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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_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)

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
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)

Using
//Load the Glyph for our character.

//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
values:

    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:

http://s14.directupl…hj76npd_jpg.htm

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)

http://s7.directuplo…38n873s_jpg.htm

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)

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?

Snoob