On Sun, 2 Apr 2017 08:19:00 +0200 Michael Everson <[email protected]> wrote:
> > On 1 Apr 2017, at 23:49, Philippe Verdy <[email protected]> wrote: > > > > I think it's all about sizing so that white or black cells will > > align, independantly of the piece that may be within it. > > A white knight may stand alone in text, in which case no variation > exists for display beyond the base glyph in the font. > > A white knight may need to be represented with specific > em-square-related metrics within the font, in two variations, one > with no fill on the background of the em-square indicating the piece > on a white square, and one with a (typically ///-shaped) fill > indicating the piece on a black square. I'm uneasy about the semantics of the sequences. To take the extreme example, <U+2657, U+FE00> would be the white bishop on a (or 'with its') white square while <U+2657, U+FE01> would be the white bishop on a black square. Perhaps someone can show me evidence from mathematical symbols or Japanese kanji that such semantic modifications are perfectly acceptable. I have related unease about the glyph of <U+2656, U+FE01>, intended for a white rook on a black square, being used in text where the meaning is 'white rook' regardless of what square it is on. If these variation sequences are acccepted, I hope that the intention that they contribute to producing presentable populated chess boards in plain text will be captured in at least the Unicode Standard. I can see issues with line-spacing, which I believe is formally out of control in true plain text. If my unease is well-founded, then I think we have a case for two combining marks akin to U+20E3 COMBINING ENCLOSING KEYCAP. Unfortunately, that would not be as simple to use (or define) as the proposed variation sequences. I'm also bothered by the purposes of the Format 14 'cmap' subtable. For each supported variation selector, it has a direct and an indirect mapping of base character to glyph. The direct mapping maps the character to the glyph to be used when qualified by the variation selector; that makes perfect sense. The indirect mapping gives a list of characters for which the 'default' glyph is to be used, i.e. the cmap subtables that do not directly support variation selectors are to be used. As I understand it, this list will in general not include all the characters supported by the font but without a non-default variation sequence mapping. Thus if a font supports the use of U+E0100 VARIATION SELECTOR-17, the table for U+E0100 will not have mention of U+0030, for <U+0030, U+E0100> is not a standardised variation sequence, and therefore no font should support it. I believe the purpose of having the indirect mapping is so that one can query whether a font explicitly supports a sequence. Thus the cmap distinguishes two cases: 1) <U+82A6, U+E0100> is explicitly catered for, and used the same glyph as unqualified U+82A6. 2) <U+82A6, U+E0100> is not catered for. If one needs to be sure of having its distinctive form, another font must be used. If I have understood the intended use correctly, then we need another variation sequence to explicitly specify a glyph of U+2656 suitable for use in plain-looking running text, analogous to <U+0032, FE0E> for a text-style '2'. A renderer can then ask whether a font supports plain text white rooks, as opposed to providing one dimensioned for assembling chess boards. Richard.
