[ft] Vertical offset at font-rendering

[Erik Unger]

Hi,

I want to render a font into a 256*256 pixel texture,
using a grid for 16*16 glyphs.

My renderGlyph() function receives a pointer to the
upper left corner of the grid-position, so I have to
move this "pen-position" by glyph->bitmap_top and glyph->bitmap_left
to align the glyph-bitmaps to the baseline of the grid:

void Font::renderGlyph(char32 character, uint8 * pixels, int 
lineStrideInBytes)
{
// ...
const FT_Bitmap& glyphBitmap = face->glyph->bitmap;
const int left = face->glyph->bitmap_left;
const int top = size - face->glyph->bitmap_top - 4;
pixels = &pixels[top * lineStrideInBytes + left];
for (int y = 0; y < glyphBitmap.rows; ++y)
{
        for (int x = 0; x < glyphBitmap.width; ++x)
        {
		pixels[y * lineStrideInBytes + x] = 
offset_cast<uint8>(glyphBitmap.buffer, y * glyphBitmap.pitch + x);
        }
}

-> Notice the "- 4". For the Arial-Font under Windows with
a font-size of 16, it seams that all glyph-bitmaps have to
be moved 4 pixels up. If I don't move them up, the last row
of glyphs in my grid exceeds the border of the texture.

So where do this 4 pixel offset come from?

There is also something strange with bitmap_left:
It's -1 for 'A'. If 'A' would be the first glyph in
the 16*16 grid, some pixels would be written before the
texture-memory.

Thanks in advance,
-Erik