[hb-style] Fix the sign of slant ratios

We want negative slant angles to yield
positive slant ratios. Fix that.

Test included.

src/hb-style.cc

@@ -46,13 +46,13 @@
 static inline float
 _hb_angle_to_ratio (float a)
 {
-  return tanf (a * float (M_PI / 180.));
+  return - tanf (a * float (M_PI / 180.));
 }
 
 static inline float
 _hb_ratio_to_angle (float r)
 {
-  return atanf (r) * float (180. / M_PI);
+  return - atanf (r) * float (180. / M_PI);
 }
 
 /**

src/hb-style.h

@@ -43,8 +43,10 @@ HB_BEGIN_DECLS
  * @HB_STYLE_TAG_SLANT_ANGLE: Used to vary between upright and slanted text. Values
  * must be greater than -90 and less than +90. Values can be interpreted as
  * the angle, in counter-clockwise degrees, of oblique slant from whatever the
- * designer considers to be upright for that font design.
+ * designer considers to be upright for that font design. Typical right-leaning
+ * Italic fonts have a negative slant angle (typically around -12)
  * @HB_STYLE_TAG_SLANT_RATIO: same as @HB_STYLE_TAG_SLANT_ANGLE expression as ratio.
+ * Typical right-leaning Italic fonts have a positive slant ratio (typically around 0.2)
  * @HB_STYLE_TAG_WIDTH: Used to vary width of text from narrower to wider.
  * Non-zero. Values can be interpreted as a percentage of whatever the font
  * designer considers “normal width” for that font design.

test/api/test-style.c

@@ -161,6 +161,18 @@ test_synthetic_slant (void)
 
   hb_font_destroy (font);
   hb_face_destroy (face);
+
+  face = hb_test_open_font_file ("fonts/notosansitalic.ttf");
+  font = hb_font_create (face);
+
+  /* We expect a negative angle for a typical italic font,
+   * which should give us a positive ratio
+   */
+  assert_cmpfloat (hb_style_get_value (font, HB_STYLE_TAG_SLANT_ANGLE), -12);
+  assert_cmpfloat (hb_style_get_value (font, HB_STYLE_TAG_SLANT_RATIO), 0.21);
+
+  hb_font_destroy (font);
+  hb_face_destroy (face);
 }
 
 int