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• avcodec/put_bits : Make skip_put_bits() less dangerous

  31 juillet 2020, par Andreas Rheinhardt

  avcodec/put_bits : Make skip_put_bits() less dangerous
  Before c63c303a1f2b58677d480505ec93a90f77dd25b5 (the commit which
  
  introduced a typedef for the type of the buffer of a PutBitContext)
  
  skip_put_bits() was as follows :
  static inline void skip_put_bits(PutBitContext *s, int n)
  
  
  
      s->bit_left -= n ;
  
      s->buf_ptr  -= 4 * (s->bit_left >> 5) ;
  
      s->bit_left &= 31 ;
  
  
  If s->bit_left was negative after the first subtraction, then the next
  
  line will divide this by 32 with rounding towards -inf and multiply by
  
  four ; the result will be negative, of course.
  The aforementioned commit changed this to :
  static inline void skip_put_bits(PutBitContext *s, int n)
  
  
  
      s->bit_left -= n ;
  
      s->buf_ptr  -= sizeof(BitBuf) * ((unsigned)s->bit_left / BUF_BITS) ;
  
      s->bit_left &= (BUF_BITS - 1) ;
  
  
  Casting s->bit_left to unsigned meant that the rounding is still towards
  
   inf ; yet the right side is now always positive (it transformed the
  
  arithmetic shift into a logical shift), so that s->buf_ptr will always
  
  be decremented (by about UINT_MAX / 8 unless n is huge) which leads to
  
  segfaults on further usage and is already undefined pointer arithmetic
  
  before that. This can be reproduced with the mpeg4 encoder with the
  
  AV_CODEC_FLAG2_NO_OUTPUT flag set.
  Furthermore, the earlier version as well as the new version share
  
  another bug : s->bit_left will be in the range of 0..(BUF_BITS - 1)
  
  afterwards, although the assumption throughout the other PutBitContext
  
  functions is that it is in the range of 1..BUF_BITS. This might lead to
  
  a shift by BUF_BITS in little-endian mode. This has been fixed, too.
  
  The new version is furthermore able to skip zero bits, too.
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
  

  • [DH] libavcodec/put_bits.h

• CGO : How do you use pointers in Golang to access data from an array in C

  23 avril 2018, par nevernew

  I’m writing an app for the windows platform using FFmpeg and it’s golang wrapper goav, but I’m having trouble understanding how to use the C pointers to gain access to the data array they point to.

  I’m trying to get the data stored in the AVFrame class and use Go to write it to a file, and eventually a texture in OpenGl to make a video player with cool transitions.

  I think understanding how to cast and access the C data will make coding this a lot easier.

  I’ve stripped out all the relevant parts of the C code, the wrapper and my code, shown below :

  C code - libavutil/frame.h

  #include 
  
  
  
  typedef struct AVFrame {
  
  #define AV_NUM_DATA_POINTERS 8
  
      uint8_t *data[AV_NUM_DATA_POINTERS];
  
  }

  Golang goav wrapper - I don’t really know whats going on here with the unsafe.Pointers and casting but it gives me access to the underlying C code

  package avutil
  
  
  
  /*
  
      #cgo pkg-config: libavutil
  
      #include <libavutil></libavutil>frame.h>
  
      #include 
  
  */
  
  import "C"
  
  import (
  
      "unsafe"
  
  )
  
  
  
  type Frame C.struct_AVFrame
  
  
  
  func AvFrameAlloc() *Frame {
  
      return (*Frame)(unsafe.Pointer(C.av_frame_alloc()))
  
  }
  
  
  
  func Data(f *Frame) *uint8 {
  
      return (*uint8)(unsafe.Pointer((*C.uint8_t)(unsafe.Pointer(&amp;f.data))))
  
  }

  My Golang code

  package main
  
  
  
  import "github.com/giorgisio/goav/avutil"
  
  
  
  func main() {
  
      videoFrame := avutil.AvFrameAlloc()
  
  
  
      data := avutil.Data(videoFrame)
  
  
  
      fmt.Println(data) // here i want the values from data[0] to data[7], but how?
  
  }

• Ideal bitrates for different video resolutions

  15 octobre 2018, par Ramesh Navi

  I am building a Video-on-demand service for a closed community. I using FFMPEG for video processing and dash.js for adaptive bitrate player with custom resolution selector. Can somebody please suggest what ideal bitrates should I use while video/audio transcoding ?

  I am talking about -b:v and -ab option

  ffmpeg -i vid.mp4 -c:v libvpx-vp9 -keyint_min 150 \
  
  -g 150 -tile-columns 4 -frame-parallel 1  -f webm -dash 1 \
  
  -an -vf scale=144:-1 -b:v 120k -dash 1 video_1.webm \
  
  -an -vf scale=240:-1 -b:v 250k -dash 1 video_2.webm \
  
  -an -vf scale=360:-1 -b:v 500k -dash 1 video_3.webm \
  
  -an -vf scale=480:-1 -b:v 750k -dash 1 video_4.webm \
  
  -an -vf scale=720:-1 -b:v 1500k -dash 1 video_5.webm

  And

  ffmpeg -i vid.mp4 -vn -acodec libvorbis -ab 96k -dash 1 audio_96k.webm

  Any suggestions/hacks or examples to tackle real-world network situations are appreciated.