bugThe FreeType Project - Bugs: bug #54969, Document FT_Face_Rec_::bbox as...

 
 

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bug #54969: Document FT_Face_Rec_::bbox as unreliable / deprecated.

Submitter:  bungeman <bungeman>
Submitted:  Tue 06 Nov 2018 03:44:29 PM UTC
   
 
Severity:  3 - Normal Item Group:  None
Status:  Fixed Privacy:  Public
Assigned to:  wl Open/Closed:  Closed
Planned Release:  2.10.3

Fri 10 Jul 2020 04:09:07 PM UTC, comment #4: 

Thanks a lot!  Pushed to git repository.

Werner LEMBERG <wl>
Group administrator
Thu 09 Jul 2020 06:50:57 PM UTC, comment #3: 

Attaching a patch which more or less states the same thing the OpenType docs state.

I've been thinking for a while if there is anything else which should be done (like having a specific face_flag for if the bounding box is valid or not). There is already a note on the bounding box that states it is only relevant to scalable formats (which itself has a face_flag). Unfortunately since the flags are default 0, I think the flag would need to be something like FT_FACE_FAG_BBOX_INVALID. On the other hand at the moment I think that flag would more or less means the same thing as FT_FACE_FLAG_VARIATION || !FT_FACE_FLAG_SCALABLE (though not quite, since it could have been set to the default variation).

For the time being just documenting this is fine though.

(file #49453)

bungeman <bungeman>
Sat 06 Apr 2019 05:11:49 AM UTC, comment #2: 

Ping!

Werner LEMBERG <wl>
Group administrator
Tue 06 Nov 2018 04:22:08 PM UTC, comment #1: 

Ben, since you are a native speaker, can you prepare a patch?

Werner LEMBERG <wl>
Group administrator
Tue 06 Nov 2018 03:44:29 PM UTC, original submission:  

As pointed out in the Note in the section https://docs.microsoft.com/en-us/typography/opentype/spec/otvaroverview#variation-data-tables-and-miscellaneous-requirements

"... the xMin, yMin, xMax, yMax, macStyle and lowestRecPPEM fields in the font header ('head') table are not supported by variation data and should only be used in relation to the default instance for the font. Also, variations for values in the kerning ('kern') table are not supported; variable fonts should handle kerning using the GPOS table."

Unfortunately, this means that there is no longer any reliable 'fast union of all glyph bounds'  available in an OpenType font. As a result, any user of these values may get very unexpected results if they are relying on this bbox having a sensible value.

Obviously, this field cannot be removed or given bad values for now, but this should be noted so this field isn't carried over into a future FreeType 3 and so existing users know its limitations (can't be used a a generic fast bounding box for quick rejection).

bungeman <bungeman>

 

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    Follow 5 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2020-07-10 wl StatusNone Fixed
        Assigned toNone wl
        Open/ClosedOpen Closed
        Planned ReleaseNone 2.10.3
    2020-07-09 bungeman Attached File- Added 0001-Document-FT_Face_Rec_-box-with-variable-fonts.patch, #49453

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