[HarfBuzz] harfbuz uniscribe backend and firefox.

[Behdad Esfahbod]

Hi Tom,

On 15-08-07 01:15 AM, Tom Hindle wrote:
> Hello,
> 
> Recently I created a experimental patch for Firefox to allow use of harfbuz's
> uniscribe backend, selectable via a about:config option.
> 
> The patch can be seen here:
> https://hg.mozilla.org/try/rev/08eef5e76a40

I'm curious.  Why did you do this?


> In order to get it work I had to make a couple of changes to hb-uniscribe.cc
> 
> This was because when Firefox code calls
> 
> hb_font_set_scale(mHBFont, scale, scale);
> 
> Its scale values are "16.16 fixed point" format (floats converted to int by
> multiplying by 65536) and so it expects values returns by
> hb_buffer_get_glyph_positions to be the same format.

Yes, I had become aware of this, and that's why I stopped building the Win32
binaries with uniscribe backend enabled, until I fix it.


> I made hb-uniscribe.cc comply with this by scaling by ratio between
> font->(x|y)_ppem and font->(x|y)_scale.
> 
> Is this something hb-uniscribe.cc should already be doing or is the Firefox
> code making unsafe assumptions?

The uniscribe backend should do this indeed.  I did this in the CoreText
backend.  Will check your code and fix it eventually.

Thanks,
behdad

> The latest gfxHarfBuzzShaper.cpp source code can be seen here:
> https://hg.mozilla.org/mozilla-central/file/03e3d77d1b6b/gfx/thebes/gfxHarfBuzzShaper.cpp
> 
> 
> 
> Thanks
> Tom
> 
> 
> 
> 
> 
> 
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