Re: [ft-devel] FT_Slot_LoadFunc load_flags info?

[Ian Britten]

Werner LEMBERG wrote:

> you are obviously talking about an FT_GlyphSlotRec structure:

Ya, sorry if I wasn't clear about that.

Many thanks for the info - very helpful!  If you can put up with me
a bit more, I'd like to clarify just a couple more points though,
mostly about what the fields represent, since you've largely
covered scaling, 26.6, etc...

Just to recap:  My font driver is generally working correctly, in
both our custom rendering code, and when the font is rendered via
Cairo.  There's just a couple of rendering cases that aren't working
correctly, and my suspicion is that in those cases, Cairo is using
some fields of my FT_GlyphSlotRec that aren't populated correctly.

For clarity, lets imagine a simple glyph/example, and work with real
numbers:
- The font has no hinting, so the geometry is always just it's points
  (Besides, I always call load_glyph() with the NO_HINTING flag).
- It's cover is [(-800, -1000), (800, 1000)], as an example.
- Imagine NO_SCALE is passed in, so all(?) values are in font units.
  Obviously, the code needs to be updated accordingly for this flag.

> >         // These are obvious
> >         slot->metrics.width = 1600;
> >         slot->metrics.height = 2000;

> >         slot->metrics.horiBearingX = -800;
> >         slot->metrics.horiBearingY = 1000;

Since my origin is in the centre of the glyph, these are basically
half the width/height, right?

> >         // Advance #1 (Hinted)
> >         slot->metrics.horiAdvance = ;
> >         slot->metrics.vertAdvance = 0;
>
> The horizontal advance is the amount you have to move the cursor to
> the right (or down for vertical layouts);

This is where I start to scratch my head ...
- Should this be the width of my glyph?
- Or maybe half the width (From the Origin to the right edge)?
- Or should it be zero?  Zero seems to work in _some_ cases...

This also seems to be one of the key fields that is giving me grief.
In some cases, zero seems right, but in other cases, 'width' seems
right.  I suspect something else may be wrong elsewhere, but I'd
like to ensure I've got these numbers/fields right first ...

> >         // Advance #2 (Unhinted)
> >         slot->linearHoriAdvance = slot->metrics.horiAdvance;
> >         slot->linearVertAdvance = slot->metrics.vertAdvance;
>
> These are *unhinted* values, as needed for device-independent layout.

Since I have no hinting, these just the same as the
slot->metrics values, right?

> >         // Advance #3
> >         slot->advance.x = slot->metrics.horiAdvance;
> >         slot->advance.y = 0;
>
> This is a shorthand: Depending on FT_LOAD_VERTICAL_LAYOUT, this value
> contains either metrics.horiAdvance or metrics.vertAdvance;


FYI...
I also seem to be getting a FT_LOAD_IGNORE_GLOBAL_ADVANCE_WIDTH flag?
Do I need to alter anything due to this, or is it something the caller
is responsible for?  The comments in src/truetype/ttgload.c almost
suggest I shouldn't be seeing this flag ...

> Hopefully, I've got everything right :-)

Any info you can provide will hopefully make my stuff "more right"
than it currently is!  I must confess that getting this stuff to work
correctly through all 3 codebases (Me, FT, Cairo) is pretty
confusing ...
[ And actually, it's *4*, since we use ICU for layout/kerning/etc ]

Again, many thanks!
Ian