2013/10/20 Asmus Freytag <[email protected]> > Incidentally, the dotted circle shown in the Unicode Code charts is *not* > 25CC, and if I were to implement a "show dotted circle" feature in a > program I would not use 25CC for this - that character has a standard glyph > of rather unsuitable metrics for the purpose, never mind that many people > have co-opted it. >
In fact the OpenType specs suggests assigning a glyph for the dotted circle so that renderers can use a glyph with the correct metrics for combining marks that are mapped in the font. The font may also assign distinct glyphs for some pairs with that base character, if the font supports multiple scripts. Many OpenType fonts are built like that so that the combining marks in isolation will not show on top of an unknown glyph with incorrect metrics, or worse over a zero-width space where they will collide with everything else on both sides. OpenType fonts however do not determine themselves is sequence is ill-formed : it is the renderer that parse these contexts and which then infer when to insert the base placeholder (it gets the glyph to insert by looking in the same font for the U+25CC mapping, otherwise it will use random or default or builtin font to get the dotted circle glyph, but it won't be able to correctly position that glyph and the combining mark from the font).
