And thing about arbitrary art is that they never fit any kind of monitor resolution - some images are tall, others are wide, all have to be scaled, etc - and that processing has to be done somewhere.
Most WMs/DEs seem to be cropping largest aspect-correct rectangle from the center of the image and scaling that, which doesn't work well for tall images on wide displays and generally can be improved upon.
Used my aura project for that across ~10 years, which did a lot of custom processing using GIMP plugin, as it was the only common image-processing thing supporting seam carving (or liquid rescale / lqr) algo at the time (around 2011) for neat content-aware image resizing.
It always worked fairly slowly, mostly due to GIMP startup times and various inefficiencies in the process there, and by now it is also heavily deprecated due to using Python2 (which is no longer supported in any way past April 2020), as well as GIMP's Python-Fu API, which will probably also be gone in GIMP 3.0+ (with its migration to gobject-introspection bindings).
Wanted to document how it was working somewhere for myself, which was useful for fgbg rewrite (see below), and maybe it might be useful to cherry-pick ideas from to someone else who'd randomly stumble upon this list :)
Tool was doing roughly this:
aura.sh script running as a daemon, with some wakeup interval to update backgrounds.
Most details of the process were configurable in ~/.aurarc.
xprintidle was used to check if desktop is idle - no need to change backgrounds if so.
Number of displays to run lqr_wpset.py for was checked via xrandr cli tool.
Image was picked mostly-randomly, but with bias towards "favorites" and ignoring blacklisted ones.
Both faves and blacklist was supported and updated via cli options (-f/--fave and -b/--blacklist), allowing to easily set "like" or "never use again" status for current image, with lists of these stored in ~/.aura.
Haven't found much use for these honestly - all images work great with proper processing, and there seem to be little use in limiting their variety that way.
GIMP was run in batch mode and parameters passed via env, using lqr_wpset.py plugin to either set background on specified display or print some "WPS-ERR:" string to pick some other image (on errors or some sanity-checks failing there).
Image picks and all GIMP output were stored in ~/.aura/picker.log (overridable via aurarc, same as most other params), with a special file for storing just currently-used image source path(s).
Command-line options to wake up daemon via signal or print currently-used image source were also supported and quite useful.
Actual heavy-lifting was done in lqr_wpset.py GIMP plugin, which handled image processing, some caching to speed things up when re-using same source image, as well as actual background-setting.
Uses old dbus and pygtk modules to set background in various DEs at the last step.
Solid-color edges are stripped from the image - e.g. black stripes on the top/bottom - to get actual image size and contents to use.
This is done by making a "mask" layer from image, which gets blurred and then contrast-adjusted to average-out any minor color fluctuations in these edges, and then cropped by gimp to remove them.
Resulting size/position of cropped remains of that "mask" is then used to crop relevant part out of the original image.
50% random chance to flip image horizontally for more visual variety.
Given that parts of my desktop background are occluded by conky and terminals, this is actually very useful, as it will show diff parts of same image(s) from under these.
Only works weirdly with text on images, which is unreadable when mirrored, but that's very rare and definitely not a big deal, as it's often there for signage and such, not for actual reading.
If image is way too large or small - e.g. 6x diff by area or 3x diff by width/height, abort processing, as it'll be either too expensive cpu-wise or won't get nice result anyway (for tiny images).
If image aspect is too tall compared to display's - scale it smartly to one side of the screen.
This is somewhat specific to my use-case, as my most used virtual desktop is #1 with transparent conky system-monitor on the left and terminal window on the right.
So background shows through on the left there, and tall images can easily fill that space, but "gravity" value can be configured in the script to position such image anywhere horizontally (0-100, default - 25 for "center at 25% left-to-right").
Processing in this case is a bit complicated:
Render specified bg color (if any) on display-sized canvas, e.g. just black.
Scale/position image in there using specified "gravity" value as center point, or against left/right side, if it'd go beyond these.
Pick specified number of "edge" pixels (e.g. 25px) on left/right sides of the image, which aren't bumping into canvas edge, and on a layer in-between solid-color background (first step) and scaled/positioned image, do:
- Scale this edge to fill rest of the canvas in empty direction.
- Blur result a lot, so it'd look vague and indistinct, like background noise.
- Use some non-100% opacity for it, something like 70%, to blend-in with bg color.
This would produce a kind of "blurred halo" stretching from tall image sides, and filling otherwise-empty parts of canvas very nicely.
Gradient-blend above "edge" pixels with produced stretched/blurred background.
Arrived at this process after some experimentation, I think something like that with scaling and blur is common way to make fitting bg underlay for sharp centered images in e.g. documentaries and such.
If image is at least 30% larger by area, scale it preserving aspect with the regular "cubic" algo.
This turns out to be very important pre-processing step for LQR scaling later - on huge source images, such scaling can take many minutes, e.g. when scaling 4k image to 1080p.
And also - while this tool didn't do that (fixed later in fgbg script) - it's also important to scale ALL images as close to final resolution as possible, so that seam carving algo will add as little distortion as possible.
Generally you want LQR to do as little work as possible, if other non-distorting options are available, like this aspect-scaling option.
Run seam carving / lqr algo to match image aspect to display size exactly.
Cache produced result, to restart from this step when using same source image and h-flip-chance next time.
Text added on top in the next step can vary with current date/time, so intermediate result is cached here.
This helps a lot with performance, obviously.
Add text plaque in the image corner with its filename, timestamps and/or some tag metadata.
This is mostly useful when proper image titles stored in EXIF tags, as well as creation time/place for photos.
Metadata from exiv2 (used via pyexiv2) has a ton of various keys for same things, so script does its best to include ~20 potential keys for each useful field like "author", "creation date" or "title".
Font used to be rendered in a contrasting color, picked via L*a*b* colorspace against "averaged" background color (via blur or such).
This produced too wild and still bad results on busy backgrounds, so eventually switched to a simplier and better "light text with dark outline" option.
Outline is technically rendered as a "glow" - a soft gradient shadow (solid dark color to full trasparency) expanding in all directions from font outline.
Try all known background-setting options, skipping expected errors, as most won't work with one specific DE running.
Can ideally be configured via env (from ~/.aurarc) to skip unnecessary work here, but they all are generally easy/quick to try anyway.
GNOME/Unity - gsettings set org.gnome.desktop.background picture-uri file://... command.
Older GNOME/XFCE - use "gconf" python module to set "/desktop/gnome/background/picture_filename" path.
XFCE - set via DBus call to /org/xfce/Xfconf [org.xfce.Xfconf].
Has multiple different props to set there.
Enlightenment (E17+) - DBus calls to /org/enlightenment/wm/RemoteObject [org.enlightenment.wm.service].
Can have many images there, for each virtual desktop and such.
Paint X root window via pygtk!
This works for many ancient window managers, and is still showing through in some DEs too, occasionally.
Collected and added these bg-setting steps via experiments with different WMs/DEs over the years, and it's definitely nowhere near exhaustive list.
These days there might be some more uniform way to do it, especially with wayland compositors.
At some point, mostly due to everything in this old tool being deprecated out of existance, did a full rewrite with all steps above in some form, as well as major improvements, in the form of modern fgbg script (in mk-fg/de-setup repo).
It uses ImageMagick and python3 Wand module, which also support LQR and all these relatively-complex image manipulations these days, but work few orders of magnitude faster than old "headless GIMP" for such automated processing purpose.
New script is much less complicated, as well as self-contained daemon, with only optional extra wand-py and xprintidle (see above) dependencies (when e.g. image processing is enabled via -p/--process option).
Also does few things more and better, drawing all lessions from that old aura project, which can finally be scrapped, I guess.
Actually, one missing bit there atm (2020-05-09) is various background-setting methods from different DEs, as I've only used it with Enlightement so far, where it can set multiple background images in configurable ways via DBus (using xrandr and sd-bus lib from systemd via ctypes).
Should be relatively trivial to support more DEs there by adding specific commands for these, working more-or-less same as in the old script (and maybe just copying methods from there), but these just need to be tested, as my limited knowledge of interfaces in all these DEs is probably not up to date.