Linearly interpolate weight values

Rest of Part of https://bugs.freedesktop.org/show_bug.cgi?id=81453

Adds new API:

	FcWeightFromOpenType()
	FcWeightToOpenType()

doc/Makefile.am

@@ -85,6 +85,7 @@ DOC_FUNCS_FNCS =		\
 	fcstring.fncs		\
 	fcstrset.fncs		\
 	fcvalue.fncs		\
+	fcweight.fncs		\
 	$(NULL)
 SGML_FILES =			\
 	fontconfig-user.sgml	\

doc/fcweight.fncs

@@ -0,0 +1,47 @@
+/*
+ * fontconfig/doc/fcweight.fncs
+ *
+ * Permission to use, copy, modify, distribute, and sell this software and its
+ * documentation for any purpose is hereby granted without fee, provided that
+ * the above copyright notice appear in all copies and that both that
+ * copyright notice and this permission notice appear in supporting
+ * documentation, and that the name of the author(s) not be used in
+ * advertising or publicity pertaining to distribution of the software without
+ * specific, written prior permission.  The authors make no
+ * representations about the suitability of this software for any purpose.  It
+ * is provided "as is" without express or implied warranty.
+ *
+ * THE AUTHOR(S) DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE,
+ * INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO
+ * EVENT SHALL THE AUTHOR(S) BE LIABLE FOR ANY SPECIAL, INDIRECT OR
+ * CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE,
+ * DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
+ * TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+ * PERFORMANCE OF THIS SOFTWARE.
+ */
+@RET@		int
+@FUNC@		FcWeightFromOpenType
+@TYPE1@		int				@ARG1@		ot_weight
+@PURPOSE@	Convert from OpenType weight values to fontconfig ones
+@DESC@
+<function>FcWeightFromOpenType</function> returns an integer value
+to use with FC_WEIGHT, from an integer in the 1..1000 range, resembling
+the numbers from OpenType specification's OS/2 usWeight numbers, which
+are also similar to CSS font-weight numbers.  If input is negative,
+zero, or greater than 1000, returns -1.  This function linearly interpolates
+between various FC_WEIGHT_* constants.  As such, the returned value does not
+necessarily match any of the predefined constants.
+@SINCE@		2.11.91
+@@
+
+@RET@		int
+@FUNC@		FcWeightToOpenType
+@TYPE1@		int				@ARG1@		ot_weight
+@PURPOSE@	Convert from fontconfig weight values to OpenType ones
+@DESC@
+<function>FcWeightToOpenType</function> is the inverse of
+<function>FcWeightFromOpenType</function>.  If the input is less than
+FC_WEIGHT_THIN or greater than FC_WEIGHT_EXTRABLACK, returns -1.  Otherwise
+returns a number in the range 1 to 1000.
+@SINCE@		2.11.91
+@@

fontconfig/fontconfig.h

@@ -908,6 +908,13 @@ FcRangeDestroy (FcRange *range);
 FcPublic FcRange *
 FcRangeCopy (const FcRange *r);
 
+/* fcweight.c */
+
+FcPublic int
+FcWeightFromOpenType (int ot_weight);
+
+FcPublic int
+FcWeightToOpenType (int fc_weight);
 
 /* fcstr.c */
 

src/Makefile.am

@@ -154,6 +154,7 @@ libfontconfig_la_SOURCES = \
 	fcserialize.c \
 	fcstat.c \
 	fcstr.c \
+	fcweight.c \
 	fcwindows.h \
 	fcxml.c \
 	ftglue.h \

src/fcfreetype.c

@@ -1628,32 +1628,7 @@ FcFreeTypeQueryFace (const FT_Face  face,
 
     if (os2 && os2->version != 0xffff)
     {
-	if (os2->usWeightClass == 0)
+	weight = FcWeightFromOpenType (os2->usWeightClass);
-	    ;
-	else if (os2->usWeightClass < 150)
-	    weight = FC_WEIGHT_THIN;
-	else if (os2->usWeightClass < 250)
-	    weight = FC_WEIGHT_EXTRALIGHT;
-	else if (os2->usWeightClass < 325)
-	    weight = FC_WEIGHT_LIGHT;
-	else if (os2->usWeightClass < 365)
-	    weight = FC_WEIGHT_SEMILIGHT;
-	else if (os2->usWeightClass < 390)
-	    weight = FC_WEIGHT_BOOK;
-	else if (os2->usWeightClass < 450)
-	    weight = FC_WEIGHT_REGULAR;
-	else if (os2->usWeightClass < 550)
-	    weight = FC_WEIGHT_MEDIUM;
-	else if (os2->usWeightClass < 650)
-	    weight = FC_WEIGHT_SEMIBOLD;
-	else if (os2->usWeightClass < 750)
-	    weight = FC_WEIGHT_BOLD;
-	else if (os2->usWeightClass < 850)
-	    weight = FC_WEIGHT_EXTRABOLD;
-	else if (os2->usWeightClass < 925)
-	    weight = FC_WEIGHT_BLACK;
-	else if (os2->usWeightClass < 1000)
-	    weight = FC_WEIGHT_EXTRABLACK;
 	if ((FcDebug() & FC_DBG_SCANV) && weight != -1)
 	    printf ("\tos2 weight class %d maps to weight %d\n",
 		    os2->usWeightClass, weight);

src/fcweight.c

@@ -0,0 +1,84 @@
+/*
+ * fontconfig/src/fcweight.c
+ *
+ * Permission to use, copy, modify, distribute, and sell this software and its
+ * documentation for any purpose is hereby granted without fee, provided that
+ * the above copyright notice appear in all copies and that both that
+ * copyright notice and this permission notice appear in supporting
+ * documentation, and that the name of the author(s) not be used in
+ * advertising or publicity pertaining to distribution of the software without
+ * specific, written prior permission.  The authors make no
+ * representations about the suitability of this software for any purpose.  It
+ * is provided "as is" without express or implied warranty.
+ *
+ * THE AUTHOR(S) DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE,
+ * INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO
+ * EVENT SHALL THE AUTHOR(S) BE LIABLE FOR ANY SPECIAL, INDIRECT OR
+ * CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE,
+ * DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
+ * TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+ * PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include "fcint.h"
+
+static const struct {
+  int ot;
+  int fc;
+} map[] = {
+    {   0, FC_WEIGHT_THIN },
+    { 100, FC_WEIGHT_THIN },
+    { 200, FC_WEIGHT_EXTRALIGHT },
+    { 350, FC_WEIGHT_DEMILIGHT },
+    { 300, FC_WEIGHT_LIGHT },
+    { 380, FC_WEIGHT_BOOK },
+    { 400, FC_WEIGHT_REGULAR },
+    { 500, FC_WEIGHT_MEDIUM },
+    { 600, FC_WEIGHT_DEMIBOLD },
+    { 700, FC_WEIGHT_BOLD },
+    { 800, FC_WEIGHT_EXTRABOLD },
+    { 900, FC_WEIGHT_BLACK },
+    {1000, FC_WEIGHT_EXTRABLACK },
+};
+
+static int lerp(int x, int x1, int x2, int y1, int y2)
+{
+  int dx = x2 - x1;
+  int dy = y2 - y1;
+  assert (dx > 0 && dy > 0 && x1 <= x && x <= x2);
+  return y1 + (dy*(x-x1) + dx/2) / dx;
+}
+
+FcPublic int
+FcWeightFromOpenType (int ot_weight)
+{
+	int i;
+	if (ot_weight <= 0 || ot_weight > 1000)
+	    return -1;
+
+	for (i = 1; ot_weight > map[i].ot; i++)
+	  ;
+
+	if (ot_weight == map[i].ot)
+	  return map[i].fc;
+
+	/* Interpolate between two items. */
+	return lerp (ot_weight, map[i-1].ot, map[i].ot, map[i-1].fc, map[i].fc);
+}
+
+FcPublic int
+FcWeightToOpenType (int fc_weight)
+{
+	int i;
+	if (fc_weight < 0 || fc_weight > FC_WEIGHT_EXTRABLACK)
+	    return -1;
+
+	for (i = 1; fc_weight > map[i].fc; i++)
+	  ;
+
+	if (fc_weight == map[i].fc)
+	  return map[i].ot;
+
+	/* Interpolate between two items. */
+	return lerp (fc_weight, map[i-1].fc, map[i].fc, map[i-1].ot, map[i].ot);
+}