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OpenType GPOS LookupType 8 - skipping marks

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I am trying to understand the rendering of "بڑ" (Urdu - Unicode 1576, 1681) with a font Jameel_Noori_Nastaleeq.ttf.

The string is converted into glyphs [607, 460, 471, 1651] by the GSUB table. I can detect the correct anchor-attachment of the second glyph under the first one. But I can not find an appropriate GSUB subtable, that would position the third glyph on top of the first one. Here, the left one is correct, the right one is what my program does at the moment.

Also, I don't quite understand the LookupType 8 of GSUB. Some LookupTable can have LookupFlags including a bit 8 - ignoreMarks. When matching the Backtrack, Input and Lookahead sequences, should I take these flags into account, i.e. skip marks? What exactly is the mechanism of matching and applying LookupType 8?

fonts true-type-fonts opentype

share|improve this question

asked Mar 27 at 23:54

Ivan KuckirIvan Kuckir

1,39111 gold badge1515 silver badges3333 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  You 100% want to ask this question over on typedrawers.com, rather than here. OpenType programming is a super niche subject, and typedrawers.com is full of literally all the people that worked on shaping the opentype spec itself, knowledge exactly what you're asking about and able to give you a way more detailed answer than almost anyone on SO could give you.
  
  – Mike 'Pomax' Kamermans
  Mar 28 at 0:54
  
  

  
  
  
  

add a comment | 

0

I am trying to understand the rendering of "بڑ" (Urdu - Unicode 1576, 1681) with a font Jameel_Noori_Nastaleeq.ttf.

The string is converted into glyphs [607, 460, 471, 1651] by the GSUB table. I can detect the correct anchor-attachment of the second glyph under the first one. But I can not find an appropriate GSUB subtable, that would position the third glyph on top of the first one. Here, the left one is correct, the right one is what my program does at the moment.

Also, I don't quite understand the LookupType 8 of GSUB. Some LookupTable can have LookupFlags including a bit 8 - ignoreMarks. When matching the Backtrack, Input and Lookahead sequences, should I take these flags into account, i.e. skip marks? What exactly is the mechanism of matching and applying LookupType 8?

fonts true-type-fonts opentype

share|improve this question

asked Mar 27 at 23:54

Ivan KuckirIvan Kuckir

1,39111 gold badge1515 silver badges3333 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  You 100% want to ask this question over on typedrawers.com, rather than here. OpenType programming is a super niche subject, and typedrawers.com is full of literally all the people that worked on shaping the opentype spec itself, knowledge exactly what you're asking about and able to give you a way more detailed answer than almost anyone on SO could give you.
  
  – Mike 'Pomax' Kamermans
  Mar 28 at 0:54
  
  

  
  
  
  

add a comment | 

I am trying to understand the rendering of "بڑ" (Urdu - Unicode 1576, 1681) with a font Jameel_Noori_Nastaleeq.ttf.

The string is converted into glyphs [607, 460, 471, 1651] by the GSUB table. I can detect the correct anchor-attachment of the second glyph under the first one. But I can not find an appropriate GSUB subtable, that would position the third glyph on top of the first one. Here, the left one is correct, the right one is what my program does at the moment.

Also, I don't quite understand the LookupType 8 of GSUB. Some LookupTable can have LookupFlags including a bit 8 - ignoreMarks. When matching the Backtrack, Input and Lookahead sequences, should I take these flags into account, i.e. skip marks? What exactly is the mechanism of matching and applying LookupType 8?

fonts true-type-fonts opentype

share|improve this question

asked Mar 27 at 23:54

Ivan KuckirIvan Kuckir

1,39111 gold badge1515 silver badges3333 bronze badges

share|improve this question

asked Mar 27 at 23:54

Ivan KuckirIvan Kuckir

1,39111 gold badge1515 silver badges3333 bronze badges

share|improve this question

asked Mar 27 at 23:54

Ivan KuckirIvan Kuckir

1,39111 gold badge1515 silver badges3333 bronze badges

asked Mar 27 at 23:54

Ivan KuckirIvan Kuckir

1,39111 gold badge1515 silver badges3333 bronze badges

asked Mar 27 at 23:54

Ivan KuckirIvan Kuckir

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1 Answer
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1

The positioning of both marks (the small tah, and dot of 'beh') is done through a lookup in the Mark Positioning ('mark') feature of the GPOS table, which is applied after the GSUB rules are applied. There is no GSUB-only way to get the correct final positioning. The GPOS must be processed (after the GSUB).

As to the ignoreMarks flag: the flag isn't specific to GSUB LookupType8. Any lookup (GSUB or GPOS) can set this flag. It tells the layout engine to ignore marks in the sequence under consideration, for purposes of matching context. This allows defining substitution contexts with only the "root" glyphs of a sequence, so if the context rule is A B C, a lookup with the ignoreMarks flag set would match A (mark) B C, A B (mark) C, A B C, etc.

It comes into play in this font because first the two input characters are decomposed (in the GSUB) to a sequence of base + mark glyphs, then recomposed (also in the GSUB), then the marks are positioned (in the GPOS).

(as an aside: why are you doing text layout yourself, as opposed to using an existing layout engine, such as HarfBuzz or engines built in to other operating systems?)

share|improve this answer

edited Mar 28 at 1:27

answered Mar 28 at 1:19

djangodudedjangodude

3,34111 gold badge2020 silver badges3333 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Hi, I am making a photo editor www.Photopea.com, that works in a browser and is used by 100 000 people every day. Sadly, there is no OpenType layout engine in Javascript, so I had to make my own called Typr.js . It is already quite advanced, but as my photo editor expands, I have to gradually add missing parts (like rendering Urdu etc).
  
  – Ivan Kuckir
  Mar 28 at 10:21
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  so could you tell me, which lookup table is responsible for repositioning the "tiny b"? The "mark" feature mentions 15 lookup records, and I can not match it with any of them :(
  
  – Ivan Kuckir
  Mar 28 at 11:14
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Have a look at GPOS lookupList index #20. That is the lookup referenced by the 'mark' feature which positions both the dot and the small tah ("tiny b"). It's actually a mark-to-ligature lookup (GPOS lookupType 5).
  
  – djangodude
  Mar 28 at 13:21
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Oh, and check out github.com/prezi/harfbuzz-js which claims to be a JavaScript implementation of HarfBuzz (via Emscripten). Sounds like the project could use some help, and the hard part (OpenType Layout logic) is already done.
  
  – djangodude
  Mar 28 at 13:22
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Oh, thanks a lot! I have been discussing it here in parallel: typedrawers.com/discussion/comment/40179#Comment_40179
  
  – Ivan Kuckir
  Mar 28 at 14:24
  

  
  
  
  

 | 
show 4 more comments

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1

The positioning of both marks (the small tah, and dot of 'beh') is done through a lookup in the Mark Positioning ('mark') feature of the GPOS table, which is applied after the GSUB rules are applied. There is no GSUB-only way to get the correct final positioning. The GPOS must be processed (after the GSUB).

As to the ignoreMarks flag: the flag isn't specific to GSUB LookupType8. Any lookup (GSUB or GPOS) can set this flag. It tells the layout engine to ignore marks in the sequence under consideration, for purposes of matching context. This allows defining substitution contexts with only the "root" glyphs of a sequence, so if the context rule is A B C, a lookup with the ignoreMarks flag set would match A (mark) B C, A B (mark) C, A B C, etc.

It comes into play in this font because first the two input characters are decomposed (in the GSUB) to a sequence of base + mark glyphs, then recomposed (also in the GSUB), then the marks are positioned (in the GPOS).

(as an aside: why are you doing text layout yourself, as opposed to using an existing layout engine, such as HarfBuzz or engines built in to other operating systems?)

share|improve this answer

edited Mar 28 at 1:27

answered Mar 28 at 1:19

djangodudedjangodude

3,34111 gold badge2020 silver badges3333 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Hi, I am making a photo editor www.Photopea.com, that works in a browser and is used by 100 000 people every day. Sadly, there is no OpenType layout engine in Javascript, so I had to make my own called Typr.js . It is already quite advanced, but as my photo editor expands, I have to gradually add missing parts (like rendering Urdu etc).
  
  – Ivan Kuckir
  Mar 28 at 10:21
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  so could you tell me, which lookup table is responsible for repositioning the "tiny b"? The "mark" feature mentions 15 lookup records, and I can not match it with any of them :(
  
  – Ivan Kuckir
  Mar 28 at 11:14
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Have a look at GPOS lookupList index #20. That is the lookup referenced by the 'mark' feature which positions both the dot and the small tah ("tiny b"). It's actually a mark-to-ligature lookup (GPOS lookupType 5).
  
  – djangodude
  Mar 28 at 13:21
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Oh, and check out github.com/prezi/harfbuzz-js which claims to be a JavaScript implementation of HarfBuzz (via Emscripten). Sounds like the project could use some help, and the hard part (OpenType Layout logic) is already done.
  
  – djangodude
  Mar 28 at 13:22
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Oh, thanks a lot! I have been discussing it here in parallel: typedrawers.com/discussion/comment/40179#Comment_40179
  
  – Ivan Kuckir
  Mar 28 at 14:24
  

  
  
  
  

 | 
show 4 more comments

1

The positioning of both marks (the small tah, and dot of 'beh') is done through a lookup in the Mark Positioning ('mark') feature of the GPOS table, which is applied after the GSUB rules are applied. There is no GSUB-only way to get the correct final positioning. The GPOS must be processed (after the GSUB).

As to the ignoreMarks flag: the flag isn't specific to GSUB LookupType8. Any lookup (GSUB or GPOS) can set this flag. It tells the layout engine to ignore marks in the sequence under consideration, for purposes of matching context. This allows defining substitution contexts with only the "root" glyphs of a sequence, so if the context rule is A B C, a lookup with the ignoreMarks flag set would match A (mark) B C, A B (mark) C, A B C, etc.

It comes into play in this font because first the two input characters are decomposed (in the GSUB) to a sequence of base + mark glyphs, then recomposed (also in the GSUB), then the marks are positioned (in the GPOS).

(as an aside: why are you doing text layout yourself, as opposed to using an existing layout engine, such as HarfBuzz or engines built in to other operating systems?)

share|improve this answer

edited Mar 28 at 1:27

answered Mar 28 at 1:19

djangodudedjangodude

3,34111 gold badge2020 silver badges3333 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Hi, I am making a photo editor www.Photopea.com, that works in a browser and is used by 100 000 people every day. Sadly, there is no OpenType layout engine in Javascript, so I had to make my own called Typr.js . It is already quite advanced, but as my photo editor expands, I have to gradually add missing parts (like rendering Urdu etc).
  
  – Ivan Kuckir
  Mar 28 at 10:21
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  so could you tell me, which lookup table is responsible for repositioning the "tiny b"? The "mark" feature mentions 15 lookup records, and I can not match it with any of them :(
  
  – Ivan Kuckir
  Mar 28 at 11:14
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Have a look at GPOS lookupList index #20. That is the lookup referenced by the 'mark' feature which positions both the dot and the small tah ("tiny b"). It's actually a mark-to-ligature lookup (GPOS lookupType 5).
  
  – djangodude
  Mar 28 at 13:21
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Oh, and check out github.com/prezi/harfbuzz-js which claims to be a JavaScript implementation of HarfBuzz (via Emscripten). Sounds like the project could use some help, and the hard part (OpenType Layout logic) is already done.
  
  – djangodude
  Mar 28 at 13:22
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Oh, thanks a lot! I have been discussing it here in parallel: typedrawers.com/discussion/comment/40179#Comment_40179
  
  – Ivan Kuckir
  Mar 28 at 14:24
  

  
  
  
  

 | 
show 4 more comments

The positioning of both marks (the small tah, and dot of 'beh') is done through a lookup in the Mark Positioning ('mark') feature of the GPOS table, which is applied after the GSUB rules are applied. There is no GSUB-only way to get the correct final positioning. The GPOS must be processed (after the GSUB).

As to the ignoreMarks flag: the flag isn't specific to GSUB LookupType8. Any lookup (GSUB or GPOS) can set this flag. It tells the layout engine to ignore marks in the sequence under consideration, for purposes of matching context. This allows defining substitution contexts with only the "root" glyphs of a sequence, so if the context rule is A B C, a lookup with the ignoreMarks flag set would match A (mark) B C, A B (mark) C, A B C, etc.

It comes into play in this font because first the two input characters are decomposed (in the GSUB) to a sequence of base + mark glyphs, then recomposed (also in the GSUB), then the marks are positioned (in the GPOS).

(as an aside: why are you doing text layout yourself, as opposed to using an existing layout engine, such as HarfBuzz or engines built in to other operating systems?)

share|improve this answer

edited Mar 28 at 1:27

answered Mar 28 at 1:19

djangodudedjangodude

3,34111 gold badge2020 silver badges3333 bronze badges

share|improve this answer

edited Mar 28 at 1:27

answered Mar 28 at 1:19

djangodudedjangodude

3,34111 gold badge2020 silver badges3333 bronze badges

answered Mar 28 at 1:19

djangodudedjangodude

3,34111 gold badge2020 silver badges3333 bronze badges

answered Mar 28 at 1:19

djangodudedjangodude

3,34111 gold badge2020 silver badges3333 bronze badges