ) )
+ /*
+ * Computing the ascender/descender/height is tricky.
+ *
+ * The OpenType specification v1.8.3 says:
+ *
+ * [OS/2's] sTypoAscender, sTypoDescender and sTypoLineGap fields
+ * are intended to allow applications to lay out documents in a
+ * typographically-correct and portable fashion.
+ *
+ * This is somewhat at odds with the decades of backwards
+ * compatibility, operating systems and applications doing whatever
+ * they want, not to mention broken fonts.
+ *
+ * Not all fonts have an OS/2 table; in this case, we take the values
+ * in the horizontal header, although there is nothing stopping the
+ * values from being unreliable. Even with a OS/2 table, certain fonts
+ * set the sTypoAscender, sTypoDescender and sTypoLineGap fields to 0
+ * and instead correctly set usWinAscent and usWinDescent.
+ *
+ * As an example, Arial Narrow is shipped as four files ARIALN.TTF,
+ * ARIALNI.TTF, ARIALNB.TTF and ARIALNBI.TTF. Strangely, all fonts have
+ * the same values in their sTypo* fields, except ARIALNB.ttf which
+ * sets them to 0. All of them have different usWinAscent/Descent
+ * values. The OS/2 table therefore cannot be trusted for computing the
+ * text height reliably.
+ *
+ * As a compromise, do the following:
+ *
+ * 1. If the OS/2 table exists and the fsSelection bit 7 is set
+ * (USE_TYPO_METRICS), trust the font and use the sTypo* metrics.
+ * 2. Otherwise, use the `hhea' table's metrics.
+ * 3. If they are zero and the OS/2 table exists,
+ * 1. use the OS/2 table's sTypo* metrics if they are non-zero.
+ * 2. Otherwise, use the OS/2 table's usWin* metrics.
+ */
+
+ if ( face->os2.version != 0xFFFFU && face->os2.fsSelection & 128 )
{
- if ( face->os2.version != 0xFFFFU )
- {
- if ( face->os2.sTypoAscender || face->os2.sTypoDescender )
- {
- root->ascender = face->os2.sTypoAscender;
- root->descender = face->os2.sTypoDescender;
+ root->ascender = face->os2.sTypoAscender;
+ root->descender = face->os2.sTypoDescender;
+ root->height = root->ascender - root->descender +
+ face->os2.sTypoLineGap;
+ }
+ else
+ {
+ root->ascender = face->horizontal.Ascender;
+ root->descender = face->horizontal.Descender;
+ root->height = root->ascender - root->descender +
+ face->horizontal.Line_Gap;
- root->height = root->ascender - root->descender +
- face->os2.sTypoLineGap;
- }
- else
+ if ( !( root->ascender || root->descender ) )
+ {
+ if ( face->os2.version != 0xFFFFU )
{
- root->ascender = (FT_Short)face->os2.usWinAscent;
- root->descender = -(FT_Short)face->os2.usWinDescent;
-
- root->height = root->ascender - root->descender;
+ if ( face->os2.sTypoAscender || face->os2.sTypoDescender )
+ {
+ root->ascender = face->os2.sTypoAscender;
+ root->descender = face->os2.sTypoDescender;
+ root->height = root->ascender - root->descender +
+ face->os2.sTypoLineGap;
+ }
+ else
+ {
+ root->ascender = (FT_Short)face->os2.usWinAscent;
+ root->descender = -(FT_Short)face->os2.usWinDescent;
+ root->height = root->ascender - root->descender;
+ }
}
}
}