GIMP was not really an option for my use-case since I wanted to convert a bunch of files in the command line. What worked was using the latest version of ImageMagick (7.1.0). I used the portable, static build which was running like a charm on my Ubuntu install
I know this problem has already been solved, but I thought I’d mention this.
You may not know it, but GIMP has a batch mode and a couple of scripting interfaces. It is possible to write scripts in GIMP that can batch convert files, or apply effects/filters/transformations to batches of files.
The following link documents a way to batch convert using Gimps default script-fu scripting interface - which is scheme based: (basically a dialect of lisp)
Introduction¶ GIMP comes with a so-called batch mode that allows you to do image processing from the command line. It also makes it easy to apply the same set of operations to a number of images. We have got a lot of questions on the mailing-lists on how to use …
www.gimp.org
And there is the much more user friendly Python based Python-fu scripting interface:
Using one of Gimps scripting interfaces, you could create a script that will batch convert files in gimp - however, this route will probably only be useful if you already have some prior programming experience.
But if you do use Gimp a lot, then learning a bit about at least one of it’s scripting interfaces is time well spent.
Gimp doesn’t have any kind of macro recording/playback functionality, but for any repetitive actions you perform on files, or even particular layers in files - you can use the scripting interfaces to write your own scripts/plugins/extensions.
Again, it may take some time to learn how to use gimps scripting features, especially if you have little, or no programming experience. But if you spend a lot of time using Gimp, then adding your own extensions/plugins can be worthwhile in the long run.
I haven’t used Gimp for much recently, but I’ve used both of the scripting interfaces in the past. Some years back I helped write some scripts/plugins that one of my artist friends needed, in order to speed up his workflow for some of his projects. Including a few scripts that did batch conversion, or applied a series of effects/transformations to images, or even named layers in batches of files.
Unfortunately, I don’t know where any of those scripts are. They might be on one of my many backup drives, or they might even be on a data CD/DVD in the loft!! :/