Re: [Devel] Getting accurate advances

[David Bevan]

Werner LEMBERG wrote:
> > Note that the rounding occurs in the calculation for dim_x.
>
>
> Oops, I missed that.
>
>
> > Changing this to:
> >
> >   dim_x = ( char_width * horz_resolution + 36 ) / 72;
> >
> >   metrics->x_ppem  = (FT_UShort)( ( dim_x + 32 ) >> 6 );
> >
> >   metrics->x_scale = FT_DivFix( dim_x, face->units_per_EM );
> >
> > produces the correct advances when used with your suggested change -
> > *but* this changes the x_scale value used throughout all resizing,
> > rendering, hinting, etc.  Is this change correct or dangerous?
>
>
> I think it is correct, and the optical appearance will become better.
> Have you actually done some testing already?

I have done only very limited testing (with no problems). However, our 
product works at printer resolutions (e.g. 300 or 600dpi) monochrome, so 
I won't be testing at low (screen) resolutions or using grey-scale.

It may be tomorrow before I can make the change properly and begin more 
thorough testing.

Other similar code (producing rounded x_scale values) can be found at 
ftobjs.c line 2081, pfdrivr.c line 102, and ttdrivr.c line 224. I 
presume those should also be changed?

> > Has the x_scale value always been wrong or does it need to be
> > rounded for some reason?
>
>
> I couldn't find an entry in the ChangeLog file thus I assume that it
> has always been wrong.  There was an improvement to round correctly
> (the `36+72*72' stuff) but the rounding itself was always there.
>
>
>     Werner


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 David Bevan, Emtex Ltd.
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