CFF2: interpret empty glyphs from non-default sparse master as missing, like with gvar

[anthrotype]

in #2082, @justvanrossum added support in varLib for interpreting empty glyphs as missing (thus not participating) if they occur in non-default sparse masters, but this only works for TrueType-flavored VFs:

fonttools/Lib/fontTools/varLib/__init__.py

Lines 320 to 326 in 7fce344

	if allData[defaultMasterIndex][1].numberOfContours != 0:
	# If the default master is not empty, interpret empty non-default masters
	# as missing glyphs from a sparse master
	allData = [
	d if d is not None and d[1].numberOfContours != 0 else None
	for d in allData
	]

Ideally we would do the same thing for variable CFF2 fonts, but I am not familiar enough with the varLib.cff module to know exactly how/where this change should best be placed.

If one attempts to use this pattern (default master has a non-empty glyph, non-default master has an empty glyph with the same name) one would get the following error:

fontTools.varLib.errors.VarLibCFFPointTypeMergeError: Glyph '.notdef': 'rlineto' at point index 4 in master index 1 ...

Maybe @behdad or @justvanrossum can help?

[behdad]

Good news is that it's supposed to work from glancing at the code:

def merge_charstrings(glyphOrder, num_masters, top_dicts, masterModel): 
 
    vsindex_dict = {} 
    vsindex_by_key = {} 
    varDataList = [] 
    masterSupports = [] 
    default_charstrings = top_dicts[0].CharStrings 
    for gid, gname in enumerate(glyphOrder): 
        all_cs = [_get_cs(td.CharStrings, gname) for td in top_dicts] 
        if len([gs for gs in all_cs if gs is not None]) == 1: 
            continue 
        model, model_cs = masterModel.getSubModel(all_cs) 

[behdad]

We just need to change it to filter out emtpy glyphs as well. I think I can do that.

[anthrotype]

well.. it does not as I have shown above. I get an VarLibCFFPointTypeMergeError if I try to pass such a sparse empty glyph setup. That code you linked, _get_cs, returns None if the glyph is not present in a master; here it is present, but empty

[behdad]

well.. it does not as I have shown above. I get an VarLibCFFPointTypeMergeError if I try to pass such a sparse empty glyph setup. That code you linked, _get_cs, returns None if the glyph is not present in a master; here it is present, but empty

Yeah. We just need to change _get_cs to filter out empty (for CFF2) or just endchar (for CFF1) and return None.

[anthrotype]

right.. detecting when a charstring is empty is what I couldn't figure it out. Checking its program length doesn't work because it has to be decompiled or something