It uses handbrake.
Best way is to experiment manually with a handful of videos and watch out for compression artefacts (the color-bleed-out blocks on faces and other smooth color transitions). When you are satisfied, create a profile with the settings, name it e.g. MyProfile. You can use these settings by invoking the cli version of handbrake inside the batch script with --preset="MyProfile"
Where or how do you store the originals?
Don't throw away information lightheartedly. This is what you do when compressing.
I copy the original folders of my canon camera manually to my external drive, additionally to importing stuff to iphoto.
So iphotos videos are rather.... dependable.
I will modify the script to directly access the originals inside the photo library. This a really bad thing to do, normally.
If you want try to contact me 1'2016. appstackex-imov4m.hofma at the service 0ox with domain .com ;)
If you try this, too, please backup before tampering with the package!
Read http://osxdaily.com/2015/04/22/show-original-file-from-photos-app-mac-finder-osx/ to see the part/video where they open the package in finder.
In finder, you can locate your library, for me and just e.g.it is:
(photoslibrary is the new one since migration of the iphoto database to photos database. Just look in finder, right-click "open library content" or whatever the menue entry reads in english, might be "packet content" (it is "Paketinhalt anzeigen" for me) then open the subfolder "masters", and drag the masters-folder to terminal, then you have text on the command line representing the correct location...
When you do a change directory to the path of masters, you can thereafter find all originals, e.g. by:
find . -type f \( -iname "*.mov" -o -name "*.mp4" -o -name "*.jpg" \)
This is nice for me, because I have a bunch of empty folders in this directory structure created by iphoto / photos. Find does show only results with a nice relative path.