stbtt_InitFont

stbtt_ScaleForMappingEmToPixels x 3
stbtt_ScaleForPixelHeight
stbtt_BakeFontBitmap
stbtt_GetFontVMetrics x 2

struct RenImage {
  RenColor *pixels;
  int width, height;
};

typedef struct {
  RenImage *image;
  stbtt_bakedchar glyphs[256];
} GlyphSet;

struct RenFont {
  void *data;
  stbtt_fontinfo stbfont;
  GlyphSet *sets[MAX_GLYPHSET];
  float size;
  int height;
};

typedef struct {
   unsigned short x0, y0, x1, y1; // coordinates of bbox in bitmap
   float xoff, yoff, xadvance;
} stbtt_bakedchar;

The function stbtt_BakeFontBitmap is used to write bitmap data into set->image->pixels (where set is a GlyphSet).
Note that set->image->pixels need data in RGB format. After stbtt_BakeFontBitmap call the bitmap data are converted into RGB.
With a single call many glyphs corresponding to a range of codepoints, all in a
single image.