[font-types] Fixed types: round towards infinity, not away from zero

[behdad]

There are multiple hints in the OpenType spec that rounding of fixed coordinates should be done towards infinity. Currently font-types implements round away from zero.

https://learn.microsoft.com/en-us/typography/opentype/otspec181/otvaroverview

"for fractional values of 0.5 and higher, take the next higher integer; for other fractional values, truncate"

I think this should do the trick:

diff --git a/font-types/src/fixed.rs b/font-types/src/fixed.rs
index af00c765..4033738b 100644
--- a/font-types/src/fixed.rs
+++ b/font-types/src/fixed.rs
@@ -180,7 +180,7 @@ macro_rules! fixed_mul_div {
             #[inline(always)]
             fn mul(self, other: Self) -> Self::Output {
                 let ab = self.0 as i64 * other.0 as i64;
-                Self(((ab + 0x8000 - i64::from(ab < 0)) >> 16) as i32)
+                Self(((ab + 0x8000) >> 16) as i32)
             }
         }
 

Both HarfBuzz and FontTools round towards infinity and don't rely on system round.

Please discuss. @rsheeter @dfrg @cmyr @anthrotype

[dfrg]

Agreed that this is out of spec. Unfortunately, making this change would break FreeType compatibility across the board: https://github.com/freetype/freetype/blob/e3a0652b6d706ee1ce77d4dda606b6597dd8b5c4/src/base/ftcalc.c#L224

Notably, I believe this is the only place where FT uses round away from zero.

[behdad]

Would it make sense to make some types, eg. F2Dot14, do the towards-infinity? Feel free to close.

[dfrg]

I think the current state is that all rounding is towards infinity except for the result of 16.16 multiplication but I haven’t verified.