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• Why is ffmpeg faster than this minimal example ?

  23 juillet 2022, par Dave Ceddia

  I'm wanting to read the audio out of a video file as fast as possible, using the libav libraries. It's all working fine, but it seems like it could be faster.

  


  To get a performance baseline, I ran this ffmpeg command and timed it :

  


  time ffmpeg -threads 1 -i file -map 0:a:0 -f null -


  


  On a test file (a 2.5gb 2hr .MOV with pcm_s16be audio) this comes out to about 1.35 seconds on my M1 Macbook Pro.

  


  On the other hand, this minimal C code (based on FFmpeg's "Demuxing and decoding" example) is consistently around 0.3 seconds slower.

  


  #include <libavcodec></libavcodec>avcodec.h>&#xA;#include <libavformat></libavformat>avformat.h>&#xA;&#xA;static int decode_packet(AVCodecContext *dec, const AVPacket *pkt, AVFrame *frame)&#xA;{&#xA;    int ret = 0;&#xA;&#xA;    // submit the packet to the decoder&#xA;    ret = avcodec_send_packet(dec, pkt);&#xA;&#xA;    // get all the available frames from the decoder&#xA;    while (ret >= 0) {&#xA;        ret = avcodec_receive_frame(dec, frame);&#xA;        av_frame_unref(frame);&#xA;    }&#xA;&#xA;    return 0;&#xA;}&#xA;&#xA;int main (int argc, char **argv)&#xA;{&#xA;    int ret = 0;&#xA;    AVFormatContext *fmt_ctx = NULL;&#xA;    AVCodecContext  *dec_ctx = NULL;&#xA;    AVFrame *frame = NULL;&#xA;    AVPacket *pkt = NULL;&#xA;&#xA;    if (argc != 3) {&#xA;        exit(1);&#xA;    }&#xA;&#xA;    int stream_idx = atoi(argv[2]);&#xA;&#xA;    /* open input file, and allocate format context */&#xA;    avformat_open_input(&amp;fmt_ctx, argv[1], NULL, NULL);&#xA;&#xA;    /* get the stream */&#xA;    AVStream *st = fmt_ctx->streams[stream_idx];&#xA;&#xA;    /* find a decoder for the stream */&#xA;    AVCodec *dec = avcodec_find_decoder(st->codecpar->codec_id);&#xA;&#xA;    /* allocate a codec context for the decoder */&#xA;    dec_ctx = avcodec_alloc_context3(dec);&#xA;&#xA;    /* copy codec parameters from input stream to output codec context */&#xA;    avcodec_parameters_to_context(dec_ctx, st->codecpar);&#xA;&#xA;    /* init the decoder */&#xA;    avcodec_open2(dec_ctx, dec, NULL);&#xA;&#xA;    /* allocate frame and packet structs */&#xA;    frame = av_frame_alloc();&#xA;    pkt = av_packet_alloc();&#xA;&#xA;    /* read frames from the specified stream */&#xA;    while (av_read_frame(fmt_ctx, pkt) >= 0) {&#xA;        if (pkt->stream_index == stream_idx)&#xA;            ret = decode_packet(dec_ctx, pkt, frame);&#xA;&#xA;        av_packet_unref(pkt);&#xA;        if (ret &lt; 0)&#xA;            break;&#xA;    }&#xA;&#xA;    /* flush the decoders */&#xA;    decode_packet(dec_ctx, NULL, frame);&#xA;&#xA;    return ret &lt; 0;&#xA;}&#xA;

  


  I tried measuring parts of this program to see if it was spending a lot of time in the setup, but it's not – at least 1.5 seconds of the runtime is the loop where it's reading frames.

  


  So I took some flamegraph recordings (using cargo-flamegraph) and ran each a few times to make sure the timing was consistent. There's probably some overhead since both were consistently higher than running normally, but they still have the 0.3 second delta.

  


  # 1.812 total
time sudo flamegraph ./minimal file 1

# 1.542 total
time sudo flamegraph ffmpeg -threads 1 -i file -map 0:a:0 -f null - 2>&1


  


  Here are the flamegraphs stacked up, scaled so that the faster one is only 85% as wide as the slower one. (click for larger)

  


  

  


  The interesting thing that stands out to me is how long is spent on read in the minimal example vs. ffmpeg :

  


  

  


  The time spent on lseek is also a lot longer in the minimal program – it's plainly visible in that flamegraph, but in the ffmpeg flamegraph, lseek is a single pixel wide.

  


  What's causing this discrepancy ? Is ffmpeg actually doing less work than I think it is here ? Is the minimal code doing something naive ? Is there some buffering or other I/O optimizations that ffmpeg has enabled ?

  


  How can I shave 0.3 seconds off of the minimal example's runtime ?

  


• Add a sample "CAMM" data to specific PTS in a MP4 file

  11 janvier 2023, par Cheloute

  For testing purpose, I need to add programatically some samples to an existing MP4 video file but I can't understand how.
My goal is to add some CAMM1 records (exposure data) associated to each PTS multiple of 33366, that's to say : PTS=0, PTS=33366, PTS=66732, ...

  


  I already have an existing camm track atom in my file, so the idea is to duplicate all the existing CAMM samples (CAMM2 and CAMM3 data), add my CAMM1 samples to the existing CAMM data, and write all the samples again at the end of the mdat atom (moov atom is located after mdat atom in my case). Next I'll update the CAMM track atom (stts, stsc, stsz and stco) to index all the samples correctly.

  


  But I have 1 question I can't understand. STSC and STCO are dealing with offsets and STTS with time and delta, how can I do to link my CAMM1 records to PTS multiple of 33366 ?

  


  Thanks

  


• ffmpeg way to encode h265 video that allows fast-forward play without jumps

  27 octobre 2022, par SysHex

  Let me start of by saying that this issue presents itself on a Android headset, while running a Unity app with unity video player component that plays a video on it at 2 times the speed. Hence, the issue might be related to these components and not the video file itself.

  


  I have a library of video files with properties as I will describe further down this post.

  


  When I play these files at 2x the speed , the become extremelly jumpy to the point they can't be watched.

  


  I've played with configurations such as : min-keyint=30:keyint=30:scenecut=0:open-gop=0:lossless=1 but I have to be honest and state that I have almost no clue as to what they do. I've read the documentation and understand gop, I-Frames , and the need to exist reference frames for fast seek, and almost for sure Fast playing of the file (?! maybe not ?) , but don't know how to fine tune this.

  


  Maybe I'm missing out something completely and/or someone as any advice as to what I could try ?

  


  The goal is to play these files at 2x its speed, or possibly faster.

  


  Now for the details on the mpeg4 files :

  


  Video
ID                                       : 1
Format                                   : HEVC
Format/Info                              : High Efficiency Video Coding
Format profile                           : Main@L5@Main
Codec ID                                 : hev1
Codec ID/Info                            : High Efficiency Video Coding
Duration                                 : 15 min 0 s
Bit rate                                 : 6 578 kb/s
Width                                    : 3 840 pixels
Height                                   : 2 160 pixels
Display aspect ratio                     : 16:9
Frame rate mode                          : Constant
Frame rate                               : 30.000 FPS
Color space                              : YUV
Chroma subsampling                       : 4:2:0 (Type 0)
Bit depth                                : 8 bits
Scan type                                : Progressive
Bits/(Pixel*Frame)                       : 0.026
Stream size                              : 706 MiB (98%)
Writing library                          : x265 3.5+37-07b011400:[Windows][GCC 11.2.0][64 bit] 8bit+10bit+12bit
Encoding settings                        : cpuid=1111039 / frame-threads=3 / numa-pools=8 / wpp / no-pmode / no-pme / no-psnr / no-ssim / log-level=2 / input-csp=1 / input-res=3840x2160 / interlace=0 / total-frames=0 / level-idc=0 / high-tier=1 / uhd-bd=0 / ref=4 / no-allow-non-conformance / no-repeat-headers / annexb / no-aud / no-eob / no-eos / no-hrd / info / hash=0 / no-temporal-layers / open-gop / min-keyint=25 / keyint=250 / gop-lookahead=0 / bframes=4 / b-adapt=2 / b-pyramid / bframe-bias=0 / rc-lookahead=25 / lookahead-slices=4 / scenecut=40 / no-hist-scenecut / radl=0 / no-splice / no-intra-refresh / ctu=64 / min-cu-size=8 / rect / no-amp / max-tu-size=32 / tu-inter-depth=1 / tu-intra-depth=1 / limit-tu=0 / rdoq-level=2 / dynamic-rd=0.00 / no-ssim-rd / signhide / no-tskip / nr-intra=0 / nr-inter=0 / no-constrained-intra / strong-intra-smoothing / max-merge=3 / limit-refs=3 / limit-modes / me=3 / subme=3 / merange=57 / temporal-mvp / no-frame-dup / no-hme / weightp / no-weightb / no-analyze-src-pics / deblock=0:0 / sao / no-sao-non-deblock / rd=4 / selective-sao=4 / no-early-skip / rskip / no-fast-intra / no-tskip-fast / no-cu-lossless / no-b-intra / no-splitrd-skip / rdpenalty=0 / psy-rd=2.00 / psy-rdoq=1.00 / no-rd-refine / no-lossless / cbqpoffs=0 / crqpoffs=0 / rc=crf / crf=20.0 / qcomp=0.60 / qpstep=4 / stats-write=0 / stats-read=0 / ipratio=1.40 / pbratio=1.30 / aq-mode=2 / aq-strength=1.00 / cutree / zone-count=0 / no-strict-cbr / qg-size=32 / no-rc-grain / qpmax=69 / qpmin=0 / no-const-vbv / sar=0 / overscan=0 / videoformat=5 / range=0 / colorprim=1 / transfer=1 / colormatrix=1 / chromaloc=1 / chromaloc-top=0 / chromaloc-bottom=0 / display-window=0 / cll=0,0 / min-luma=0 / max-luma=255 / log2-max-poc-lsb=8 / vui-timing-info / vui-hrd-info / slices=1 / no-opt-qp-pps / no-opt-ref-list-length-pps / no-multi-pass-opt-rps / scenecut-bias=0.05 / hist-threshold=0.03 / no-opt-cu-delta-qp / no-aq-motion / no-hdr10 / no-hdr10-opt / no-dhdr10-opt / no-idr-recovery-sei / analysis-reuse-level=0 / analysis-save-reuse-level=0 / analysis-load-reuse-level=0 / scale-factor=0 / refine-intra=0 / refine-inter=0 / refine-mv=1 / refine-ctu-distortion=0 / no-limit-sao / ctu-info=0 / no-lowpass-dct / refine-analysis-type=0 / copy-pic=1 / max-ausize-factor=1.0 / no-dynamic-refine / no-single-sei / no-hevc-aq / no-svt / no-field / qp-adaptation-range=1.00 / scenecut-aware-qp=0conformance-window-offsets / right=0 / bottom=0 / decoder-max-rate=0 / no-vbv-live-multi-pass
Language                                 : English
Color range                              : Limited
Color primaries                          : BT.709
Transfer characteristics                 : BT.709
Matrix coefficients                      : BT.709
Codec configuration box                  : hvcC

Audio
ID                                       : 2
Format                                   : AAC LC
Format/Info                              : Advanced Audio Codec Low Complexity
Codec ID                                 : mp4a-40-2
Duration                                 : 15 min 0 s
Source duration                          : 15 min 0 s
Source_Duration_LastFrame                : -12 ms
Bit rate mode                            : Constant
Bit rate                                 : 128 kb/s
Channel(s)                               : 2 channels
Channel layout                           : L R
Sampling rate                            : 48.0 kHz
Frame rate                               : 46.875 FPS (1024 SPF)
Compression mode                         : Lossy
Stream size                              : 13.8 MiB (2%)
Source stream size                       : 13.8 MiB (2%)
Default                                  : Yes
Alternate group                          : 1
mdhd_Duration                            : 900233