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Subject:	Re: mintty overstrokes some fonts unexpectedly
To:	cygwin AT cygwin DOT com
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From:	Thomas Wolff <towo AT towo DOT net>
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Date:	Mon, 26 Apr 2021 20:31:38 +0200
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Am 26.04.2021 um 01:14 schrieb Lemures Lemniscati via Cygwin:
> On Sun, 25 Apr 2021 22:33:57 +0200, Thomas Wolff
>> Am 25.04.2021 um 15:41 schrieb Lemures Lemniscati via Cygwin:
>>> Hi!
>>>
>>> mintty overstrokes some fonts unexpectedly.
>>> https://gitlab.com/test.cases/mintty-test/-/tree/54ae800e695ecd1741851cab57320a9d0e95a6fd
>>>
>>> I got a result mintty-sample-msgothic.png.
>>> https://gitlab.com/test.cases/mintty-test/-/blob/54ae800e695ecd1741851cab57320a9d0e95a6fd/mintty-sample-msgothic.png
>>>
>>> In the 4th line of the output, fonts (of u+25cb) were overstruck
>>> unexpectedly.
>>> And there are other characters also, which are similarly overstruck.
>> This is a Windows bug. Mintty clearly instruct Windows to apply equidistant spacing to achieve fixed-width character cell behaviour. But for certain character ranges, Windows ignores that. Another example for such misbehaviour is the Tibetan block (U+0F00-U+0FFF). Mintty could work around that by rendering characters separately, at a significant penalty for output speed however. Or it could do that only for affected ranges, but criteria to identify them are obscure.
> Thank you, Thomas.
>
> I tried some earlier versions of mintty:
>
> * mintty-3.1.0-1 has the same issue
> * mintty-2.9.6-0 works expectedly in this case.
My previous comment was wrong, sorry. The cause of the issue is that the 
Windows CJK fonts are designed for CJK ambiguous-wide layout. If you run 
mintty with option Charwidth=ambig-wide, or better in an ambiguous-wide 
locale like LC_CTYPE=C DOT utf8 AT cjkwide, width handling and font rendering 
will match.
Mintty applies auto-narrowing to some characters that would render far 
out of their character cell, squeezing them into the cell, in order to 
optimise the trade-off between readability and authentic rendering.
Some character ranges were taken out of that mechanism in 2.9.7, 
including the Geometric Shapes which you've encountered, in this case 
because they are "geometric" and the assumption was that they should not 
be tampered for rendering.

Thomas

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