freetype/truetype: implement GC and MD instructions.

These instructions are found in Adobe Source Sans Pro fonts.

R=golang-dev, nigeltao, bsiegert
CC=golang-dev
https://codereview.appspot.com/16440043

Committer: Nigel Tao <nigeltao@golang.org>

freetype/truetype/hint.go

@@ -627,6 +627,33 @@ func (h *Hinter) run(program []byte, pCurrent, pUnhinted, pInFontUnits []Point,
 		case opRCVT:
 			h.stack[top-1] = int32(h.getScaledCVT(h.stack[top-1]))
 
+		case opGC0, opGC1:
+			i := h.stack[top-1]
+			if opcode == opGC0 {
+				p := h.point(2, current, i)
+				h.stack[top-1] = int32(dotProduct(f26dot6(p.X), f26dot6(p.Y), h.gs.pv))
+			} else {
+				p := h.point(2, unhinted, i)
+				// Using dv as per C Freetype.
+				h.stack[top-1] = int32(dotProduct(f26dot6(p.X), f26dot6(p.Y), h.gs.dv))
+			}
+
+		case opMD0, opMD1:
+			top--
+			i, j := h.stack[top-1], h.stack[top]
+			if opcode == opMD1 {
+				p := h.point(0, current, i)
+				q := h.point(1, current, j)
+				h.stack[top-1] = int32(dotProduct(f26dot6(p.X-q.X), f26dot6(p.Y-q.Y), h.gs.pv))
+			} else {
+				// TODO: do we need to check (h.gs.zp[0] == 0 || h.gs.zp[1] == 0)
+				// as C Freetype does, similar to the MDRP instructions?
+				p := h.point(0, unhinted, i)
+				q := h.point(1, unhinted, j)
+				// Use dv for MD0 as in C Freetype.
+				h.stack[top-1] = int32(dotProduct(f26dot6(p.X-q.X), f26dot6(p.Y-q.Y), h.gs.dv))
+			}
+
 		case opMPPEM, opMPS:
 			if top >= len(h.stack) {
 				return errors.New("truetype: hinting: stack overflow")

freetype/truetype/opcodes.go

@@ -80,11 +80,11 @@ const (
 	opRS        = 0x43 // Read Store
 	opWCVTP     = 0x44 // Write Control Value Table in Pixel units
 	opRCVT      = 0x45 // Read Control Value Table entry
-	opGC0       = 0x46
-	opGC1       = 0x47
+	opGC0       = 0x46 // Get Coordinate projected onto the projection vector
+	opGC1       = 0x47 // .
 	opSCFS      = 0x48
-	opMD0       = 0x49
-	opMD1       = 0x4a
+	opMD0       = 0x49 // Measure Distance
+	opMD1       = 0x4a // .
 	opMPPEM     = 0x4b // Measure Pixels Per EM
 	opMPS       = 0x4c // Measure Point Size
 	opFLIPON    = 0x4d // set the auto FLIP Boolean to ON
@@ -275,7 +275,7 @@ var popCount = [256]uint8{
 	1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 1, 0, 1, 1, 1, 1, // 0x10 - 0x1f
 	1, 1, 0, 2, 0, 1, 1, q, q, q, 2, 1, 1, 0, 1, 1, // 0x20 - 0x2f
 	0, 0, q, q, q, q, 1, 1, 1, 0, q, q, 0, 0, 2, 2, // 0x30 - 0x3f
-	0, 0, 2, 1, 2, 1, q, q, q, q, q, 0, 0, 0, 0, 0, // 0x40 - 0x4f
+	0, 0, 2, 1, 2, 1, 1, 1, q, 2, 2, 0, 0, 0, 0, 0, // 0x40 - 0x4f
 	2, 2, 2, 2, 2, 2, 1, 1, 1, 0, 2, 2, 1, q, 1, 1, // 0x50 - 0x5f
 	2, 2, 2, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, // 0x60 - 0x6f
 	q, q, q, q, q, q, 1, 1, 2, 2, 0, q, 0, 0, 1, 1, // 0x70 - 0x7f