Merge pull request #32 from edgeware/clearer-doc

Improved documentation

README.md

@@ -6,13 +6,16 @@
 [![Go Report Card](https://goreportcard.com/badge/github.com/edgeware/mp4ff)](https://goreportcard.com/report/github.com/edgeware/mp4ff)
 [![license](https://img.shields.io/github/license/edgeware/mp4ff.svg)](https://github.com/edgeware/mp4ff/blob/master/LICENSE.md)
 
-MP4/ISOBMFF media file parser and writer. Focused on fragmented files as used for streaming in DASH, MSS and HLS fMP4.
+Package mp4ff implements MP4 media file parser and writer for AVC video, AAC audio and stpp/wvtt subtitles.
+Focused on fragmented files as used for streaming in DASH, MSS and HLS fMP4.
 
 ## Library
 
-The library has functions for parsing (called Decode) and writing (Encode).
+The library has functions for parsing (called Decode) and writing (Encode) in the package `mp4ff/mp4`.
+It also contains codec specific parsing of of AVC/H.264 including complete parsing of
+SPS and PPS in the package `mp4ff.avc`.
 
-Traditional multiplexed non-fragmented mp4 files can be parsed and decoded, see `examples/segment`.
+Traditional multiplexed non-fragmented mp4 files can also be parsed and decoded, see `examples/segment`.
 
 The focus is, however, on non-multiplexed single-track fragmented mp4 files as used in DASH, HLS, and CMAF.
 
@@ -114,7 +117,7 @@ it can be calculated. It is set to `MoofBox.Size()+8`.
 
 Some simple command line tools are available in `cmd`.
 
-1. `mp4ff-pslister` extracts and displays pps and sps for AVC in an mp4 file
+1. `mp4ff-pslister` extracts and displays pps and sps for AVC in an mp4 file.
 
 ## Example code
 
@@ -133,7 +136,7 @@ The APIs should be fairly stable, but minor non-backwards-compatible tweaks may
 
 MIT, see [LICENSE.md](LICENSE.md).
 
-Some code in pkg/mp4, es from or is based on https://github.com/jfbus/mp4 which has
+Some code in pkg/mp4, comes from or is based on https://github.com/jfbus/mp4 which has
 `Copyright (c) 2015 Jean-François Bustarret`.
 
 Some code in pkg/bits comes from or is based on https://github.com/tcnksm/go-casper/tree/master/internal/bits

avc/doc.go

@@ -0,0 +1,4 @@
+/*
+Package avc implements parsing of AVC(H.264) nal headers, slice headers and complete SPS and PPS.
+*/
+package avc

bits/bits.go

@@ -5,11 +5,12 @@ import (
 	"io"
 )
 
-// Writer writes bits into underlying io.Writer. Stops at first error
+// Writer writes bits into underlying io.Writer. Stops writing at first error.
+// Errors that have occured can later be checked with Error().
 type Writer struct {
 	n   int   // current number of bits
 	v   uint  // current accumulated value
-	err error // Has a write caused an error
+	err error // The first error caused by any write operation
 
 	wr io.Writer
 }
@@ -130,11 +131,12 @@ func (r *Reader) MustRead(n int) uint {
 	return v
 }
 
-// MustReadFlag - read 1 bit into flag
+// MustReadFlag - read 1 bit into flag and panic if not possible
 func (r *Reader) MustReadFlag() bool {
 	return r.MustRead(1) == 1
 }
 
+// mask - n-bit binary mask
 func mask(n int) uint {
 	return (1 << uint(n)) - 1
 }

bits/doc.go

@@ -1,8 +1,8 @@
 /*
-Package bits implements bit reading and writing.
+Package bits implements bit reading and writing including EBSP.
 
 Beyond plain bit reading and writing, it includes reading of ebsp (Encapsulated Byte Sequence Packets)
-Golomb codes as usid in the AVC/H.264 video coding standard.
+Golomb codes as used in the AVC/H.264 and HEVC video coding standards.
 
 */
 package bits

bits/ebsp.go

@@ -7,6 +7,10 @@ import (
 	"io"
 )
 
+const (
+	startCodeEmulationPreventionByte = 0x03
+)
+
 var ErrNotReedSeeker = errors.New("Reader does not support Seek")
 
 // NewEBSPReader - return a new Reader.
@@ -39,7 +43,7 @@ func (r *EBSPReader) MustRead(n int) uint {
 			panic("Reading error")
 		}
 		r.pos++
-		if r.zeroCount == 2 {
+		if r.zeroCount == 2 && b == startCodeEmulationPreventionByte {
 			err = binary.Read(r.rd, binary.BigEndian, &b)
 			if err != nil {
 				panic("Reading error")
@@ -70,7 +74,7 @@ func (r *EBSPReader) MustReadFlag() bool {
 	return r.MustRead(1) == 1
 }
 
-// MustReadExpGolomb - Read one unsigned exponential golomb code
+// MustReadExpGolomb - Read one unsigned exponential golomb code. Panic if not possible
 func (r *EBSPReader) MustReadExpGolomb() uint {
 	leadingZeroBits := 0
 
@@ -88,7 +92,7 @@ func (r *EBSPReader) MustReadExpGolomb() uint {
 	return res + endBits
 }
 
-// MustReadSignedGolomb - Read one signed exponential golomb code
+// MustReadSignedGolomb - Read one signed exponential golomb code. Panic if not possible
 func (r *EBSPReader) MustReadSignedGolomb() int {
 	unsignedGolomb := r.MustReadExpGolomb()
 	if unsignedGolomb%2 == 1 {
@@ -108,13 +112,13 @@ func (r *EBSPReader) NrBitsReadInCurrentByte() int {
 	return 8 - r.n
 }
 
-// EBSP2rbsp - convert from EBSP to RBSP by removing escape 0x03 after two 0x00
+// EBSP2rbsp - convert from EBSP to RBSP by removing start code emulation prevention bytes
 func EBSP2rbsp(ebsp []byte) []byte {
 	zeroCount := 0
 	output := make([]byte, 0, len(ebsp))
 	for i := 0; i < len(ebsp); i++ {
 		b := ebsp[i]
-		if zeroCount == 2 && b == 3 {
+		if zeroCount == 2 && b == startCodeEmulationPreventionByte {
 			zeroCount = 0
 		} else {
 			if b != 0 {
@@ -140,7 +144,7 @@ func (r *EBSPReader) Read(n int) (uint, error) {
 			return 0, err
 		}
 		r.pos++
-		if r.zeroCount == 2 && b <= 3 {
+		if r.zeroCount == 2 && b == startCodeEmulationPreventionByte {
 			err = binary.Read(r.rd, binary.BigEndian, &b)
 			if err != nil {
 				return 0, err
@@ -220,7 +224,7 @@ func (r *EBSPReader) IsSeeker() bool {
 }
 
 // MoreRbspData - false if next bit is 1 and last 1 in fullSlice
-// Underlying reader must support ReadSeeker interface
+// Underlying reader must support ReadSeeker interface to reset after check
 func (r *EBSPReader) MoreRbspData() (bool, error) {
 	if !r.IsSeeker() {
 		return false, ErrNotReedSeeker
@@ -239,7 +243,7 @@ func (r *EBSPReader) MoreRbspData() (bool, error) {
 		}
 		return true, nil
 	}
-	// Must check if all remaining bits are zero
+	// If all remainging bits are zero, there is no more rbsp data
 	more := false
 	for {
 		b, err := r.Read(1)
@@ -261,6 +265,7 @@ func (r *EBSPReader) MoreRbspData() (bool, error) {
 	return more, nil
 }
 
+// reset EBSPReader based on copy of previous state
 func (r *EBSPReader) reset(prevState EBSPReader) error {
 	rdSeek, ok := r.rd.(io.ReadSeeker)
 

bits/esbp_test.go

@@ -56,6 +56,7 @@ func TestGolomb(t *testing.T) {
 	})
 }
 
+// TestEbspParser including startCodeEmulationPrevention removal
 func TestEbspParser(t *testing.T) {
 
 	cases := []struct{ name, start, want string }{

cmd/mp4ff-pslister/main.go

@@ -59,7 +59,7 @@ func main() {
 			avcC := stsd.AvcX.AvcC
 			trackID := trak.Tkhd.TrackID
 			if *verbose {
-				fmt.Printf("Videavo track ID=%d\n", trackID)
+				fmt.Printf("Video track ID=%d\n", trackID)
 			}
 			var spsInfo *avc.SPS
 			var err error

doc.go

@@ -1,5 +1,6 @@
 /*
-Package mp4ff implements MP4 media file parser and writer.
+Package mp4ff implements MP4 media file parser and writer for AVC video, AAC audio and stpp/wvtt subtitles.
+Focused on fragmented files as used for streaming in DASH, MSS and HLS fMP4.
 
 MP4 library
 

examples/resegmenter/main.go

@@ -1,10 +1,10 @@
-// mp4ff-resegmenter resegments mp4 files into concatenated segments with new duration.
+// resegmenter resegments mp4 files into concatenated segments with new duration.
 //
-// The mp4 files must look like a CMAF track with init segment and media segments.
+// The input file must be a CMAF track with init segment and media segments.
 //
 //   Usage:
 //
-//    mp4ff-resegmenter -f <input.mp4> -o <output.mp4> -b <chunk_dur>
+//    resegmenter -f <input.mp4> -o <output.mp4> -b <chunk_dur>
 //    -b int
 //         Required: chunk duration (ticks)
 //    -f string

examples/segmenter/main.go

@@ -1,9 +1,9 @@
 // segmenter segments a progressive mp4 file into audio and video segments
 //
-// There should be at most one audio and one video track in the file.
+// There should be at most one audio and one video track in the input.
 // The output files will be named as
-//   init segments: <output>_a.mp4 and <output>_v.mp4
-//   media segments: <output>_a_<n>.m4s and <output>_v_<n>.m4s where n >= 1
+// init segments: <output>_a.mp4 and <output>_v.mp4
+// media segments: <output>_a_<n>.m4s and <output>_v_<n>.m4s where n >= 1
 //
 //   Usage:
 //

examples/tree/main.go

@@ -1,4 +1,4 @@
-// mp4ff-tree print a tree of the box structure of a file.
+// tree prints a tree of the box structure of a file using the Dump method of boxes.
 //
 //   Usage:
 //
@@ -30,5 +30,11 @@ func main() {
 	}
 	defer ifd.Close()
 	parsedMp4, err := mp4.DecodeFile(ifd)
-	parsedMp4.Dump(os.Stdout, "  ")
+	if err != nil {
+		log.Fatal(err)
+	}
+	err = parsedMp4.Dump(os.Stdout, "  ")
+	if err != nil {
+		log.Fatal(err)
+	}
 }