Print SVG path from GDI's HFONT using GetGlyphOutlineW

a.cc

// gcc a.cc -lgdi32 && a.exe

#include <stdint.h>
#include <stdio.h>

#include <math.h>

#include <windows.h>

int
main ()
{
  HFONT hFont = CreateFont (1000, 0, 0, 0, FW_NORMAL, 0, 0, 0, ANSI_CHARSET,
			    OUT_DEFAULT_PRECIS, CLIP_DEFAULT_PRECIS,
			    DEFAULT_QUALITY, DEFAULT_PITCH, TEXT ("Tahoma"));

  HDC hdc = GetDC (NULL);
  SelectObject (hdc, hFont);

  GLYPHMETRICS gm = {0};
  MAT2 mat2 = {{0, 1}, {0, 0}, {0, 0}, {0, 1}};

  unsigned gid = 34;
  unsigned flags = GGO_GLYPH_INDEX | GGO_NATIVE;
  DWORD buffer_size = GetGlyphOutlineW (hdc, gid, flags, &gm, 0, NULL, &mat2);
  if (buffer_size == GDI_ERROR) return 1;

  unsigned char *buffer = (unsigned char *) malloc (buffer_size);
  buffer_size =
      GetGlyphOutlineW (hdc, gid, flags, &gm, buffer_size, buffer, &mat2);
  if (buffer_size < 0) return 1;

  uint8_t *ptr = buffer;
  /* https://github.com/freedesktop/cairo/blob/3a03c1b/src/win32/cairo-win32-font.c#L1690-L1804
   */
  while (ptr < buffer + buffer_size)
  {
    auto &header = *(const TTPOLYGONHEADER *) ptr;
    uint8_t *end_poly = ptr + header.cb;
    ptr += sizeof (TTPOLYGONHEADER);
#define fixed_to_float(fixed) ((float) fixed.value + fixed.fract / 65536.f)
#define fixed_round(fixed) (int) roundf (fixed_to_float (fixed))
    printf ("M%d,%d", fixed_round (header.pfxStart.x),
	    fixed_round (header.pfxStart.y));

    while (ptr < end_poly)
    {
      auto &curve = *(const TTPOLYCURVE *) ptr;
      ptr += sizeof (TTPOLYCURVE) + sizeof (POINTFX) * (curve.cpfx - 1);
      switch (curve.wType)
      {
      case TT_PRIM_LINE:
	for (int i = 0; i < curve.cpfx; ++i)
	  printf ("L%d,%d",
		  fixed_round (curve.apfx[i].x),
		  fixed_round (curve.apfx[i].y));
	break;
      case TT_PRIM_QSPLINE:
	for (int i = 0; i < curve.cpfx - 1; ++i)
	{
	  float cx = fixed_to_float (curve.apfx[i].x),
		cy = fixed_to_float (curve.apfx[i].y), x, y;
	  if (i + 1 == curve.cpfx - 1)
	  {
	    x = fixed_to_float (curve.apfx[i + 1].x);
	    y = fixed_to_float (curve.apfx[i + 1].y);
	  }
	  else
	  {
	    x = (cx + fixed_to_float (curve.apfx[i + 1].x)) / 2.f;
	    y = (cy + fixed_to_float (curve.apfx[i + 1].y)) / 2.f;
	  }
	  printf ("Q%d,%d %d,%d",
		  (int) roundf (cx), (int) roundf (cy),
		  (int) roundf (x), (int) roundf (y));
	}
	break;
      case TT_PRIM_CSPLINE:
	for (int i = 0; i < curve.cpfx - 2; i += 2)
	  printf ("C%d,%d %d,%d %d,%d", fixed_round (curve.apfx[i + 0].x),
		  fixed_round (curve.apfx[i + 0].y),
		  fixed_round (curve.apfx[i + 1].x),
		  fixed_round (curve.apfx[i + 1].y),
		  fixed_round (curve.apfx[i + 2].x),
		  fixed_round (curve.apfx[i + 2].y));
#undef fixed_to_float
#undef fixed_round
	break;
      }
    }
    printf ("Z");
  }

  free (buffer);
  printf ("\n");
  return 0;
}