Re: FT_Set_Char_Size() with TrueType fonts

[David Turner]

Hi Yamano-Uchi,

  I'm beginning to wonder wether I correctly understood you ?!
  Could you tell me if the following is correct:

    - there is a difference between what is called the "character size"
      of scalable formats, and the "cell size" of most bitmapped formats

      in one hand, the "character size" is the size of the EM square,
      be it expressed in pixels, points, mm, etc.. it's not directly
      related to the pixel dimensions of the bitmaps embedded in a
      TrueType font.

      on the other hand, most bitmap formats use "cells" of constant
height
      to store glyph images. this "cell height" is normally the sum of
the
      ascent and descent metrics for a given "character size".

    - FT_Set_Char_Size and FT_Set_Pixel_Sizes are only used to set the
      "character size" of fonts (specifying it either in points or
pixels).

      the original reason why there are to distinct functions to perform
what
      is basically the same thing, comes from small oddities that I
won't
      explain there (some stupid flag in the font header + some weird
opcode
      definned to retrieve the character size in _points_ and push it on
the
      stack).
    
    - I now believe that you'd like a function that let's you select
      bitmap strikes according to their "cell height", rather than their
      "character size".

If this is so, I propose to introduce a new function rather than
modifying the
existing one. Choosing a good name to avoid ambiguity being important, I
would
propose something like:

  /*******************************************************************
   *
   * <Function> FT_Set_Bitmap_Cell_Size
   *
   * <Description>
   *    chooses a given size for a face containing bitmaps,
   *    based on the bitmap cell height.
   *
   * <Input>
   *    face   :: handle to target face object
   *    width  :: cell width in integer pixels. 0 means ignore
   *    height :: cell height in integer pixels
   *
   * <Return>
   *    Error code. non-0 if the face doesn't contain a bitmap strike
   *    of the relevant cell height
   *
   * <Note>
   *    this function can also be used on a scalable face, as long
   *    as it provides embedded bitmaps for the relevant dimensions
   */
   FT_EXPORT_DEF( FT_Error )   FT_Set_Bitmap_Cell_Size( FT_Face  face,
                                                        FT_UInt  width,
                                                        FT_UInt  height
);

there is also the problem of knowing what cell sizes are present in a
font. By default, "face->available_sizes" returns a table of character
sizes for scalable formats like TrueType, and of cell sizes for the
"winfonts" driver.

Maybe we could change FT_Bitmap_Size to

  typedef struct FT_Bitmap_Size_
  {
     FT_UShort  height;       /* character height */
     FT_UShort  width;        /* character width  */
     FT_UShort  cell_height;  /* cell height      */

  } FT_Bitmap_Size;

for a bitmapped format, we would probably return "height == cell_height"
== cell_height"

I'd like your input on this topic ??

Cheers,

- David


"Yamano-uchi, Hidetoshi ($@;3Fb(J address@hidden(J)" a écrit :
> 
> This problem is for embedded bitmaps(a.k.a. sbit) but not for handling
> collect "face (x-)height" as David said.
> 
> From: Werner LEMBERG <address@hidden>
> Subject: Re: FT_Set_Char_Size() with TrueType fonts
> Date: Tue, 31 Oct 2000 14:29:45 +0100 (CET)
> Message-ID: <address@hidden>
> 
> > > or in the derivative SFNT-based format that only contains embedded
> > > bitmaps, that we now support thanks to Yamano-Uchi's excellent
> > > work..?
> >
> > The former -- Hidetoshi has explained that he needs to set the size of
> > embedded bitmaps independently from the font's outlines.
> 
> But now, I think we should open other face like following style when
> sbit's size does not depend on outline's size.  Because many
> developpers may confuse the convention as I said.
> 
> FT_New_Face(library, &outline_face, same_filename);
> FT_New_Face(library, &sbit_face, same_filename);
> 
> FT_Set_Char_Sizes(outline_face, outline_si
ze,....);
> // Set_Pixel_Sizes is not impremented yet.
> FT_Set_Pixel_Sizes(sbit_face, sbit_size,....);
> 
> FT_Load_Glyph(outline_face, index1, FT_LOAD_NO_BITMAP);
> FT_Load_Glyph(sbit_face, index2, FT_LOAD_NO_OUTLINE);
> 
> // synthesys double striked(i.e. psude-bolding) sbit.
> .....
> 
> Oops, it is difficult for me.
> 
> ----- Yamano-uchi, Hidetoshi