More notes on font rendering

notes.md

@@ -73,6 +73,87 @@ float s = ScaleForMappingEmToPixels(1) / ScaleForPixelHeight(1);
 so 's' is actually equal to (ascent - descent) / unitsPerEm.
 
 Then BakeFontBitmap is called and `font->size * s` is used for the pixel_height argument.
-So BakeFontBitmap gets, for pixel_height, (ascent - descent) * font->size / unitsPerEm.
+So BakeFontBitmap gets
+
+```c
+pixel_height = (ascent - descent) * font->size / unitsPerEm;
+```
+
 This is equal almost equal to font->height except the 0.5, the missing linegap calculation
 and the fact that this latter is an integer instead of a float.
+
+## AGG Font Engine
+
+Calls
+
+FT_Init_FreeType (initialize the library)
+
+In load_font() method:
+FT_New_Face or FT_New_Memory_Face (use FT_Done_Face when done)
+
+FT_Attach_File
+FT_Select_Charmap
+
+In update_char_size() method:
+FT_Set_Char_Size or FT_Set_Pixel_Sizes
+
+In prepare_glyph() method:
+FT_Get_Char_Index
+FT_Load_Glyph
+FT_Render_Glyph (if glyph_render_native_mono or native_gray8) 
+
+in add_kerning() method
+FT_Get_Kerning
+
+FT_Done_FreeType (end with library)
+
+## Freetype2's metrics related function and structs
+
+FT_New_Face
+
+```c
+typedef struct  FT_FaceRec_
+{
+  FT_Long           num_faces;
+  FT_Long           face_index;
+
+  FT_Long           face_flags;
+  FT_Long           style_flags;
+
+  FT_Long           num_glyphs;
+
+  FT_String*        family_name;
+  FT_String*        style_name;
+
+  FT_Int            num_fixed_sizes;
+  FT_Bitmap_Size*   available_sizes;
+
+  FT_Int            num_charmaps;
+  FT_CharMap*       charmaps;
+
+  FT_Generic        generic;
+
+  /*# The following member variables (down to `underline_thickness`) */
+  /*# are only relevant to scalable outlines; cf. @FT_Bitmap_Size    */
+  /*# for bitmap fonts.                                              */
+  FT_BBox           bbox;
+
+  FT_UShort         units_per_EM;
+  FT_Short          ascender;
+  FT_Short          descender;
+  FT_Short          height;
+
+  FT_Short          max_advance_width;
+  FT_Short          max_advance_height;
+
+  FT_Short          underline_position;
+  FT_Short          underline_thickness;
+
+  FT_GlyphSlot      glyph;
+  FT_Size           size;
+  FT_CharMap        charmap;
+
+  /* private part omitted. */
+
+} FT_FaceRec;
+```