126 lines
3.2 KiB
Markdown
126 lines
3.2 KiB
Markdown
```c
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stbtt_InitFont
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stbtt_ScaleForMappingEmToPixels x 3
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stbtt_ScaleForPixelHeight
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stbtt_BakeFontBitmap
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stbtt_GetFontVMetrics x 2
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typedef struct {
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unsigned short x0, y0, x1, y1; // coordinates of bbox in bitmap
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float xoff, yoff, xadvance;
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} stbtt_bakedchar;
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struct RenImage {
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RenColor *pixels;
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int width, height;
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};
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typedef struct {
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RenImage *image;
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stbtt_bakedchar glyphs[256];
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} GlyphSet;
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struct RenFont {
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void *data;
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stbtt_fontinfo stbfont;
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GlyphSet *sets[MAX_GLYPHSET];
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float size;
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int height;
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};
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```
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The function stbtt_BakeFontBitmap is used to write bitmap data into set->image->pixels (where set is a GlyphSet).
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Note that set->image->pixels need data in RGB format. After stbtt_BakeFontBitmap call the bitmap data are converted into RGB.
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With a single call many glyphs corresponding to a range of codepoints, all in a
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single image.
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## STB truetype font metrics
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stbtt_ScaleForPixelHeight takes a float 'height' and returns height / (ascent - descent).
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stbtt_ScaleForMappingEmToPixels take a float 'pixels' and returns pixels / unitsPerEm.
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### Computing RenFont
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When loading a font, in renderer.c, the font->height is determined as:
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```c
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int ascent, descent, linegap;
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stbtt_GetFontVMetrics(&font->stbfont, &ascent, &descent, &linegap);
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float scale = stbtt_ScaleForMappingEmToPixels(&font->stbfont, font->size);
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font->height = (ascent - descent + linegap) * scale + 0.5;
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```
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so, mathematically
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```c
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font->height = (ascent - descent + linegap) * font->size / unitsPerEm + 0.5;
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```
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**TO DO**: find out for what font->height is actually used.
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### Call to BakeFontBitmap
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In the same file, renderer.c, to create the glyphset image it computes:
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```c
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// Using stbtt functions
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float s = ScaleForMappingEmToPixels(1) / ScaleForPixelHeight(1);
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```
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so 's' is actually equal to (ascent - descent) / unitsPerEm.
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Then BakeFontBitmap is called and `font->size * s` is used for the pixel_height argument.
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So BakeFontBitmap gets
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```c
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pixel_height = (ascent - descent) * font->size / unitsPerEm;
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```
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In turns BakeFontBitmap passes pixel_height to ScaleForPixelHeight() and uses the
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resulting 'scale' to scale the glyphs by passing it to MakeGlyphBitmap().
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This is equal almost equal to font->height except the 0.5, the missing linegap calculation
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and the fact that this latter is an integer instead of a float.
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## AGG Font Engine
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Calls
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`FT_Init_FreeType` (initialize the library)
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In `load_font()` method:
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`FT_New_Face` or `FT_New_Memory_Face` (use `FT_Done_Face` when done)
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`FT_Attach_File`
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`FT_Select_Charmap`
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In `update_char_size()` method:
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`FT_Set_Char_Size` or `FT_Set_Pixel_Sizes`
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In `prepare_glyph()` method:
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`FT_Get_Char_Index`
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`FT_Load_Glyph`
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`FT_Render_Glyph` (if glyph_render_native_mono or native_gray8)
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in `add_kerning()` method
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`FT_Get_Kerning`
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`FT_Done_FreeType` (end with library)
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## Freetype2's metrics related function and structs
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`FT_New_Face` return a `FT_Face` (a pointer) with font face information.
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## AGG font engine's font size
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The variable `m_height` is the size of the font muliplied by 64.
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It will be used to set font size with:
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- `FT_Set_Char_Size` if m_resolution is set (> 0)
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- `FT_Set_Pixel_Sizes`, divided by 64, if m_resolution is not set (= 0)
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The method height() returns m_height / 64;
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