RE: [Freetype] aligning glyphs from different fonts

[Turner David]

Hello,

> I want to draw glyphs from several different font faces to a bitmap
> image (sample bitmap rendering code below).  I pass in x and y to the
> function code below to place the glyph in the bitmap.  I would like
> the fonts to have a common vertical origin, so that glyphs from one
> face appear to be on the same line as glyphs from the other.
> 

You should basically use an algorithm like the following:

  let Y be the top row of your target bitmap. growing Y numbers
  correspond to downwards movement.

  for each line of text, you need to:

    - compute the maximum ascender, descender & line_gap for all
      font sizes used in the line. (note: the maximum descender
      in absolute value !).

    - Y+max_ascender corresponds to the baseline you'll need to
      use

    - the "top" of the next line is taken by adding max_descender+max_linegap,
      i.e.:

     Y = 0
     foreach line 
     {
        ... compute max_ascent, max_descent, max_linegap

        baseline = Y + max_ascent

        ... draw glyphs

        Y = baseline + max_descent + max_linegap
     }

This should be sufficient. Note that things get slightly more
complex if you want to perform device-independent text layout.
I won't help you there.

Note also that the reason you need to use the max_linegap is
because you don't know what max font sizes will be used on the
next line.

Hope this helps,

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



> Right now I am working with two font files (ftdump info below).  The
> font files share many of the same properties: type, direction, and EM
> size.  The differ, however, in their ascender, descender and
> text_height, mainly in their descender and text_height.  The glyphs
> from the font with the large height and low descender are appearing
> too low in the rendered bitmap with respect to the glyphs from the
> other font.  Currently, I'm passing in a fixed y and incrementing x by
> advance.x; I assume I need to modify y by one of the global font
> metrics.
> 
> Pointers to docs or examples appreciated.
> 
> Thanks,
> John Hunter
> 
> 
> freetype-2.1.3
> 
> sample code
>   error = FT_Glyph_To_Bitmap(&self->glyphs[glyph->glyph_num],
>                            FT_RENDER_MODE_NORMAL,
>                            0,  //no additional translation
>                            0  //don't destroy image
>                            );
> 
>   //handle error
>   FT_BitmapGlyph bitmap = 
> (FT_BitmapGlyph)self->glyphs[glyph->glyph_num];
> 
>   draw_bitmap( &bitmap->bitmap, 
>              &self->image,
>              x + bitmap->left,
>              y-bitmap->top);
> 
> 
> ftdump
> hunter:~/src/ft2demos-2.1.3/bin> ./ftdump ~/src/ttf/cmmi10.ttf
> There is 1 face in this file.
>  
> ----- Face number: 0 -----
>  
> font name entries
>    family:     cmmi10
>    style:      Regular
>    postscript: Cmmi10
>  
> font type entries
>    FreeType driver: truetype
>    sfnt wrapped:    yes
>    type:            scalable
>    direction:       horizontal
>    fixed width:     no
>    glyph names:     yes
>    EM size:         2048
>    global BBox:     (-70,-512):(2146,1536)
>    ascent:          1444
>    descent:         -440
>    text height:     2167
>  
> charmaps
>    0: platform: 1, encoding: 0
>    1: platform: 3, encoding: 1
> hunter:~/src/ft2demos-2.1.3/bin> ./ftdump ~/src/ttf/cmex10.ttf
> There is 1 face in this file.
>  
> ----- Face number: 0 -----
>  
> font name entries
>    family:     cmex10
>    style:      Regular
>    postscript: Cmex10
>  
> font type entries
>    FreeType driver: truetype
>    sfnt wrapped:    yes
>    type:            scalable
>    direction:       horizontal
>    fixed width:     no
>    glyph names:     yes
>    EM size:         2048
>    global BBox:     (-49,-6062):(2978,1581)
>    ascent:          1581
>    descent:         -6082
>    text height:     8812
>  
> charmaps
>    0: platform: 1, encoding: 0
>    1: platform: 3, encoding: 1
> hunter:~/src/ft2demos-2.1.3/bin>
> 
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> 

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