WIP: to use AGG font renderer to create a coverage bitmap

notes.md

@@ -0,0 +1,78 @@
+```c
+stbtt_InitFont
+
+stbtt_ScaleForMappingEmToPixels x 3
+stbtt_ScaleForPixelHeight
+stbtt_BakeFontBitmap
+stbtt_GetFontVMetrics x 2
+
+typedef struct {
+   unsigned short x0, y0, x1, y1; // coordinates of bbox in bitmap
+   float xoff, yoff, xadvance;
+} stbtt_bakedchar;
+
+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;
+};
+
+```
+
+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.
+
+## STB truetype font metrics
+
+stbtt_ScaleForPixelHeight takes a float 'height' and returns height / (ascent - descent).
+
+stbtt_ScaleForMappingEmToPixels take a float 'pixels' and returns pixels / unitsPerEm.
+
+### Computing RenFont
+
+When loading a font, in renderer.c, the font->height is determined as:
+
+```c
+int ascent, descent, linegap;
+stbtt_GetFontVMetrics(&font->stbfont, &ascent, &descent, &linegap);
+float scale = stbtt_ScaleForMappingEmToPixels(&font->stbfont, font->size);
+font->height = (ascent - descent + linegap) * scale + 0.5;
+```
+
+so, mathematically
+
+```c
+font->height = (ascent - descent + linegap) * font->size / unitsPerEm + 0.5;
+```
+
+**TO DO**: find out for what font->height is actually used.
+
+### Call to BakeFontBitmap
+
+In the same file, renderer.c, to create the glyphset image it computes:
+
+```c
+// Using stbtt functions
+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.
+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.