Re: [ft-devel] New `slight' auto-hinting mode

[Werner LEMBERG]

> fractional advance width: this seems to need more thought.

Indeed :-)  Thank for you participating in this discussion.

> If that's to be used for the intended purpose, don't we need
> rendering modes with different fractional offsets?  i.e. let's say
> you want to lay out 'abc'.  'a' has a fractional width.  That means
> ideally we want to start 'b' on a fractional pixel offset also?
> etc?  So we want 3 different ways of rendering 'b', depending on
> which fractional pixel we starts it with.

It's not clear to me how you come to the value 3.  Since we are using
26.6 fractional numbers for vectors, there are actually 64
possible fractional offsets.

For modes that don't do horizontal hinting (in particular `light'
auto-hinting), simply shift the glyph by the fractional amount and
display it.

On the other hand, modes that do horizontal hinting even for
fractional positions (e.g., some ClearType modes) it has to be
investigated how to do that.

The larger the PPEM, the less fractional offsets are actually needed –
the eye no longer can detect a difference.  This is important for
caching.  For example, my rough guess is that for sizes in the range
20ppem to 30ppem it is sufficient to have four fractional values
instead of 64.

> I think toggling hinting in ftview is supposed to change layout,
> since the width changes (by up to a pixel on either side).

Yes.

Note, however, that ftview doesn't use fractional advance widths – it
simply doesn't make much sense for this application.  You should
rather have a look at the current git version of ftdiff, which now has
`light auto-hinting with sub-pixel positioning' as one of the
rendering modes.


    Werner