Modifying CFF Glyph points ( From existing .otf )

[ivangrozny]

I want to modify only the glyph shapes of otf font without modifying the rest of the font. (metadata, kerning, alt glyph, ect.)

I've managed to do it with TTF :

ttpen = TTGlyphPen( font.getGlyphSet() ) # not sur why I've passed the glyphSet...
# pen beziers ...

ttFont['glyf']['a'] = ttpen.glyph( dropImpliedOnCurves=True )

I didn't managed to do it with the CFF table, it is not working :

font = ttLib.TTFont(path("files/1.otf"))
pen = T2CharStringPen(600, None)
# pen beziers ...
font['CFF '].cff[0].CharStrings["a"] = pen.getCharString()
font.save('out.otf')

# error :
#   File "C:\Python\Python310\site-packages\fontTools\misc\psCharStrings.py", line 1142, in draw
#   self.private.nominalWidthX,
# AttributeError: 'NoneType' object has no attribute 'nominalWidthX'

maybe I should use fontTools.cffLib.CFFFontSet.decompile() / compile() or fontTools.misc.psCharStrings.T1CharString but I didn't understand these functions.

I've tried an other way with a fb = FontBuilder(font=font) but fb.setupCFF(...) seem to modify all glyphs at once and need to reset all the font metadata...
And by the way the OTF use-example fontBuilder.py produced font is not recognised on windows 11 ( out.otf is not a valid font file )

[ivangrozny]

I finnally managed to do it with recalcBBoxes = False

font = ttLib.TTFont("files/1.otf", recalcBBoxes = False )
pen = T2CharStringPen(600, None)
# pen beziers ...
font['CFF '].cff[0].CharStrings["a"] = pen.getCharString()
font.save('out.otf')

Someone know why recalcBBoxes() does not produce private.nominalWidthX ?
Should I open an issue ?

[ivangrozny]

For thoses interested there is an even better way to keep a maximum of originals data ecept vectors :
replacing only the compiled glyph bytecode into the CFF table

char_string = pen.getCharString()
char_string.compile()
font['CFF '].cff.topDictIndex[0].CharStrings[glyph].bytecode = char_string.bytecode