Re: [Freetype] Why 26.6 ?

[David Turner]

Hello,

Peter Montgomery wrote:
> Hi,
>
> Folks, you're getting things all mixed up.  26.6 refers to the placement
> of a pixel on a grid with fractional accuracy - in this case the
> fraction is 2 ^ 64.  If the 26 refers to pixels (as opposed to inches,
> centimeters, etc) , this simply means that you can position text to any
> of 64 possible sub-pixel positions.
>
> Levels of anti-aliasing has <nothing> to do with sub-pixel positioning.
> You can position text to a fractional position and then render it as 1
> bit per pixel monochrome.  This will effectively round off your
> positioning to the nearest whole pixel, but you can still specify the
> positioning as a fractional pixel.  If you render with 256 levels of
> gray, then you have that many levels of anti-aliasing available <no
> matter where you position it>!
>
> Think about it.  Suppose that FreeType only allowed you to position text
> in whole pixel increments, but you rendered the text with 256 levels of
> anti-aliasing.  Does that mean that suddenly the text would become
> monochrome?  Of course not.  The positioning accuracy and the
> anti-aliasing are completely different aspects of graphics that are not
> connected.  In creating a graphics rendering system you can
>
> 1 - Have 256 levels of anti-aliasing and no fractional positioning
> 2 - No anti-aliasing with fractional positioning
> 3  - Both
> 4 - Neither
>
> The choice is up to you when you design the renderer.  Among other
> choices, FreeType happens to allow fractional positioning with 256
> levels of anti-aliasing.
>
> The sole relationship between anti-aliasing and sub-pixel positioning is
> that if you expect to see your fractional positioning, then you better
> have some anti-aliasing going on.  FreeType uses 256 levels of grayscale
> anti-aliasing which is as much as virtually all common display systems
> on PCs allow.
Peter is right on topic. The 26.6 convention is necessary to implement
TrueType hinting correctly (with the bytecode interpreter), so using
6 bits of fractional pixel units was a minimum. Experience shows that
it is also largely sufficient to hint other kind of bezier curves, so
it staid in FreeType.

And the reason why we don't use floats are multiple:

  - most embedded systems don't have efficient FPUs, so forget
    about using floats and doubles on these platforms

  - TrueType hinting _requires_ and extensively uses 26.6 fixed
    point computations, using floats/doubles has strictly _no_
    advantage here, except slower operations, even on modern PCs

Hope this helps,

- David Turner
- The FreeType Project  (www.freetype.org)

>     Thanks,
>     PeterM
>
>
> ----- Original Message -----
> From: "Boris Letocha" <address@hidden>
> To: <address@hidden>
> Sent: Tuesday, September 17, 2002 2:32 AM
> Subject: RE: [Freetype] Why 26.6 ?
>
>
>
>
> > 26.6 the .6 refers 2^6 = 64, isn't it??
>
> Yes, but you have 64 in both axises. 64*64 is 4096 different subpixels.
>
> Boris
>
>
> _______________________________________________
> Freetype mailing list
> address@hidden
> http://www.freetype.org/mailman/listinfo/freetype