Freetype values confusing me ...

Snoob 105 Mar 12, 2012 at 13:34

I actually implement a FreeType font rendering for OpenGL. Most parts works as expected,
but on final rendering (using VBO) i got some issues which drive me nuts …

First i figured out that the Freetype font values of freetype metrics and freetype bitmap are
not the same. During initialization i create for each glyph of a font a glyph entry and set the
follwing values taken from glyph metrics:

     m_charcode 114 unsigned int
        m_advanceX  6.0000000   float
        m_advanceY  -14.000000  float
        m_width 6   unsigned int
        m_height    9   unsigned int
        m_offsetX   1.0000000   float  (m_face->glyph->metrics.horiBearingX/64.0f)
        m_offsetY   9.0000000   float (m_face->glyph->metrics.horiBearingY/64.0f)

        using FT_Load_Char(m_face, charcode, FT_LOAD_DEFAULT | FT_LOAD_FORCE_AUTOHINT))
        to load the glyph without rendering.

When i rasterize the freetype bitmap to my atlas texture i got the following values from the
Freetype FT_bitmap / FT_BitmapGlyph / Slot:

     advanceX   5.7500000   float
        advanceY    0.00000000  float
        width   4   int                  (FT_Bitmap)
        height  8   int                (FT_Bitmap)
        offsetX 1.2656250   float  (m_face->glyph->metrics.horiBearingX/64.0f)
        offsetY 7.8437500   float  (m_face->glyph->metrics.horiBearingY/64.0f)

       //Load the Glyph for our character.
       if(FT_Load_Glyph(m_face, FT_Get_Char_Index( m_face, glyph.getCharcode()), FT_LOAD_DEFAULT ))
        throw std::runtime_error("FT_Load_Glyph failed");

       //Move the face's glyph into a Glyph object.
       FT_Glyph ft_glyph;
       if(FT_Get_Glyph( m_face->glyph, &ft_glyph ))
         throw std::runtime_error("FT_Get_Glyph failed");

       //Convert the glyph to a bitmap.
       FT_Glyph_To_Bitmap( &ft_glyph, ft_render_mode_normal, 0, 1 );
       FT_BitmapGlyph bitmap_glyph = (FT_BitmapGlyph)ft_glyph;

As you can see those values are not equals but i use the equal command to determine
the offset value …

And if i take the offset values directly from bitmap i got this

    offsetX 1.0000000   float   (bitmap->left)
    offsetY 1.0000000   float   (bitmap->top)

This confusing me. Why are those values are not the same? I read the freetype documentation
multiple times, but i can’t fint a hint what is the difference between those values and which i have
to take?

This value differnces creates differnt issues when i try to render a simple text.
It looks like some of those offset values are not correct …

If I use the metric values and the FT_Bitmap dimensions i got the following output. Here have some
chars (i.e. r) some possition/offset issues:


If I use the second values intstead i got the following output. Which looks better in offset positioning,
but this create some imaging failures (chars look different colored and have some border streaks)


I tried differnt font sizes (12, 14, 16) and resolutions (72, 96, 100). This results also in other issues.
I.e. If i set the resolution for the vera.ttf font size 12 from 96 to 100 i got this text output:

http://s1.directuplo…l4pfzt8_jpg.htm (second values) (first values)

In that case the first values looks more correct in offset positioning, only some chars like the r are
also not correct…

I’am running out of ideas what could be wrong.
Can anyone help?

Thanks in advance,

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Snoob 105 Mar 13, 2012 at 11:34

Ok I’ve fixed the rendering issues including offset positioning. I still don’t know why glyph metric values
and glyph bitmap values are differnd, but when I take only the freetype values from FT_bitmap anything
is rendered correctly and the offset failures are gone …