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dts_extractor.go
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dts_extractor.go
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package h264
import (
"fmt"
"time"
"github.com/bluenviron/mediacommon/pkg/bits"
)
func getPictureOrderCount(buf []byte, sps *SPS) (uint32, error) {
if len(buf) < 6 {
return 0, fmt.Errorf("not enough bits")
}
buf = EmulationPreventionRemove(buf[:6])
buf = buf[1:]
pos := 0
_, err := bits.ReadGolombUnsigned(buf, &pos) // first_mb_in_slice
if err != nil {
return 0, err
}
_, err = bits.ReadGolombUnsigned(buf, &pos) // slice_type
if err != nil {
return 0, err
}
_, err = bits.ReadGolombUnsigned(buf, &pos) // pic_parameter_set_id
if err != nil {
return 0, err
}
_, err = bits.ReadBits(buf, &pos, int(sps.Log2MaxFrameNumMinus4+4)) // frame_num
if err != nil {
return 0, err
}
if !sps.FrameMbsOnlyFlag {
return 0, fmt.Errorf("frame_mbs_only_flag = 0 is not supported")
}
picOrderCntLsb, err := bits.ReadBits(buf, &pos, int(sps.Log2MaxPicOrderCntLsbMinus4+4))
if err != nil {
return 0, err
}
return uint32(picOrderCntLsb), nil
}
func findPictureOrderCount(au [][]byte, sps *SPS) (uint32, error) {
for _, nalu := range au {
typ := NALUType(nalu[0] & 0x1F)
if typ == NALUTypeNonIDR {
poc, err := getPictureOrderCount(nalu, sps)
if err != nil {
return 0, err
}
return poc, nil
}
}
return 0, fmt.Errorf("POC not found")
}
func getPictureOrderCountDiff(poc1 uint32, poc2 uint32, sps *SPS) int32 {
diff := int32(poc1) - int32(poc2)
switch {
case diff < -((1 << (sps.Log2MaxPicOrderCntLsbMinus4 + 3)) - 1):
diff += (1 << (sps.Log2MaxPicOrderCntLsbMinus4 + 4))
case diff > ((1 << (sps.Log2MaxPicOrderCntLsbMinus4 + 3)) - 1):
diff -= (1 << (sps.Log2MaxPicOrderCntLsbMinus4 + 4))
}
return diff
}
// DTSExtractor allows to extract DTS from PTS.
type DTSExtractor struct {
spsp *SPS
prevDTSFilled bool
prevDTS time.Duration
expectedPOC uint32
reorderedFrames int
pauseDTS int
pocIncrement int
}
// NewDTSExtractor allocates a DTSExtractor.
func NewDTSExtractor() *DTSExtractor {
return &DTSExtractor{
pocIncrement: 2,
}
}
func (d *DTSExtractor) extractInner(au [][]byte, pts time.Duration) (time.Duration, error) {
idrPresent := false
for _, nalu := range au {
typ := NALUType(nalu[0] & 0x1F)
switch typ {
case NALUTypeSPS:
var spsp SPS
err := spsp.Unmarshal(nalu)
if err != nil {
return 0, fmt.Errorf("invalid SPS: %v", err)
}
d.spsp = &spsp
case NALUTypeIDR:
idrPresent = true
}
}
if d.spsp == nil {
return 0, fmt.Errorf("SPS not received yet")
}
if d.spsp.PicOrderCntType == 2 {
return pts, nil
}
if d.spsp.PicOrderCntType == 1 {
return 0, fmt.Errorf("pic_order_cnt_type = 1 is not supported yet")
}
if idrPresent {
d.expectedPOC = 0
d.reorderedFrames = 0
d.pauseDTS = 0
d.pocIncrement = 2
return pts, nil
}
d.expectedPOC += uint32(d.pocIncrement)
d.expectedPOC &= ((1 << (d.spsp.Log2MaxPicOrderCntLsbMinus4 + 4)) - 1)
if d.pauseDTS > 0 {
d.pauseDTS--
return d.prevDTS + 1*time.Millisecond, nil
}
poc, err := findPictureOrderCount(au, d.spsp)
if err != nil {
return 0, err
}
if d.pocIncrement == 2 && (poc%2) != 0 {
d.pocIncrement = 1
d.expectedPOC /= 2
}
pocDiff := int(getPictureOrderCountDiff(poc, d.expectedPOC, d.spsp)) + d.reorderedFrames*d.pocIncrement
if pocDiff < 0 {
return 0, fmt.Errorf("invalid POC")
}
if pocDiff == 0 {
return pts, nil
}
reorderedFrames := (pocDiff - d.reorderedFrames*d.pocIncrement) / d.pocIncrement
if reorderedFrames > d.reorderedFrames {
d.pauseDTS = (reorderedFrames - d.reorderedFrames - 1)
d.reorderedFrames = reorderedFrames
return d.prevDTS + 1*time.Millisecond, nil
}
return d.prevDTS + ((pts - d.prevDTS) * time.Duration(d.pocIncrement) / time.Duration(pocDiff+d.pocIncrement)), nil
}
// Extract extracts the DTS of a access unit.
func (d *DTSExtractor) Extract(au [][]byte, pts time.Duration) (time.Duration, error) {
dts, err := d.extractInner(au, pts)
if err != nil {
return 0, err
}
if dts > pts {
return 0, fmt.Errorf("DTS is greater than PTS")
}
if d.prevDTSFilled && dts <= d.prevDTS {
return 0, fmt.Errorf("DTS is not monotonically increasing, was %v, now is %v",
d.prevDTS, dts)
}
d.prevDTS = dts
d.prevDTSFilled = true
return dts, err
}