Fix SPS parsing in some cases

Author: AlexxIT

pkg/bits/reader.go

@@ -122,9 +122,9 @@ func (r *Reader) ReadUEGolomb() uint32 {
 // ReadSEGolomb - ReadSignedExponentialGolomb
 func (r *Reader) ReadSEGolomb() int32 {
 	if b := r.ReadUEGolomb(); b%2 == 0 {
-		return -int32(b >> 1)
+		return -int32(b / 2)
 	} else {
-		return int32(b >> 1)
+		return int32((b + 1) / 2)
 	}
 }
 

pkg/h264/h264_test.go

@@ -5,7 +5,6 @@ import (
 	"encoding/hex"
 	"testing"
 
-	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
 
@@ -91,5 +90,14 @@ func TestDecodeSPS2(t *testing.T) {
 	require.Nil(t, err)
 
 	sps := DecodeSPS(b)
-	assert.Nil(t, sps) // broken SPS?
+	require.Equal(t, uint16(928), sps.Width())
+	require.Equal(t, uint16(576), sps.Height())
+
+	s = "Z2QAHq2EAQwgCGEAQwgCGEAQwgCEO1BQF/yzcBAQFAAAD6AAAXcCEA==" // unknown
+	b, err = base64.StdEncoding.DecodeString(s)
+	require.Nil(t, err)
+
+	sps = DecodeSPS(b)
+	require.Equal(t, uint16(640), sps.Width())
+	require.Equal(t, uint16(360), sps.Height())
 }

pkg/h264/sps.go

@@ -88,6 +88,8 @@ func (s *SPS) Height() uint16 {
 }
 
 func DecodeSPS(sps []byte) *SPS {
+	// https://developer.ridgerun.com/wiki/index.php/H264_Analysis_Tools
+	// ffmpeg -i file.h264 -c copy -bsf:v trace_headers -f null -
 	r := bits.NewReader(sps)
 
 	hdr := r.ReadByte()