GetGlyphOutline() Native Buffer Format

• Microsoft Windows Software Development Kit (SDK) for Windows version 3.1
• Microsoft Win32 Application Programming Interface (API) included with:

      - Microsoft Windows NT versions 3.5 and 3.51
      - Microsoft Windows 95 version 4.0
  

SUMMARY

The GetGlyphOutline function provides a method for an application to retrieve the lowest-level information about a glyph in the TrueType environment. This article describes the format of the data the GetGlyphOutline function returns.

MORE INFORMATION

A glyph outline is a series of contours that describe the glyph. Each contour is defined by a TTPOLYGONHEADER data structure, which is followed by as many TTPOLYCURVE data structures as are required to describe the contour.

Each position is described by a POINTFX data structure, which represents an absolute position, not a relative position. The starting and ending point for the glyph is given by the pfxStart member of the TTPOLYGONHEADER data structure.

The TTPOLYCURVE data structures fall into two types: a TT_PRIM_LINE record or a TT_PRIM_QSPLINE record. A TT_PRIM_LINE record is a series of points; lines drawn between the points describe the outline of the glyph. A TT_PRIM_QSPLINE record is a series of points defining the quadratic splines (q-splines) required to describe the outline of the character.

In TrueType, a q-spline is defined by three points (A, B, and C), where points A and C are on the curve and point B is off the curve. The equation for each q-spline is as follows (xA represents the x-coordinate of point A, yA represents the y-coordinate of point A, and so on)

   x(t) = (xA-2xB+xC)*t^2 + (2xB-2xA)*t + xA
   y(t) = (yA-2yB+yC)*t^2 + (2yB-2yA)*t + yA

The format of a TT_PRIM_QSPLINE record is as follows:

• Point A on the q-spline is the current position (either pfxStart in the TTPOLYGONHEADER, the starting point for the TTPOLYCURVE, or the ending point of the previous TTPOLYCURVE).
• Point B is the current point in the record.
• Point C is as follows:

      - If the record has two or more points following point B, point C
        is the midpoint between point B and the next point in the
        record.
      - Otherwise, point C is the point following point B.
  

pfxA = pfxStart;                // Starting point for this polygon

   {
   pfxB = apfx[u];              // B is always the current point
   if (u < cpfx - 2)            // If not on last spline, compute C
      {
      pfxC.x = (pfxB.x + apfx[u+1].x) / 2;  // x midpoint
      pfxC.y = (pfxB.y + apfx[u+1].y) / 2;  // y midpoint
      }
   else                         // Else, next point is C
      pfxC = apfx[u+1];

                                // Draw q-spline
   DrawQSpline(hdc, pfxA, pfxB, pfxC);
   pfxA = pfxC;                 // Update current point
   }

FIXED fx; long *pl = (long *)&fx;

// Perform all arithmetic on *pl rather than on fx

The following function converts a floating-point number into the FIXED representation:

FIXED FixedFromDouble(double d) {

   long l;

   l = (long) (d * 65536L);
   return *(FIXED *)&l;

In a production application, rather than writing a DrawQSpline function to draw each q-spline individually, it is more efficient to calculate points on the q-spline and store them in an array of POINT data structures. When the calculations for a glyph are complete, pass the POINT array to the PolyPolygon function to draw and fill the glyph.

The following example presents the data returned by the GetGlyphOutline for the lowercase "j" glyph in the 24-point Arial font of the 8514/a (Small Fonts) video driver:

GetGlyphOutline GGO_NATIVE 'j'

  dwrc            = 208      // Total native buffer size in bytes
  gmBlackBoxX, Y  = 6, 29    // Dimensions of black part of glyph
  gmptGlyphOrigin = -1, 23   // Lower-left corner of glyph
  gmCellIncX, Y   = 7, 0     // Vector to next glyph origin

TTPOLYGONHEADER #1           // Contour for dot on "j"
  cb       = 44              // Total size of dot polygon
  dwType   = 24              // TT_POLYGON_TYPE
  pfxStart = 2.000, 20.000   // Start at lower-left corner of dot

  TTPOLYCURVE #1
    wType  = TT_PRIM_LINE
    cpfx   = 3
    pfx[0] = 2.000, 23.000
    pfx[1] = 5.000, 23.000
    pfx[2] = 5.000, 20.000   // Automatically close to pfxStart


TTPOLYGONHEADER #2   // Contour for body of "j"
  cb       = 164     // Total size is 164 bytes
  dwType   = 24      // TT_POLYGON_TYPE
  pfxStart = -1.469, -5.641

  TTPOLYCURVE #1     // Finish flat bottom end of "j"
    wType  = TT_PRIM_LINE
    cpfx   = 1
    pfx[0] = -0.828, -2.813

  TTPOLYCURVE #2    // Make hook in "j" with spline
                    // Point A in spline is end of TTPOLYCURVE #1
    wType  = TT_PRIM_QSPLINE
    cpfx   = 2               // two points in spline -> one curve
    pfx[0] = -0.047, -3.000  // This is point B in spline
    pfx[1] = 0.406, -3.000   // Last point is always point C

  TTPOLYCURVE #3    // Finish hook in "j"
                    // Point A in spline is end of TTPOLYCURVE #2
    wType  = TT_PRIM_QSPLINE
    cpfx   = 3               // Three points -> two splines
    pfx[0] = 1.219, -3.000   // Point B for first spline
                             // Point C is (pfx[0] + pfx[1]) / 2
    pfx[1] = 2.000, -1.906   // Point B for second spline
    pfx[2] = 2.000, 0.281    // Point C for second spline

  TTPOLYCURVE #4    // Majority of "j" outlined by this polyline
    wType  = TT_PRIM_LINE
    cpfx   = 3
    pfx[0] = 2.000, 17.000
    pfx[1] = 5.000, 17.000
    pfx[2] = 5.000, -0.250

  TTPOLYCURVE #5    // start of bottom of hook
    wType  = TT_PRIM_QSPLINE
    cpfx   = 2               // One spline in this polycurve
    pfx[0] = 5.000, -3.266   // Point B for spline
    pfx[1] = 4.188, -4.453   // Point C for spline

  TTPOLYCURVE #6    // Middle of bottom of hook
    wType  = TT_PRIM_QSPLINE
    cpfx   = 2               // One spline in this polycurve
    pfx[0] = 3.156, -6.000   // B for spline
    pfx[1] = 0.766, -6.000   // C for spline

  TTPOLYCURVE #7    // Finish bottom of hook and glyph
    wType  = TT_PRIM_QSPLINE
    cpfx   = 2               // One spline in this polycurve
    pfx[0] = -0.391, -6.000  // B for spline
    pfx[1] = -1.469, -5.641  // C for spline