Quick Hacks: A Script to Extract a Single Image/Frame From Video

Long ago, I posted the simple way to get a frame of a video using ffmpeg. I’ve been using that technique for a long time.

It can be a bit unwieldy for iteratively finding a specific frame, as when using a terminal you have to move the cursor to the time specification. So I wrote a very small wrapper script to put the time part at or towards the end:

# f.sh - single frame

USAGE="f.sh infile timecode [outfile]"

if [ "$#" == "0" ]; then
        echo "$USAGE"
        exit 1

if [ -e "$1" ]; then
        echo "file not found: $1"
        exit 1

if [ ! -z "$2" ]; then
        echo "Need timecode!"
        exit 1

# if we have a filename write to that, else imagemagick display

if [ ! -z "$3" ]; then
        echo "ffmpeg -i \"$video\" -ss $time  -vframes 1 -f image2 \"$3\""
        ffmpeg -loglevel quiet -hide_banner -ss $time -i "$video" -vframes 1 -f image2 "$3"
        echo "ffmpeg -i \"$video\" -ss $3  -vframes 1 -f image2 - | display"
        ffmpeg -hide_banner -loglevel quiet -ss $time  -i "$video" -vframes 1 -f image2 - | display

Most of that is usage explanation, but broadly it has two modes:

  • display an image (f.sh video time)
  • write an image (f.sh video time image)

It’s more convenient to use it, hit ? and amend the time than to move the cursor into the depth of an ffmpeg command.

Rescuing a Truncated Recording Using ffmpeg

tl;dr: Well, it’s cheating because it’s not ‘rescuing’ what isn’t recorded, just retrieving and appending

Last night I played a few rounds of some cooperative gameplay with some good friends, and as usual I recorded the session for posterity.

Hans Volter's on fire and about to die
“Oh man, oh god!”, Hans is about to croak.

However, when looking over the file created, neither VLC nor ffmpeg could determine a duration. I used ffmpeg to create a new file (ffmpeg -i old.mkv -c copy new.mkv), and when it got to about the 1 hour 50 minute mark in the stream copy, it printed the following error:

invalid return value 0 for stream protocol
[matroska,webm @ 0x55eb60b609c0] Read error
Invalid return value 0 for stream protocol

It’s possible that I ran out of space during recording, so it behoves me to make sure my disks are more clear — hopefully the process of migrating to a combined fileserver will help there! — before I start recording.

Fortunately, I also stream to Twitch and so I could create a ‘highlight’ of the latter part of gameplay- about 45 minutes’ worth. The quality isn’t as good (max bitrate 3500kbit), but it’s better than nothing. So that it can be seamlessly appended, I decided to upscale the size from 720p to 1080p using ffmpeg:

$ ffmpeg -i ~/downloads/211006613-69819912-48eb80ad-009f-42d5-83aa-f68dda7b7612.mp4 -vf scale=1920x1080:flags=lanczos
-c:a copy -crf 20 -preset slow ~/mounts/storage/video/2017-12-19\ 22-04-59b.mkv

frame=105361 fps= 24 q=28.0 size= 2402802kB time=00:29:16.20 bitrate=11208.1kbits/s speed=0.408x

(Conversion in progress! Using -vf scale=1920x1080:flags=lanczos to scale and -crf 20 to ensure quality stays reasonable)

After that, it should be a simple matter of using the concat demuxer, assuming the files are similar enough (both are pixel format yuvj420p)– I previously had issues when trying to create an auto-highlighter/clip creator that minimised re-encoding.

For posterity!

An Old Video (FEAR – Stolen Kills)

I’ve been posting a fair number of videos to another blog for the folk I play games with (including the other MIA author of posts on this blog Kenny). It’s a good way to relive some moments and also a way for me to practice my cutting and splicing skills.

Long story short, I encoded an updated version of a video I created about three and a half years ago. The old one was small resolution and tiny bitrate. This version is full (source) resolution and a relatively insane bitrate.

FEAR – Stolen Kills (pop-out player if you have JS enabled, non pop-out version here)

Synopsis: I shoot Kenny in the back (with friendly fire on) and I’m rewarded with two kills. Obviously a sounds strategy!
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