问字体在freetype和Opengl ES 2 (IPhone设备)中渲染效果不佳

EN

FT_New_Face(getFTLibrary(), _filename, 0, &(_face))
FT_Select_Charmap( _face, FT_ENCODING_UNICODE );
FT_Set_Char_Size( _face, _pt<<6,_pt<<6, _dpi, _dpi);
for (i=0; i<255; i++)
{
    if (!createGlyphTexture(i))
    {
        clean();
    }
}
.....

createGlyphTexture(unsigned char ch){
    ....
    FT_Load_Glyph(_face, FT_Get_Char_Index(_face,ch), FT_LOAD_DEFAULT)
    FT_Get_Glyph(_face->glyph, &glyph)
    ....
    // *** Transform glyph to bitmap
    FT_Glyph_To_Bitmap(&glyph, FT_RENDER_MODE_NORMAL, 0, 1);

    // *** Get bitmap and glyph data
    bitmap_glyph = (FT_BitmapGlyph)glyph;
    bitmap = bitmap_glyph->bitmap;
    // ***
    width = pow2(bitmap.width);
    height = pow2(bitmap.rows);

    // *** Alloc memory for texture
    expanded_data = (GLubyte *)malloc( sizeof(GLubyte)*2*width*height );
    for (j=0; j<height;j++)
{
    for (i=0; i<width; i++)
    {
        if ( (i>=(CRuint)bitmap.width) || (j>=(CRuint)bitmap.rows) ){
            expanded_data[2*(i+j*width)] = 0;
            expanded_data[2*(i+j*width)+1] = 0;
        } else {
            expanded_data[2*(i+j*width)] = bitmap.buffer[i+bitmap.width*j];
            expanded_data[2*(i+j*width)+1] = bitmap.buffer[i+bitmap.width*j];
        }
    }
}
    // *** Load texture into memory
    glBindTexture(GL_TEXTURE_2D, _textures[ch]);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexImage2D(GL_LUMINANCE_ALPHA, width, height, 0, GL_LUMINANCE_ALPHA, GL_UNSIGNED_BYTE, expanded_data);
    ....
}