fix: correct AVC PicTiming SEI offset

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 - Parsing of AVCDecoderConfigurationRecord
+- Parsing of time offset in AVC PicTiming SEI
 
 ## [0.40.2] - 2023-11-17
 

bits/aereader.go

@@ -54,6 +54,16 @@ func (r *AccErrReader) Read(n int) uint {
 	return value
 }
 
+// ReadSigned reads a 2-complemented signed int with n bits.
+func (r *AccErrReader) ReadSigned(n int) int {
+	nr := int(r.Read(n))
+	firstBit := nr >> (n - 1)
+	if firstBit == 1 {
+		nr |= -1 << n
+	}
+	return nr
+}
+
 // ReadFlag - read 1 bit into flag. Return false if error now or previously
 func (r *AccErrReader) ReadFlag() bool {
 	bit := r.Read(1)

bits/aereader_test.go

@@ -35,6 +35,35 @@ func TestAccErrReader(t *testing.T) {
 	}
 }
 
+func TestAccErrReaderSigned(t *testing.T) {
+	input := []byte{0xff, 0x0c} // 1111 1111 0000 1100
+	rd := bytes.NewReader(input)
+	reader := NewAccErrReader(rd)
+
+	cases := []struct {
+		n    int
+		want int
+	}{
+		{2, -1}, // 11
+		{3, -1}, // 111
+		{5, -4}, // 11100
+		{3, 1},  // 001
+		{3, -4}, // 100
+	}
+
+	for _, tc := range cases {
+		got := reader.ReadSigned(tc.n)
+
+		if got != tc.want {
+			t.Errorf("Read(%d)=%b, want=%b", tc.n, got, tc.want)
+		}
+	}
+	err := reader.AccError()
+	if err != nil {
+		t.Errorf("Got accumulated error: %s", err.Error())
+	}
+}
+
 func TestBadAccErrReader(t *testing.T) {
 	// Check that reading beyond EOF provides value = 0 after acc error
 	input := []byte{0xff, 0x0f} // 1111 1111 0000 1111

sei/sei.go

@@ -605,7 +605,8 @@ func WriteSEIMessages(w io.Writer, msgs []SEIMessage) error {
 	for _, msg := range msgs {
 		bw.WriteSEIValue(msg.Type())
 		bw.WriteSEIValue(msg.Size())
-		for _, b := range msg.Payload() {
+		pl := msg.Payload()
+		for _, b := range pl {
 			bw.Write(uint(b), 8)
 		}
 	}

sei/sei1.go

@@ -75,6 +75,7 @@ func DecodePicTimingAvcSEIHRD(sd *SEIData, cbpDbpDelay *CbpDbpDelay, timeOffsetL
 		c := DecodeClockTSAvc(br, timeOffsetLen)
 		tc.Clocks = append(tc.Clocks, c)
 	}
+	tc.TimeOffsetLength = timeOffsetLen
 	return &tc, br.AccError()
 }
 
@@ -186,7 +187,7 @@ func DecodeClockTSAvc(br *bits.AccErrReader, timeOffsetLen byte) ClockTSAvc {
 			}
 		}
 		if c.TimeOffsetLength > 0 {
-			c.TimeOffsetValue = int(c.TimeOffsetLength)
+			c.TimeOffsetValue = br.ReadSigned(int(c.TimeOffsetLength))
 		}
 	}
 	return c
@@ -246,7 +247,7 @@ func (c ClockTSAvc) WriteToSliceWriter(sw bits.SliceWriter) {
 			}
 		}
 		if c.TimeOffsetLength > 0 {
-			sw.WriteBits(uint(c.TimeOffsetLength), int(c.TimeOffsetLength))
+			sw.WriteBits(uint(c.TimeOffsetValue), int(c.TimeOffsetLength))
 		}
 	}
 }

sei/sei_test.go

@@ -315,7 +315,7 @@ func TestParseSEI(t *testing.T) {
 			},
 			[]uint{1},
 			[]string{
-				`SEIPicTimingType (1), size=15, time=20:40:09:46 offset=24`,
+				`SEIPicTimingType (1), size=15, time=20:40:09:46 offset=0`,
 			},
 			nil,
 		},
@@ -425,55 +425,65 @@ func TestWriteSEI(t *testing.T) {
 }
 
 func TestParseAVCPicTimingWithHRD(t *testing.T) {
-	sei1AVCEbsp := "010d00000300000300000300021208114de180"
+	sei1AVCEbsp := "011000000300000300000300021208114de10000030080"
 	cbpDelay := sei.CbpDbpDelay{
 		CpbRemovalDelay:                    0,
 		DpbOutputDelay:                     0,
 		InitialCpbRemovalDelayLengthMinus1: 26,
 		CpbRemovalDelayLengthMinus1:        30,
 		DpbOutputDelayLengthMinus1:         31,
 	}
-	timeOffsetLen := byte(0)
+	timeOffsetLen := byte(24)
 
 	testCases := []struct {
 		name           string
 		codec          sei.Codec
-		naluHex        string
+		naluPayloadHex string
 		wantedTypes    []uint
 		wantedStrings  []string
 		expNonFatalErr error
 	}{
 		{"PicTimingWithHRD", sei.AVC, sei1AVCEbsp, []uint{1},
 			[]string{
-				`SEIPicTimingType (1), size=13, time=01:47:41:08 offset=0`,
+				`SEIPicTimingType (1), size=16, time=01:47:41:08 offset=0`,
 			},
 			sei.ErrRbspTrailingBitsMissing,
 		},
 	}
 
 	for _, tc := range testCases {
-		seiNALU, _ := hex.DecodeString(tc.naluHex)
-
-		rs := bytes.NewReader(seiNALU)
-
-		seis, err := sei.ExtractSEIData(rs)
+		seiNaluPayload, _ := hex.DecodeString(tc.naluPayloadHex)
+		r := bytes.NewReader(seiNaluPayload)
+		seis, err := sei.ExtractSEIData(r)
 		if err != nil && err != tc.expNonFatalErr {
 			t.Error(err)
 		}
 		if len(seis) != len(tc.wantedStrings) {
 			t.Errorf("%s: Not %d but %d sei messages found", tc.name, len(tc.wantedStrings), len(seis))
 		}
+		var messages []sei.SEIMessage
 		for i := range seis {
 			seiMessage, err := sei.DecodePicTimingAvcSEIHRD(&seis[i], &cbpDelay, timeOffsetLen)
 			if err != nil {
 				t.Error(err)
 			}
+			messages = append(messages, seiMessage)
 			if seiMessage.Type() != tc.wantedTypes[i] {
 				t.Errorf("%s: got SEI type %d instead of %d", tc.name, seiMessage.Type(), tc.wantedTypes[i])
 			}
 			if seiMessage.String() != tc.wantedStrings[i] {
 				t.Errorf("%s: got %q instead of %q", tc.name, seiMessage.String(), tc.wantedStrings[i])
 			}
 		}
+		buf := bytes.Buffer{}
+		err = sei.WriteSEIMessages(&buf, messages)
+		if err != nil {
+			t.Error(err)
+		}
+		output := buf.Bytes()
+		if !bytes.Equal(output, seiNaluPayload) {
+			t.Errorf("%s: wanted %s but got %s", tc.name,
+				tc.naluPayloadHex, hex.EncodeToString(output))
+		}
 	}
 }