Strange rendering errors on OS X with CoreText-enabled HarfBuzz and AAT fonts

This was "found" with mpv's OSD renderer (which uses libass to render text and other stuff).

It works when forcing fontconfig.

[Script Info]
ScriptType: v4.00+
WrapStyle: 0
PlayResX: 512
PlayResY: 288
ScaledBorderAndShadow: yes
YCbCr Matrix: TV.709

[V4+ Styles]
Format: Name, Fontname, Fontsize, PrimaryColour, SecondaryColour, OutlineColour, BackColour, Bold, Italic, Underline, StrikeOut, ScaleX, ScaleY, Spacing, Angle, BorderStyle, Outline, Shadow, Alignment, MarginL, MarginR, MarginV, Encoding
Style: progbar,sans-serif,22,&H00ffffff,&H00ffffff,&H00000000,&H7ff0f0f0,0,0,0,0,100,100,0,0,1,1.200000,0,7,10,10,8,1
Style: OSD,sans-serif,22,&H00ffffff,&H00ffffff,&H00000000,&H7ff0f0f0,0,0,0,0,100,100,0,0,1,1.200000,0,7,10,10,8,1

[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:00.00,0:00:02.00,progbar,,0,0,0,,{\an6\pos(60.150002,241.089996)}
Dialogue: 0,0:00:00.00,0:00:02.00,progbar,,0,0,0,,{\bord0\pos(64.000000,208.649994)}{\p4} m 0 0 l 15 0 l 15 72 l 0 72{\p0}
Dialogue: 0,0:00:00.00,0:00:02.00,progbar,,0,0,0,,{\pos(64.000000,236.589996)}{\p4} m -6 -6 l 3078 -6 l 3078 78 l -6 78 m 0 0 l 0 72 l 3072 72 l 3072 0 m 207 0 l 199 8 l 191 0 m 191 72 l 199 64 l 207 72 m 396 0 l 387 8 l 379 0 m 379 72 l 387 64 l 396 72 m 1375 0 l 1367 8 l 1358 0 m 1358 72 l 1367 64 l 1375 72 m 2828 0 l 2820 8 l 2811 0 m 2811 72 l 2820 64 l 2828 72 m 3017 0 l 3008 8 l 3000 0 m 3000 72 l 3008 64 l 3017 72{\p0}
Dialogue: 0,0:00:00.00,0:00:03.00,OSD,,0,0,0,,osd text

http://0x0.st/HmJ.png

PS: the linked image was made with the libass test program.

Also, if you move the last event to the top, it renders correctly.

[astiob]

Reduced further:

Dialogue: 0,0:00:00.00,0:00:02.00,progbar,,0,0,0,,{\p1} {\p0}
Dialogue: 0,0:00:00.00,0:00:03.00,OSD,,0,0,0,,osd text

[astiob]

Reduced: {\fnHelvetica\b301\p1} {\p0}text in any style. \b can have any valid argument* except numbers in the range from 100 to 300 inclusive (otherwise HelveticaLight.ttf is selected instead of Helvetica.dfont)**. The drawing text must be nonempty (otherwise the drawing will be skipped entirely by the parser), but it doesn’t have to correspond to an actual nonempty drawing. The drawing must be the first thing set in Helvetica (regardless of the \b value).

* Note: valid \b arguments are 0, 1 and integers in the range from 100 to INT_MAX, modulo parsing (libass currently reads a floating-point number and rounds it) and overflow. Invalid arguments, as well as a lack of arguments, cause the font weight to be reset to the weight specified for the current style.

** At least on Snow Leopard.

[astiob]

Works correctly without harfbuzz.

Coincides with users reporting that disabling harfbuzz made it work. Not sure if I could reproduce that.

[Nyx0uf]

Yes I can confirm that disabling harfbuzz solved the problem for me (10.11, / latest libass & mpv)

[astiob]

In fact, this bug does happen with Fontconfig as well, but the \b value must be 0, 1 or at least 350. The reason it seemed to work for @wm4 is that Fontconfig selects Bitstream Vera Sans for sans-serif, but only Helvetica (or possibly anything.dfont?) seems to be affected.

Right now I’m thinking this might be a FreeType bug.

[astiob]

Indeed, this happens for every .dfont font.

We force a font size of 256 for all text, but drawings keep the real \fs value (scaled from script resolution). When we ask FreeType for the metrics for the first time, it gives us correct metrics for the first glyph’s font size—if the first glyph is a drawing, this is \fs—and for some reason, when we ask it again for later glyphs, it keeps returning the same values instead of the correct values for whatever size the later glyphs have.

[astiob]

No, FreeType is fine, or at least not broken like that. HarfBuzz never requests the metrics for any of the individual glyphs! It does with a non-dfont.

[astiob]

This happens if and only if HarfBuzz is built with Core Text support. It detects that the font has a morx table meant for Apple Advanced Typography and hence delegates shaping to Core Text.