proposal: x/image/font: add sTypoAscent and sTypoDescent

[zhuowenchen]

Proposal Details

Now we will parse the ascender and descender in the hhea table of the font file, but this is not enough. According to the protocol description, the ascender and descender in the hhea table are applied to the apple platform, while the applications developed based on the windows platform require the ascender and descender to parse the sTypoAscender and sTypoDescender in the os2 table. （https://learn.microsoft.com/en-us/typography/opentype/spec/hhea）

However, our library does not provide such support. Can the sTypoAscender and sTypoDescender in the os2 table be added to the parse in the sfnt/sfnt.go file of golang.org/x/image/font/sfnt/?
The structure of the os2 table is described in this document, which contains the required sTypoAscender and sTypoDescender.（https://learn.microsoft.com/da-dk/typography/opentype/spec/os2）

[zhuowenchen]

We can obtain sTypoAscent and sTypoDescent as follows.And exponse them to golang.org/x/images/font/font.go.

[ianlancetaylor]

CC @nigeltao

[nigeltao]

What does FreeType do when you ask it for a font's ascent?

[zhuowenchen]

oh，I gave it a try and it would return the same value to me.It seems to be working correctly.

[zhuowenchen]

But my point is not to indicate that there is a problem with obtaining ascent. I mean that in some occasions we may need sTypoAscent.Should we provide an API to obtain sTypoAscender?

[zhuowenchen]

I tried to view the FreeType source code, and it seems that they have parsed sTypoAscender.

[nigeltao]

Sorry for not being clearer. FreeType parses os2.sTypoAscender (https://github.com/freetype/freetype/blob/1452355de99d22b0cba6ccc17bf5cdf3682ef61e/src/sfnt/ttload.c#L1193) but it also parses hhea.Ascender (https://github.com/freetype/freetype/blob/1452355de99d22b0cba6ccc17bf5cdf3682ef61e/src/sfnt/ttmtx.c#L143).

My question is, as it parses both, which one does FreeType prefer when you ask it for a font's ascent?

[zhuowenchen]

I tried it. The FreeType will return the value which is parsed from hhea.Ascender when i ask it for a font's ascent.

[zhuowenchen]

Now, x/image/font return the same value. There is no problem in this logic. I mean sometimes we need the sTypoAscender, but now x/image/font can not support that. Maybe we could provide a new field which is named sTypoAscender in class Face or we could provide other ways to get sTypoAscender?

[zhuowenchen]

What do you think of this proposal? If you are busy here, or let me submit the code and you help review it. I really want to be a code contributor, haha🤣