Proposed fix to allow DynamicFont kerning to be re-enabled

[follower]

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TL;DR: The Bug & Cause

• DynamicFont kerning was removed by Fix font kerning #24777 due to issues with rendering some fonts as described in Weird kerning on labels #21965.

• The underlying issue with the removed kerning-related code was that the call to FT_Get_Kerning() supplied arguments that were character codes rather than glyph indexes.

• The observed impact of the issue was that the kerning value applied was intended for a different pair of characters than the pair displayed.

  For certain font & character combinations the difference between the correct & incorrect kerning values was significant. e.g. for PetitLatin.ttf the kerning for t| ("tee pipe") was used to kern ck which resulted in character overlap.

TL;DR: The Fix

Potential fix steps:

• Revert a9b7843

• Apply this patch:

diff --git a/scene/resources/dynamic_font.cpp b/scene/resources/dynamic_font.cpp
index e165f7c34b..be6d6b517f 100644
--- a/scene/resources/dynamic_font.cpp
+++ b/scene/resources/dynamic_font.cpp
@@ -282,7 +282,7 @@ float DynamicFontAtSize::_get_kerning_advance(const DynamicFontAtSize *font, Cha
 
 	if (p_next) {
 		FT_Vector delta;
-		FT_Get_Kerning(font->face, p_char, p_next, FT_KERNING_DEFAULT, &delta);
+		FT_Get_Kerning(font->face, FT_Get_Char_Index(font->face, p_char), FT_Get_Char_Index(font->face, p_next), FT_KERNING_DEFAULT, &delta);
 		advance = (delta.x / 64.0) / oversampling;
 	}

This fix would both re-enable kerning[1] for DynamicFont and fix the previously observed kerning issue.

[1] For fonts with kern tables but still not for GPOS/kern tables.

TL;DR: Pretty pictures

Comparison screenshots based on HEAD 3.2 branch yesterday:

• The bug (incorrect kerning applied):

  

• Current situation (no kerning applied):

  

• After fix applied (correct kerning applied):

  

  Note the correct c+k kerning and kerning that reduces the gap for the u+c & t+| character pairs.

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[WIP: Intend to add additional details & code references.]

[Anutrix]

Can you make a PR?

[follower]

@Anutrix Yes, although I'm currently knee deep in yaks. :)

[akien-mga]

Fixed by #49377.

[follower]

Can confirm that as of Godot 3.4 beta 2 the official "ducky tee-pipe"(TM) kerning test now appears to pass. :)

Font file from Petit Latin Font.