harbuzzng: Remove assumption about Core Text working in 96 DPI

Core Text doesn't actually have a concept of DPI internally, as it
doesn't rasterize anything by itself, it just generates vector paths
that get passed along to Core Graphics.

In practice this means Core Text operates in the classical macOS
logical DPI of 72, with one typographic point corresponding to one
point in the Core Graphics coordinate system, which for a normal
bitmap context then corresponds to one pixel -- or two pixels for
a "retina" context with a 2x scale transform.

Scaling the font point sizes given to HarfBuzz to an assumed DPI
of 96 is problematic with this in mind, as fonts with optical
features such as 'trak' tables for tracking, or color glyphs,
will then base the metrics off of the wrong point size compared
to what the client asked for.

This in turn causes mismatches between the metrics of the shaped
text and the actual rasterization, which doesn't include the 72
to 96 DPI scaling.

If a 96 DPI is needed, such as on the Web, the scaling should be
done outside of HarfBuzz, allowing the client to keep the DPI of
the shaping in sync with the rasterization.

The recommended way to do that is by scaling the font point size,
not by applying a transform to the target Core Graphics context,
to let Core Text choose the right optical features of the target
point size, as described in WWDC 2015 session 804:

https://developer.apple.com/videos/play/wwdc2015/804/

GitHub-PR: https://github.com/harfbuzz/harfbuzz/pull/1484
Change-Id: I830f0cd7a82552422bbe09226e2d571e246fe3f4
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

1 files changed, 4 insertions, 21 deletions

diff --git a/src/3rdparty/harfbuzz-ng/src/hb-coretext.cc b/src/3rdparty/harfbuzz-ng/src/hb-coretext.cc
index 9431ba5fe1..d64cb7edbd 100644
--- a/src/3rdparty/harfbuzz-ng/src/hb-coretext.cc
+++ b/src/3rdparty/harfbuzz-ng/src/hb-coretext.cc
@@ -52,24 +52,6 @@ struct CoreTextFontEngineData {
 /* https://developer.apple.com/documentation/coretext/1508745-ctfontcreatewithgraphicsfont */
 #define HB_CORETEXT_DEFAULT_FONT_SIZE 12.f
 
-static CGFloat
-coretext_font_size_from_ptem (float ptem)
-{
-  /* CoreText points are CSS pixels (96 per inch),
-   * NOT typographic points (72 per inch).
-   *
-   * https://developer.apple.com/library/content/documentation/GraphicsAnimation/Conceptual/HighResolutionOSX/Explained/Explained.html
-   */
-  ptem *= 96.f / 72.f;
-  return ptem <= 0.f ? HB_CORETEXT_DEFAULT_FONT_SIZE : ptem;
-}
-static float
-coretext_font_size_to_ptem (CGFloat size)
-{
-  size *= 72.f / 96.f;
-  return size <= 0.f ? 0 : size;
-}
-
 static void
 release_table_data (void *user_data)
 {
@@ -104,7 +86,7 @@ _hb_cg_font_release (void *data)
 
 HB_SHAPER_DATA_ENSURE_DEFINE(coretext, face)
 HB_SHAPER_DATA_ENSURE_DEFINE_WITH_CONDITION(coretext, font,
-	fabs (CTFontGetSize((CTFontRef) data) - coretext_font_size_from_ptem (font->ptem)) <= .5
+	fabs (CTFontGetSize ((CTFontRef) data) - font->ptem) <= .5
 )
 
 static CTFontDescriptorRef
@@ -308,7 +290,8 @@ _hb_coretext_shaper_font_data_create (hb_font_t *font)
   if (unlikely (!hb_coretext_shaper_face_data_ensure (face))) return nullptr;
   CGFontRef cg_font = (CGFontRef) HB_SHAPER_DATA_GET (face);
 
-  CTFontRef ct_font = create_ct_font (cg_font, coretext_font_size_from_ptem (font->ptem));
+  CGFloat font_size = font->ptem <= 0.f ? HB_CORETEXT_DEFAULT_FONT_SIZE : font->ptem;
+  CTFontRef ct_font = create_ct_font (cg_font, font_size);
 
   if (unlikely (!ct_font))
   {
@@ -340,7 +323,7 @@ hb_coretext_font_create (CTFontRef ct_font)
   if (unlikely (hb_object_is_inert (font)))
     return font;
 
-  hb_font_set_ptem (font, coretext_font_size_to_ptem (CTFontGetSize(ct_font)));
+  hb_font_set_ptem (font, CTFontGetSize (ct_font));
 
   /* Let there be dragons here... */
   HB_SHAPER_DATA_GET (font) = (hb_coretext_shaper_font_data_t *) CFRetain (ct_font);