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

codec/mjpeg/jpeg.go

@@ -71,13 +71,19 @@ const (
 	jfifHeadLen     = 16       // Length of JFIF header segment excluding APP0 marker.
 )
 
-// SOF0 (start of scan) header fields.
+// SOF0 (start of frame) header fields.
 const (
 	sofLen            = 17
 	sofPrecision      = 8
 	sofNoOfComponents = 3
 )
 
+// SOS (start of scan) header fields.
+const (
+	sosLen              = 12
+	sosComponentsInScan = 3
+)
+
 var (
 	errNoQTable  = errors.New("no quantization table")
 	errReservedQ = errors.New("q value is reserved")
@@ -360,8 +366,10 @@ func writeHeader(p *putBuffer, _type, width, height, nbqTab, dri int, qtable []b
 
 	// Write start of scan.
 	mark(p, codeSOS)
-	p.put16(12)
-	p.put8(3)
+	p.put16(sosLen)
+	p.put8(sosComponentsInScan)
+
+	// TODO: find out what remaining fields are.
 	p.put8(1)
 	p.put8(0)
 	p.put8(2)