user26624: I'm not an expert at ffmpeg, or video tech by any means. I have very little experience with it.
I have an IP camera that produces an h264 stream. I'm trying to use ffmpeg, and nginx (with the rtmp module) to restream the live camera feed to multiple clients. I am NOT trying to re-encode anything. The less CPU-intensive that my ffmpeg command is, the better.
If I use an older version of ffmpeg (version 4.1.4), it supports 2 ways of copying the video and audio feeds from the source stream without re-encoding them: -c:av copy and -vcodec copy -acodec copy. From what I understand, these are supposed to act exactly the same, but that doesn't appear to be the case.
-c:av copy
output:
# ffmpeg -err_detect ignore_err -rtsp_transport tcp -re -i rtsp://USERNAME:PASSWORD@IP/Streaming/Channels/701/httpPreview -c:av copy -f flv rtmp://127.0.0.1/stream/stream1
ffmpeg version 4.1.4-static https://johnvansickle.com/ffmpeg/  Copyright (c) 2000-2019 the FFmpeg developers
  built with gcc 6.3.0 (Debian 6.3.0-18+deb9u1) 20170516
  configuration: --enable-gpl --enable-version3 --enable-static --disable-debug --disable-ffplay --disable-indev=sndio --disable-outdev=sndio --cc=gcc-6 --enable-fontconfig --enable-frei0r --enable-gnutls --enable-gmp --enable-libgme --enable-gray --enable-libaom --enable-libfribidi --enable-libass --enable-libvmaf --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librubberband --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libvorbis --enable-libopus --enable-libtheora --enable-libvidstab --enable-libvo-amrwbenc --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzvbi --enable-libzimg
  libavutil      56. 22.100 / 56. 22.100
  libavcodec     58. 35.100 / 58. 35.100
  libavformat    58. 20.100 / 58. 20.100
  libavdevice    58.  5.100 / 58.  5.100
  libavfilter     7. 40.101 /  7. 40.101
  libswscale      5.  3.100 /  5.  3.100
  libswresample   3.  3.100 /  3.  3.100
  libpostproc    55.  3.100 / 55.  3.100
Input #0, rtsp, from 'rtsp://USERNAME:PASSWORD@IP/Streaming/Channels/701/httpPreview':
  Metadata:
    title           : Media Server V3.4.92
    comment         : Media Server Session Description : standard
  Duration: N/A, start: 0.500000, bitrate: N/A
    Stream #0:0: Video: h264 (Main), yuvj420p(pc, bt709, progressive), 2688x1520 [SAR 1:1 DAR 168:95], 20 fps, 20 tbr, 90k tbn, 40 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> flv1 (flv))
Press [q] to stop, [?] for help
[swscaler @ 0x5f72cc0] deprecated pixel format used, make sure you did set range correctly
Output #0, flv, to 'rtmp://127.0.0.1/stream/stream1':
  Metadata:
    title           : Media Server V3.4.92
    comment         : Media Server Session Description : standard
    encoder         : Lavf58.20.100
    Stream #0:0: Video: flv1 (flv) ([2][0][0][0] / 0x0002), yuv420p, 2688x1520 [SAR 1:1 DAR 168:95], q=2-31, 200 kb/s, 20 fps, 1k tbn, 20 tbc
    Metadata:
      encoder         : Lavc58.35.100 flv
    Side data:
      cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay: -1
[flv @ 0x5f55780] Failed to update header with correct duration.ate=2762.3kbits/s dup=0 drop=5 speed=0.956x    
[flv @ 0x5f55780] Failed to update header with correct filesize.
frame=   85 fps= 19 q=24.8 Lsize=    1495kB time=00:00:04.20 bitrate=2916.0kbits/s dup=0 drop=5 speed=0.943x    
video:1494kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.108328%

resulting video:

-vcodec copy -acodec copy
output:
# ffmpeg -err_detect ignore_err -rtsp_transport tcp -re -i rtsp://USERNAME:PASSWORD@IP/Streaming/Channels/701/httpPreview -acodec copy -vcodec copy -f flv rtmp://127.0.0.1/stream/stream1
ffmpeg version 4.1.4-static https://johnvansickle.com/ffmpeg/  Copyright (c) 2000-2019 the FFmpeg developers
  built with gcc 6.3.0 (Debian 6.3.0-18+deb9u1) 20170516
  configuration: --enable-gpl --enable-version3 --enable-static --disable-debug --disable-ffplay --disable-indev=sndio --disable-outdev=sndio --cc=gcc-6 --enable-fontconfig --enable-frei0r --enable-gnutls --enable-gmp --enable-libgme --enable-gray --enable-libaom --enable-libfribidi --enable-libass --enable-libvmaf --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librubberband --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libvorbis --enable-libopus --enable-libtheora --enable-libvidstab --enable-libvo-amrwbenc --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzvbi --enable-libzimg
  libavutil      56. 22.100 / 56. 22.100
  libavcodec     58. 35.100 / 58. 35.100
  libavformat    58. 20.100 / 58. 20.100
  libavdevice    58.  5.100 / 58.  5.100
  libavfilter     7. 40.101 /  7. 40.101
  libswscale      5.  3.100 /  5.  3.100
  libswresample   3.  3.100 /  3.  3.100
  libpostproc    55.  3.100 / 55.  3.100
Input #0, rtsp, from 'rtsp://USERNAME:PASSWORD@IP/Streaming/Channels/701/httpPreview':
  Metadata:
    title           : Media Server V3.4.92
    comment         : Media Server Session Description : standard
  Duration: N/A, start: 0.350000, bitrate: N/A
    Stream #0:0: Video: h264 (Main), yuvj420p(pc, bt709, progressive), 2688x1520 [SAR 1:1 DAR 168:95], 20 fps, 20 tbr, 90k tbn, 40 tbc
Output #0, flv, to 'rtmp://127.0.0.1/stream/stream1':
  Metadata:
    title           : Media Server V3.4.92
    comment         : Media Server Session Description : standard
    encoder         : Lavf58.20.100
    Stream #0:0: Video: h264 (Main) ([7][0][0][0] / 0x0007), yuvj420p(pc, bt709, progressive), 2688x1520 [SAR 1:1 DAR 168:95], q=2-31, 20 fps, 20 tbr, 1k tbn, 90k tbc
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
Press [q] to stop, [?] for help
[flv @ 0x6670a40] Failed to update header with correct duration.ate=3111.6kbits/s speed=1.03x    
[flv @ 0x6670a40] Failed to update header with correct filesize.
frame=   38 fps= 25 q=-1.0 Lsize=     589kB time=00:00:01.55 bitrate=3111.7kbits/s speed=1.01x    
video:588kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.192649%

resulting video:

As you can clearly see, the output of -vcodec copy -acodec copy is clearly far less pixelated. While that sounds like a good thing, it seems like FFMPEG does more work to process the image, which may explain why when nginx re-streams the feed, and I try to connect to it with VLC, the -c:av copy feed (worse quality) works almost every time, whereas the -vcodec copy -acodec copy feed sometimes has VLC throw an error that it can't connect, and other times connects, but just shows a video that's all black.
The problem with sticking with -c:av copy is that when I use a more-recent version of ffmpeg (like the 4.2.1 static build from here: https://johnvansickle.com/ffmpeg/), they don't even have the -c:av copy arg anymore, so my only option is to use -vcodec copy -acodec copy, and I experience the same problems with VLC. This means right now, I can only use the old FFMPEG version.
Am I doing something wrong? I want ffmpeg to do as little processing as possible - I just want it to copy the video stream to an flv container as-is, and publish it to nginx. I don't care if this results in a bit of pixelation - the quality isn't a massive deal for me.

user1871: -c:av copy is not valid syntax and re-encodes the video. Only the first specifier a is honoured.
You can see this in the stream mapping
For -c:av copy:
Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> flv1 (flv))

For -vcodec copy -acodec copy:
Stream mapping:
  Stream #0:0 -> #0:0 (copy)

Before version 4.2, syntax matching for this wasn't strict so ffmpeg did not abort the command, or warn.

As to your main issue, the camera is producing a 2688x1520 resolution H.264 stream with an unknown (to me) keyframe interval. The former part means the decoder has to support at least AVC Level 5 - your target player may not. If the keyframe interval is large enough then the player will fail to decode the stream till it receives a KF.
To keep CPU low, your best bet is to tweak the camera output - a) a low keyframe interval (< 4 seconds), b) smaller frame size (smaller the better, but at most 1920x1080).
If those can't be done, you'll have to re-encode. Your -c:av copy command is using the old flv encoder, which is an older generation codec. I suggest using x264.
-vf scale=1920:1080:force_original_aspect_ratio=decrease -c:v libx264 -profile:v main -preset superfast
I inserted a scaler to keep the output frame size to under 1920x1080.

26624: <p>I'm not an expert at ffmpeg, or video tech by any means. I have very little experience with it.</p>

<p>I have an IP camera that produces an h264 stream. I'm trying to use ffmpeg, and nginx (with the rtmp module) to restream the live camera feed to multiple clients. I am NOT trying to re-encode anything. The less CPU-intensive that my ffmpeg command is, the better.</p>

<p>If I use an older version of ffmpeg (version 4.1.4), it supports 2 ways of copying the video and audio feeds from the source stream without re-encoding them: <code>-c:av copy</code> and <code>-vcodec copy -acodec copy</code>. From what I understand, these are supposed to act exactly the same, but that doesn't appear to be the case.</p>

<p><strong>-c:av copy</strong></p>

<p>output:</p>

<pre><code># ffmpeg -err_detect ignore_err -rtsp_transport tcp -re -i rtsp://USERNAME:PASSWORD@IP/Streaming/Channels/701/httpPreview -c:av copy -f flv rtmp://127.0.0.1/stream/stream1
ffmpeg version 4.1.4-static https://johnvansickle.com/ffmpeg/  Copyright (c) 2000-2019 the FFmpeg developers
  built with gcc 6.3.0 (Debian 6.3.0-18+deb9u1) 20170516
  configuration: --enable-gpl --enable-version3 --enable-static --disable-debug --disable-ffplay --disable-indev=sndio --disable-outdev=sndio --cc=gcc-6 --enable-fontconfig --enable-frei0r --enable-gnutls --enable-gmp --enable-libgme --enable-gray --enable-libaom --enable-libfribidi --enable-libass --enable-libvmaf --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librubberband --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libvorbis --enable-libopus --enable-libtheora --enable-libvidstab --enable-libvo-amrwbenc --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzvbi --enable-libzimg
  libavutil      56. 22.100 / 56. 22.100
  libavcodec     58. 35.100 / 58. 35.100
  libavformat    58. 20.100 / 58. 20.100
  libavdevice    58.  5.100 / 58.  5.100
  libavfilter     7. 40.101 /  7. 40.101
  libswscale      5.  3.100 /  5.  3.100
  libswresample   3.  3.100 /  3.  3.100
  libpostproc    55.  3.100 / 55.  3.100
Input #0, rtsp, from 'rtsp://USERNAME:PASSWORD@IP/Streaming/Channels/701/httpPreview':
  Metadata:
    title           : Media Server V3.4.92
    comment         : Media Server Session Description : standard
  Duration: N/A, start: 0.500000, bitrate: N/A
    Stream #0:0: Video: h264 (Main), yuvj420p(pc, bt709, progressive), 2688x1520 [SAR 1:1 DAR 168:95], 20 fps, 20 tbr, 90k tbn, 40 tbc
Stream mapping:
  Stream #0:0 -&gt; #0:0 (h264 (native) -&gt; flv1 (flv))
Press [q] to stop, [?] for help
[swscaler @ 0x5f72cc0] deprecated pixel format used, make sure you did set range correctly
Output #0, flv, to 'rtmp://127.0.0.1/stream/stream1':
  Metadata:
    title           : Media Server V3.4.92
    comment         : Media Server Session Description : standard
    encoder         : Lavf58.20.100
    Stream #0:0: Video: flv1 (flv) ([2][0][0][0] / 0x0002), yuv420p, 2688x1520 [SAR 1:1 DAR 168:95], q=2-31, 200 kb/s, 20 fps, 1k tbn, 20 tbc
    Metadata:
      encoder         : Lavc58.35.100 flv
    Side data:
      cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay: -1
[flv @ 0x5f55780] Failed to update header with correct duration.ate=2762.3kbits/s dup=0 drop=5 speed=0.956x    
[flv @ 0x5f55780] Failed to update header with correct filesize.
frame=   85 fps= 19 q=24.8 Lsize=    1495kB time=00:00:04.20 bitrate=2916.0kbits/s dup=0 drop=5 speed=0.943x    
video:1494kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.108328%
</code></pre>

<p>resulting video:</p>

<p><a href="https://i.stack.imgur.com/kBLSQ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/kBLSQ.png" alt="enter image description here"></a></p>

<p><strong>-vcodec copy -acodec copy</strong></p>

<p>output:</p>

<pre><code># ffmpeg -err_detect ignore_err -rtsp_transport tcp -re -i rtsp://USERNAME:PASSWORD@IP/Streaming/Channels/701/httpPreview -acodec copy -vcodec copy -f flv rtmp://127.0.0.1/stream/stream1
ffmpeg version 4.1.4-static https://johnvansickle.com/ffmpeg/  Copyright (c) 2000-2019 the FFmpeg developers
  built with gcc 6.3.0 (Debian 6.3.0-18+deb9u1) 20170516
  configuration: --enable-gpl --enable-version3 --enable-static --disable-debug --disable-ffplay --disable-indev=sndio --disable-outdev=sndio --cc=gcc-6 --enable-fontconfig --enable-frei0r --enable-gnutls --enable-gmp --enable-libgme --enable-gray --enable-libaom --enable-libfribidi --enable-libass --enable-libvmaf --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librubberband --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libvorbis --enable-libopus --enable-libtheora --enable-libvidstab --enable-libvo-amrwbenc --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzvbi --enable-libzimg
  libavutil      56. 22.100 / 56. 22.100
  libavcodec     58. 35.100 / 58. 35.100
  libavformat    58. 20.100 / 58. 20.100
  libavdevice    58.  5.100 / 58.  5.100
  libavfilter     7. 40.101 /  7. 40.101
  libswscale      5.  3.100 /  5.  3.100
  libswresample   3.  3.100 /  3.  3.100
  libpostproc    55.  3.100 / 55.  3.100
Input #0, rtsp, from 'rtsp://USERNAME:PASSWORD@IP/Streaming/Channels/701/httpPreview':
  Metadata:
    title           : Media Server V3.4.92
    comment         : Media Server Session Description : standard
  Duration: N/A, start: 0.350000, bitrate: N/A
    Stream #0:0: Video: h264 (Main), yuvj420p(pc, bt709, progressive), 2688x1520 [SAR 1:1 DAR 168:95], 20 fps, 20 tbr, 90k tbn, 40 tbc
Output #0, flv, to 'rtmp://127.0.0.1/stream/stream1':
  Metadata:
    title           : Media Server V3.4.92
    comment         : Media Server Session Description : standard
    encoder         : Lavf58.20.100
    Stream #0:0: Video: h264 (Main) ([7][0][0][0] / 0x0007), yuvj420p(pc, bt709, progressive), 2688x1520 [SAR 1:1 DAR 168:95], q=2-31, 20 fps, 20 tbr, 1k tbn, 90k tbc
Stream mapping:
  Stream #0:0 -&gt; #0:0 (copy)
Press [q] to stop, [?] for help
[flv @ 0x6670a40] Failed to update header with correct duration.ate=3111.6kbits/s speed=1.03x    
[flv @ 0x6670a40] Failed to update header with correct filesize.
frame=   38 fps= 25 q=-1.0 Lsize=     589kB time=00:00:01.55 bitrate=3111.7kbits/s speed=1.01x    
video:588kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.192649%
</code></pre>

<p>resulting video:</p>

<p><a href="https://i.stack.imgur.com/THFxZ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/THFxZ.png" alt="enter image description here"></a></p>

<p>As you can clearly see, the output of <code>-vcodec copy -acodec copy</code> is clearly far less pixelated. While that sounds like a good thing, it seems like FFMPEG does more work to process the image, which may explain why when nginx re-streams the feed, and I try to connect to it with VLC, the <code>-c:av copy</code> feed (worse quality) works almost every time, whereas the <code>-vcodec copy -acodec copy</code> feed sometimes has VLC throw an error that it can't connect, and other times connects, but just shows a video that's all black.</p>

<p>The problem with sticking with <code>-c:av copy</code> is that when I use a more-recent version of ffmpeg (like the 4.2.1 static build from here: <a href="https://johnvansickle.com/ffmpeg/" rel="nofollow noreferrer">https://johnvansickle.com/ffmpeg/</a>), they don't even have the <code>-c:av copy</code> arg anymore, so my only option is to use <code>-vcodec copy -acodec copy</code>, and I experience the same problems with VLC. This means right now, I can only use the old FFMPEG version.</p>

<p>Am I doing something wrong? I want ffmpeg to do as little processing as possible - I just want it to copy the video stream to an flv container as-is, and publish it to nginx. I don't care if this results in a bit of pixelation - the quality isn't a massive deal for me.</p>
 1871: <p><code>-c:av copy</code> is not valid syntax and re-encodes the video. Only the first specifier <code>a</code> is honoured.</p>

<p>You can see this in the stream mapping</p>

<p>For <code>-c:av copy</code>:</p>

<pre><code>Stream mapping:
  Stream #0:0 -&gt; #0:0 (h264 (native) -&gt; flv1 (flv))
</code></pre>

<p>For <code>-vcodec copy -acodec copy</code>:</p>

<pre><code>Stream mapping:
  Stream #0:0 -&gt; #0:0 (copy)
</code></pre>

<p>Before version 4.2, syntax matching for this wasn't strict so ffmpeg did not abort the command, or warn.</p>

<hr>

<p>As to your main issue, the camera is producing a <code>2688x1520</code> resolution H.264 stream with an unknown (to me) keyframe interval. The former part means the decoder has to support at least AVC Level 5 - your target player may not. If the keyframe interval is large enough then the player will fail to decode the stream till it receives a KF.</p>

<p>To keep CPU low, your best bet is to tweak the camera output - a) a low keyframe interval (&lt; 4 seconds), b) smaller frame size (smaller the better, but at most 1920x1080).</p>

<p>If those can't be done, you'll have to re-encode. Your <code>-c:av copy</code> command is using the old flv encoder, which is an older generation codec. I suggest using x264.</p>

<p><code>-vf scale=1920:1080:force_original_aspect_ratio=decrease -c:v libx264 -profile:v main -preset superfast</code></p>

<p>I inserted a scaler to keep the output frame size to under 1920x1080.</p>
