codec/mjpeg/jpeg.go: defining consts for SOF0 header fields

codec/mjpeg/jpeg.go

@@ -55,8 +55,8 @@ const (
 // Density units.
 const (
 	unitNone = iota
-	unitPxIN
-	unitPxCM
+	unitPxIN // Pixels per inch.
+	unitPxCM // Pixels per centimeter.
 )
 
 // JFIF header fields.
@@ -71,6 +71,13 @@ const (
 	jfifHeadLen     = 16       // Length of JFIF header segment excluding APP0 marker.
 )
 
+// SOF0 (start of scan) header fields.
+const (
+	sofLen            = 17
+	sofPrecision      = 8
+	sofNoOfComponents = 3
+)
+
 var (
 	errNoQTable  = errors.New("no quantization table")
 	errReservedQ = errors.New("q value is reserved")
@@ -334,11 +341,13 @@ func writeHeader(p *putBuffer, _type, width, height, nbqTab, dri int, qtable []b
 		mtxNo = 1
 	}
 
-	p.put16(17)
-	p.put8(8)
+	p.put16(sofLen)
+	p.put8(sofPrecision)
 	p.put16(uint16(height))
 	p.put16(uint16(width))
-	p.put8(3)
+	p.put8(sofNoOfComponents)
+
+	// TODO: find meaning of these fields.
 	p.put8(1)
 	p.put8(uint8((2 << 4) | sample))
 	p.put8(0)