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mkmk blocked by base+mark ligature formation #1109
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Right. Thanks for the great report. I think we should do this:
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That sounds good. We always set the GDEF for this kind of ligature to base because it's not meant to act like a ligature with components. If we expect that kind of behavior we make sure to set the GDEF to ligature. |
Do you happen to have a font I can use for testing and add to test suite? |
I sent you a font on June 28 with test strings. I can send a new one if you need. This isn't delivered yet so can't post here. |
Got it. Thanks.
…On Wed, Jul 25, 2018 at 11:14 AM, Zachary Quinn Scheuren < ***@***.***> wrote:
I sent you a font on June 28 with test strings. I can send a new one if
you need. This isn't delivered yet so can't post here.
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The situation is not so unusual. It's one way of dealing with Tai Tham <NA, TONE-2, SIGN AA, MAI KANG> 'water'. NA and SIGN AA ligate, and then TONE-2 and MAI KANG interact. SIGN AA is a spacing post-base mark that mostly acts like a base, and the other two marks are marks above. |
@Richard57 SIGN AA is a base, isn't it? I know it's not a consonant, but it takes up space and it doesn't 'attach' as such. Would cursive attachment help at all with the original issue? |
So far as I can tell, it is entirely a matter of choice as to whether U+1A63 TAI THAM SIGN AA is a base or a mark with an advance width to be restored by the |
Fixed in 9efddb9 However, @punchcutter the font you sent me has the iMatra_gran as a base in GDEF, not mark. So we couldn't verify the fix. Please test, and send me the corrected font so I can add a test to the test suite. Thanks. |
After some discussion @behdad summed it up nicely:
With sequence base1,mark1,base2,mark2 that makes sense that the marks both apply to the resulting ligature of base1+base2. However, what we have here is base1,mark1,mark2,mark3. A ligature is formed from base1+mark2 and then mark1 should attach to that with mark feature and mark3 should then attach to mark1 with mkmk feature. This is probably an unusual situation, but in this case all marks are post-base marks. They are actually all Spacing Marks, but we can't classify them as bases in the GDEF because we need them to be marks to allow for other following marks (including Vedic marks). That puts us in this situation of relying on mark and mkmk attachment to place them appropriately.
Here's a little visual explanation showing what happens in various situations. The third one shows how the ligature stops the mkmk. The fourth shows what should happen. There mark2 doesn't ligate so it's showing base1, mark1 (post-base), mark2 (top), mark3 (mkmk attached to mark1).
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