Re: [Devel] pixel size, hinting and scaling

[David Turner]

Hi Graham,

Graham Asher wrote:
> Can someone explain how, and if, the problem of pixel size and scaling is
> solved, when a font transformation has some scaling component? I noticed
> that when TrueType glyphs are rendered certain important parameters
> affecting hinting depend on the number of pixels per em - that is, the pixel
> size. These parameters are set when FT_Set_Char_Size is called but not when
> FT_Set_Transform is called. FT_Set_Transform can also affect the pixel size.
> Thus a transform applied to a font can prevent hinting from working as
> intended.
>
> There are other, more difficult issues when pixels are not square or when
> the transformation causes some skewing or rotation.
>
> I hope I have simply misunderstood some part of FreeType. If not, there is a
> defect here.

Here are some clarifications: up to three distinct transforms can be used
when loading a glyph within FreeType 2:

  * the "scaling transform", used to scale glyph coordinates from
    "font" to "device" units. It is entirely determined by the character
    sizes set through FT_Set_Pixel_Sizes or FT_Set_Char_Size. The so-called
    "device" space is where all hinting computations are performed.

  * the internal "glyph transform", that can be applied by glyph loaders
    for certain formats (e.g. artificially obliqued Type 1 fonts through
    a shearing transform). It is located in "glyph->internal.transform"
    as far as I remember.

    this transform is a bit specific because:

      # it is applied _after_ the scaled outlines have been hinted

      # except that its inverse translation is applied to the outline
        _before_ hinting.

    the last point ensures that grid fitting is preserved for the final
    hinted+transformed outline, whenever this makes sense.



  * the additional "user-level transform", set with FT_Set_Transform, which
    is always applied _after_ scaling and hinting. It has _zero_ interaction
    with the other transforms..


So, if all you have is a "master" to "device" transform of your own, you'll
need to perform the following steps to use FreeType with it:

  - decompose your transform into a two distinct ones: a scaling one, and
    a shearing / rotating one. Equivalent to finding the matrix's eigenvalues

  - use your scaling transform to call FT_Set_Pixel_Sizes() or 
FT_Set_Char_Size()

  - use your shearing/rotation one to call FT_Set_Transform()

  - load the glyph



Hope this helps,

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