Re: [ft-devel] Re: rounding bug in MS TT rasterizer

[David Turner]

Hi Werner,

that's fabulous news. I've investigated the issue, and storing the 
original font unit
coordinates is not too trivial at the moment. I hope to hack the fix 
after the 2.2
release, which I'm currently preparing by fixing the documentation comments,
plus other things I'll talk about later.

Thanks again, this is extremely cool :-)

- David



Werner LEMBERG a écrit :
> With the help of Greg Hitchcock from Microsoft I can now reproduce the
> way how the MS rasterizer computes the IP operation.  They use
> *unscaled* values!
>
>   original x value of reference point 1:  606 ( 9.46875)      [O1]
>   original x value of reference point 2: 1194 (18.65625)      [O2]
>
>   current x value of reference point 1:  608 ( 9.5)           [C1]
>   current x value of reference point 2: 1216 (19.00)          [C2]
>
>   unscaled x value of reference point 1:  969                 [U1]
>   unscaled x value of reference point 2: 1911                 [U2]
>
>   original x value of point 25:  884 (13.8125)                [OX]
>   unscaled x value of point 25: 1415                          [UX]
>
> We do this:
>
>      C2 - C1               608
>   -> ------- * (OX - O1) = --- * 278  ->  4.4914966
>      O2 - O1               588
>
> The MS rasterizer does this:
>
>      C2 - C1               608
>   -> ------- * (UX - U1) = --- * 446  ->  4.4978769
>      U2 - U1               942
>
> This is exactly the value Greg has reported.
>
> David, can you incorporate this information into the bytecode
> interpreter?
>
> Looking through the various bytecode instructions, the following
> commands might be affected similarly (this is, the computations use
> original values which are rounded and/or scaled afterwards) -- this
> information is based entirely on the available descriptions, searching
> for some keywords.  It is not based on knowledge :-)
>
>   GC
>   MD
>   MDRP
>   MIAP
>   MIRP
>   SDPVTL
>   SHC
>   SHP
>   SHZ
>
> The descriptions of the above keywords in the Apple specification
> mention `original outline'.  On the other hand, only the description
> of MDRP talks about the `original uninstructed outline' (as does the
> description of `IP') -- so maybe only IP and MDRP have to be checked.
>
> BTW, in the MS specification, the dual projection vector is set
> `parallel to the points as they appeared in the original outline
> before any grid-fitting took place'.  This looks like another thing to
> control...
>
>
>     Werner
>
>
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> http://lists.nongnu.org/mailman/listinfo/freetype-devel
>
>   


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