Add compression

1 files changed, 8 insertions, 0 deletions

diff --git a/2021/prepare.md b/2021/prepare.md
index 5316536f..9b16f933 100644
--- a/2021/prepare.md
+++ b/2021/prepare.md
@@ -100,6 +100,14 @@ WebM format if possible.*
 > format.  The closer to this format submitted video files arrive in,
 > the easier it will be to process and upload them.
 
+If you would like to compress your video before uploading, the following shell script may be useful:
+
+    Q=56
+    ffmpeg -y -i "$1" -c:v libvpx-vp9 -b:v 0 -crf $Q -aq-mode 2 -an -tile-columns 0 -tile-rows 0 -frame-parallel 0 -cpu-used 8 -auto-alt-ref 1 -lag-in-frames 25 -g 240 -pass 1 -f webm -threads 8 /dev/null &&
+    ffmpeg -y -i "$1" -c:v libvpx-vp9 -b:v 0 -crf $Q -c:a copy -tile-columns 2 -tile-rows 2 -frame-parallel 0 -cpu-used -5 -auto-alt-ref 1 -lag-in-frames 25 -pass 2 -g 240 -threads 8 "$2"
+
+If you put it in a file called `compress-video.sh`, you can execute it from the command line with something like `sh compress-video.sh input-file.webm output-file.webm`.
+
 If you're planning to do live Q&A, you can check your system setup at
 https://test.bigbluebutton.org/ .  We'll set up a tech-check call with
 you, too.  We'll post details here closer to that time.