Differences

This shows you the differences between two versions of the page.

--- computers:ffmpeg [2023/02/27 18:17] – jon
+++ computers:ffmpeg [2025/12/15 16:40] (current) – jon
@@ Line 36: / Line 36: @@
 ==== Scripting ====
+=== Bash Scripting ===
+When building out bash scripting, you will need to run your ffmpeg commands with -nostdin if you are piping in variables with paths((https://stackoverflow.com/questions/21634088/execute-ffmpeg-command-in-a-loop)). ffmpeg uses stdin for its commands, which will break your 'while read' commands.
+<note tip>
+-stdin
+Enable interaction on standard input. On by default unless standard input is used as an input. To explicitly disable interaction you need to specify -nostdin.
+Disabling interaction on standard input is useful, for example, if ffmpeg is in the background process group. Roughly the same result can be achieved with ffmpeg ... < /dev/null but it requires a shell.</note>
 === Batch Filing ===
@@ Line 64: / Line 73: @@
 === Powershell Coding ===
+==== Frequent Commands ====
+https://www.videoproc.com/resource/ffmpeg-commands.htm
+==== Listing out features ====
+https://dev.to/drsensor/list-of-helpful-ffmpeg-command-for-checking-capabilities-1110
 ==== Watermarking ====
@@ Line 81: / Line 98: @@
 Example using Powershell:
 <code>ffmpeg -ss 00:53:00 -i 'input' -avoid_negative_ts 1 -to 07:19:17 -c copy 'output'</code>
+=== Caveats to cropping ===
+When you do cut with -c copy (no re-encode), and the cut point is not exactly on a keyframe, and/or the video uses B-frames (re-ordered frames), ffmpeg can’t rewrite frames to make a clean boundary. It copies packets but the decode/display timestamps (DTS/PTS) can become inconsistent around the cut boundary. Then when you concat, ffmpeg sees timestamps going “backwards” and prints: "Non-monotonic DTS... This may result in incorrect timestamps."
+Players often still play linearly, but seeking near the splice can fail hard (especially around the crop point). mp4 is particularly sensitive because it stores timing/indexing in a way that makes sloppy cuts more painful than in some other workflows.
+Workaround is to use a copy-cut and regenerate timestamps
+<code>ffmpeg -ss 0 -to 02:03:12 -i ogvideo.mp4 \
+  -map 0 -c copy \
+  -fflags +genpts -avoid_negative_ts make_zero \
+  croppedvid.mp4</code>
+Then, "remux normalize" the video to give clean metadata. This is a nice to do on all the videos that you're working on if concat-ing them togther
+<code>
+ffmpeg -i croppedvid.mp4 -map 0 -c copy -movflags +faststart fixed1.mp4
+</code>
 ==== Joining video files together (Concatenate) ====
@@ Line 86: / Line 123: @@
 https://stackoverflow.com/questions/7333232/how-to-concatenate-two-mp4-files-using-ffmpeg
 https://stackoverflow.com/questions/18474312/concatenate-join-mp4-files-using-ffmpeg-and-windows-command-line-batch-not-lin
+Make a text file in the same directory. Each video needs the word 'file' at the beginning of the line to delineate.
+<code>
+file videofile.mp4
+file videfile2.mp4
+</code>
+Here's how I've scripted this. Note, this pertains more to macOS with the sed statement since the single quotes are needed, but the tweak for linux is to just remove the single quotes and the -e
+<code>
+find . -type f > concat.txt && sed -i '' -e '/^\.\/\.DS_Store$/d' -e '/^\.\/concat\.txt$/d' -e 's/^\.\///' -e 's/^/file /' concat.txt
+</code>
+Then, reference the text file as your input.
+<code>
+ffmpeg -f concat -safe 0 -i concat.txt -c copy output.mp4
+</code>
 ==== Splitting a large video into smaller clips ====