glyf - Glyph Data (OpenType 1.4)

This table contains information that describes the glyphs in the font in the TrueType outline format. Information regarding the rasterizer (scaler) refers to the TrueType rasterizer.

Each glyph begins with the following header:

Note that the bounding rectangle from each character is defined as the rectangle with a lower left corner of (xMin, yMin) and an upper right corner of (xMax, yMax).

Note: The scaler will perform better if the glyph coordinates have been created such that the xMin is equal to the lsb. For example, if the lsb is 123, then xMin for the glyph should be 123. If the lsb is -12 then the xMin should be -12. If the lsb is 0 then xMin is 0. If all glyphs are done like this, set bit 1 of flags field in the 'head' table.

Simple Glyph Description

This is the table information needed if numberOfContours is greater than zero, that is, a glyph is not a composite.

Note: In the glyf table, the position of a point is not stored in absolute terms but as a vector relative to the previous point. The delta-x and delta-y vectors represent these (often small) changes in position.

Each flag is a single byte. Their meanings are shown below.

Composite Glyph Description

This is the table information needed for composite glyphs (numberOfContours is -1). A composite glyph starts with two USHORT values ("flags" and "glyphIndex," i.e. the index of the first contour in this composite glyph); the data then varies according to "flags").

The C pseudo-code fragment below shows how the composite glyph information is stored and parsed; definitions for "flags" bits follow this fragment:

do {
	USHORT flags;
	USHORT glyphIndex;
	if ( flags & ARG_1_AND_2_ARE_WORDS) {
	(SHORT or FWord) argument1;
	(SHORT or FWord) argument2;
	} else {
		USHORT arg1and2; /* (arg1 << 8) | arg2 */
	}
	if ( flags & WE_HAVE_A_SCALE ) {
		F2Dot14  scale;    /* Format 2.14 */
	} else if ( flags & WE_HAVE_AN_X_AND_Y_SCALE ) {
		F2Dot14  xscale;    /* Format 2.14 */
		F2Dot14  yscale;    /* Format 2.14 */
	} else if ( flags & WE_HAVE_A_TWO_BY_TWO ) {
		F2Dot14  xscale;    /* Format 2.14 */
		F2Dot14  scale01;   /* Format 2.14 */
		F2Dot14  scale10;   /* Format 2.14 */
		F2Dot14  yscale;    /* Format 2.14 */
	}
} while ( flags & MORE_COMPONENTS ) 
if (flags & WE_HAVE_INSTR){
	USHORT numInstr
	BYTE instr[numInstr]

Argument1 and argument2 can be either x and y offsets to be added to the glyph or two point numbers. In the latter case, the first point number indicates the point that is to be matched to the new glyph. The second number indicates the new glyph’s "matched" point. Once a glyph is added, its point numbers begin directly after the last glyphs (endpoint of first glyph + 1).

When arguments 1 and 2 are an x and a y offset instead of points and the bit ROUND_XY_TO_GRID is set to 1, the values are rounded to those of the closest grid lines before they are added to the glyph. X and Y offsets are described in FUnits.

If the bit WE_HAVE_A_SCALE is set, the scale value is read in 2.14 format-the value can be between -2 to almost +2. The glyph will be scaled by this value before grid-fitting.

The bit WE_HAVE_A_TWO_BY_TWO allows for an interrelationship between the x and y coordinates. This could be used for 90-degree rotations, for example.

These are the constants for the flags field:

The purpose of USE_MY_METRICS is to force the lsb and rsb to take on a desired value. For example, an i-circumflex (U+00EF) is often composed of the circumflex and a dotless-i. In order to force the composite to have the same metrics as the dotless-i, set USE_MY_METRICS for the dotless-i component of the composite. Without this bit, the rsb and lsb would be calculated from the hmtx entry for the composite (or would need to be explicitly set with TrueType instructions).

Note that the behavior of the USE_MY_METRICS operation is undefined for rotated composite components.