diff --git a/codec/mjpeg/jpeg.go b/codec/mjpeg/jpeg.go
index 85aef03c..7ad82398 100644
--- a/codec/mjpeg/jpeg.go
+++ b/codec/mjpeg/jpeg.go
@@ -94,6 +94,14 @@ var (
 	ErrNoFrameStart         = errors.New("missing start of frame")
 )
 
+// n values required for huffman table generation.
+var (
+	nDCLum = deriveN(bitsDCLum)
+	nDCChr = deriveN(bitsDCChr)
+	nACLum = deriveN(bitsACLum)
+	nACChr = deriveN(bitsACChr)
+)
+
 // Slices used in the creation of huffman tables.
 var (
 	bitsDCLum = []byte{0, 0, 1, 5, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0}
@@ -291,7 +299,7 @@ func (c *Context) ParsePayload(p []byte, m bool) error {
 	return nil
 }
 
-// writeHeader writes a JPEG header to the writer w.
+// writeHeader writes a JPEG header to the writer slice p.
 func writeHeader(p []byte, _type, width, height, nbqTab int, dri uint16, qtable []byte) int {
 	width <<= 3
 	height <<= 3
@@ -305,8 +313,7 @@ func writeHeader(p []byte, _type, width, height, nbqTab int, dri uint16, qtable
 	binary.BigEndian.PutUint16(p[idx+4:], jfifHeadLen)
 	idx += 6
 
-	copy(p[idx:], jfifLabel)
-	idx += len(jfifLabel)
+	idx += copy(p[idx:], jfifLabel)
 
 	binary.BigEndian.PutUint16(p[idx:], jfifVer)
 	p[idx+2] = jfifDensityUnit
@@ -336,7 +343,7 @@ func writeHeader(p []byte, _type, width, height, nbqTab int, dri uint16, qtable
 		p[idx] = byte(i)
 		idx++
 
-		copy(p[idx:], qtable[64*i:(64*i)+64])
+		idx += copy(p[idx:], qtable[64*i:(64*i)+64])
 	}
 
 	// Define huffman table.
@@ -345,10 +352,10 @@ func writeHeader(p []byte, _type, width, height, nbqTab int, dri uint16, qtable
 	lenIdx := idx
 	binary.BigEndian.PutUint16(p[idx:], 0)
 	idx += 2
-	idx += writeHuffman(p[idx:], 0, 0, bitsDCLum, valDC)
-	idx += writeHuffman(p[idx:], 0, 1, bitsDCChr, valDC)
-	idx += writeHuffman(p[idx:], 1, 0, bitsACLum, valACLum)
-	idx += writeHuffman(p[idx:], 1, 1, bitsACChr, valACChr)
+	idx += writeHuffman(p[idx:], bitsDCLum, valDC, 0, nDCLum)
+	idx += writeHuffman(p[idx:], bitsDCChr, valDC, 1, nDCChr)
+	idx += writeHuffman(p[idx:], bitsACLum, valACLum, 1<<4, nACLum)
+	idx += writeHuffman(p[idx:], bitsACChr, valACChr, 1<<4|1, nACChr)
 	binary.BigEndian.PutUint16(p[lenIdx:], uint16(idx-lenIdx))
 
 	// Start of frame.
@@ -389,21 +396,11 @@ func writeHeader(p []byte, _type, width, height, nbqTab int, dri uint16, qtable
 	return idx
 }
 
-// writeHuffman write a JPEG huffman table to w.
-func writeHuffman(p []byte, class, id int, bits, values []byte) int {
-	idx := 0
-	p[idx] = uint8(class<<4 | id)
-	idx++
-
-	var n int
-	for i := 1; i <= 16; i++ {
-		n += int(bits[i])
-	}
-
-	idx += copy(p[idx:], bits[1:17])
-	idx += copy(p[idx:], values[0:n])
-
-	return idx
+// writeHuffman write a JPEG huffman table to alice p.
+func writeHuffman(p, bits, values []byte, prefix byte, n int) int {
+	p[0] = prefix
+	i := copy(p[1:], bits[1:17])
+	return copy(p[i+1:], values[0:n]) + i + 1
 }
 
 // defaultQTable returns a default quantization table.
@@ -443,3 +440,12 @@ func clip(v, min, max int) int {
 func get24(p []byte) int {
 	return int(p[0]<<16) | int(p[1]<<8) | int(p[2])
 }
+
+// deriveN calculates n values required for huffman table generation.
+func deriveN(bits []byte) int {
+	var n int
+	for i := 1; i <= 16; i++ {
+		n += int(bits[i])
+	}
+	return n
+}