[pango/italic-correction] wip: Apply italic correction

[Matthias Clasen <matthiasc src gnome org>]

commit 390c6cdb5eebe7f21625f9590a184f14ab57cd70
Author: Matthias Clasen <mclasen redhat com>
Date:   Fri Feb 18 19:21:49 2022 -0600

    wip: Apply italic correction
    
    This is a work-in-progress attempt to implement Italic
    correction between runs of different slant.

 pango/pango-layout.c | 42 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)
---
diff --git a/pango/pango-layout.c b/pango/pango-layout.c
index 6e4bb357..5933bd13 100644
--- a/pango/pango-layout.c
+++ b/pango/pango-layout.c
@@ -6330,6 +6330,44 @@ adjust_line_letter_spacing (PangoLayoutLine *line,
     line->runs = g_slist_reverse (line->runs);
 }
 
+static void
+apply_italic_correction (PangoLayoutLine *line,
+                         ParaBreakState  *state)
+{
+  GSList *l;
+
+  if (atoi (g_getenv ("ITALC") ? : "0") == 0)
+    return;
+
+  for (l = line->runs; l; l = l->next)
+    {
+      PangoLayoutRun *run = l->data;
+      PangoLayoutRun *next_run = l->next ? l->next->data : NULL;
+      hb_font_t *font, *next_font;
+      float slant, next_slant;
+      hb_font_extents_t extents;
+      hb_position_t height;
+      hb_position_t pos;
+
+      if (!next_run)
+        break;
+
+      font = pango_font_get_hb_font (run->item->analysis.font);
+      next_font = pango_font_get_hb_font (next_run->item->analysis.font);
+
+      slant = hb_style_get_value (font, HB_STYLE_TAG_SLANT_RATIO);
+      next_slant = hb_style_get_value (next_font, HB_STYLE_TAG_SLANT_RATIO);
+
+      if (next_slant > slant)
+        continue;
+
+      hb_font_get_extents_for_direction (font, HB_DIRECTION_LTR, &extents);
+      height = extents.ascender - extents.descender;
+      pos = extents.ascender - height / 2;
+      pad_glyphstring_right (run->glyphs, state, (slant - next_slant) * pos);
+    }
+}
+
 static void
 justify_clusters (PangoLayoutLine *line,
                   ParaBreakState  *state)
@@ -6823,6 +6861,10 @@ pango_layout_line_postprocess (PangoLayoutLine *line,
 
   DEBUG ("after letter spacing", line, state);
 
+  apply_italic_correction (line, state);
+
+  DEBUG ("after italic correction", line, state);
+
   /* Distribute extra space between words if justifying and line was wrapped */
   if (line->layout->justify && (wrapped || ellipsized || line->layout->justify_last_line))
     {