Return to Oz – flying back to the emerald city

Rockingham Street Q-Park car park, Sheffield, UK.

After a partially voluntary and partially involuntary hiatus, I am back in flight. After significant work commitments, then getting married with a honeymoon in Barcelona, ES, I am back in Sheffield, UK, the greenest city in England. As this project is currently unfunded, and solo, I’m updating things as-and-when.

I promised you some performance tests and here they are. I’m am perfectly satisfied with the installations of Ubuntu Linus 8.04.1 on blue-and-white G3s and 10.04 on grey-and-white  Macs. As suspected, the GUI is a major burden on the system and opening a folder of about 3,000 jpeg images can take a full 5 minutes because of the overhad loading all the thumbnails.

The solution is simple. Don’t.

For my purposes this is no problem. I want to use these machines as workhorses and don’t need the GUI.

I’m going to be using FFmpeg to convert and compress a folder full of images into a movie. However, I came across a well-documented problem immediately. FFmpeg will not continue if a.) the first file is not numbered xxxxxx1 and will fail if there is a gap in the numbering.

Fortunately, I easily found a Bash script to sort this out, and I found it here:

http://stackoverflow.com/questions/2829113/ffmpeg-create-a-video-from-images

x=1; for i in *jpg; do counter=$(printf %03d $x); ln “$i” /tmp/img”$counter”.jpg; x=$(($x+1)); done

(Also see the revision below).

This works but note the format of the filename extensions is case-sensitive. Helpfully (not), the script just fails if you do not get this correct. It renames all the images in a folder beginning at xxxxxx1, so if you sequence does not, or it is missing one or more images in the middle, it will not cause FFmpeg to fail. I’ll deconstruct this script in a later blog post.

Even more usefully, Linux does not actually duplicate the binary files, it just creates link files which can be safely deleted afterwards.

These tests were done on a 1 GHz PowerMac G4 with 640 Mb RAM & 21 Gb HD.

Rename 2933 files – 23 seconds

Using FFmpeg:

Encode jpeg files at 1600×1200 to mjpeg – approx 9 min 30 secs – 192.4 MB
Encode jpeg files at 1600×1200 to mjpeg – approx 8 min 11 secs – 192.4 MB
Encode jpeg files at 1600×1200 to mjpeg – approx 8 min 12 secs – 192.4 MB

Encode jpeg files at 640×480 to mjpeg AVI – approx 10 min 15 secs – 40.4 MB – atrocious quality
Encode jpeg files at 640×480 to mjpeg AVI – forgot to time it but similar – 199.5 MB – quality set to 1

This last set of tests was done whilst messing about within the GUI. I don’t really know enough about what is going on under the hood, but it seems that things slow down a lot and this might be due to caching thumbnails or, err, whatever. After a restart performance increased dramatically.

Rename 2,933 items – 1’05’’
Rename 2,933 items – 1’12’’
Rename 2,933 items – 0’20’’ (after restart)

For my purposes the performance seems acceptable at this stage.

One other revision I had to make was to compensate for the original file numbering restarting. Canon Powershot cameras create jpeg images with filenames of the format “IMG_xxxx.JPG” and shooting timelapse with them quickly reaches numbers in excess of 9,999. For consumer-level snap cameras, this number is probably well in excess of a normal user’s needs but I can easily shoot that many in one day. Consequently, the image numbers cannot solely be relied upon to reference the images in the correct chronological order, hence this revision which sorts them into creation order before renaming.

x=1; for i in $(ls -t -r *JPG); do counter=$(printf %04d $x); ln “$i” /Users/richardbolam/Desktop/testout/img_”$counter”.jpg; x=$(($x+1)); done

I’m still a bit of a noob when it comes to Bash scripting, but I will write another post soon that disassembles these scripts, partly for your benefit, but also to help me learn more about what I’m doing. This script was revised with the invaluable help of the technical staff at Access Space. Stay tuned.

access-space.org

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