Finding backgrounds is no problem: I have plenty of images stored in ~/Backgrounds -- mostly photos I've taken over the years, with a smattering of downloads from sites like the APOD. So all I needed was a way to select one file at random from the directory.
This is Unix, so there's definitely a commandline way to do it, right? Well, surprisingly, I couldn't find an easy way that didn't involve any scripting. Some shells have a random number generator built in ($RANDOM in bash) but you still have to do some math on the result.
Of course, I could have googled, since I'm sure other people have written random-wallpaper scripts ... but what's the fun in that? If it has to be a script, I might as well write my own.
Rather than write a random wallpaper script, I wanted something that could be more generally useful: pick one random line from standard input and print it. Then I could pass it the output of ls -1 $HOME/Backgrounds, and at the same time I'd have a script that I could also use for other purposes, such as choosing a random quotation, or choosing a "flash card" question when studying for an exam.
The obvious approach is to read all of standard input into an array, count the lines, then pick a random number between one and $num_lines and print that array element. It took no time to whip that up in Python and it worked fine. But it's not very efficient -- what if you're choosing a line from a 10Mb file?
Then Sara Falamaki (thanks, Sara!) pointed me to a page with a neat Perl algorithm. It's Perl so it's not easy to read, but the algorithm is cute. You read through the input line by line, keeping track of the line number. For each line, the chance that this line should be the one printed at the end is the reciprocal of the line number: in other words, there's one chance out of $line_number that this line is the one to print.
So if there's only one line, of course you print that line; when you get to the second line, there's one chance out of two that you should switch; on the third, one chance out of three, and so on.
A neat idea, and it doesn't require storing the whole file in memory. In retrospect, I should have thought of it myself: this is basically the same algorithm I used for averaging images in GIMP for my silly Chix Stack Mars project, and I later described the method in the image stacking section of my GIMP book. To average images by stacking them, you give the bottom layer 100% opacity, the second layer 50% opacity, the third 33% opacity, and so on up the stack. Each layer makes an equal contribution to the final result, so what you see is the average of all layers.
The randomline script, which you can inspect here, worked fine, so I hooked it up to accomplish the original problem: setting a randomly chosen desktop background each day. Since I use a lightweight window manager (fvwm) rather than gnome or kde, and I start X manually rather than using gdm, I put this in my .xinitrc:
(xsetbg -fullscreen -border black `find $HOME/Backgrounds -name "*.*" | randomline`) &
Update: I've switched to using hsetroot, which is a little more robust than xsetbg. My new command is:
hsetroot -center `find -L $HOME/Backgrounds -name "*.*" | randomline`
So, an overlong article about a relatively trivial but nontheless nifty algorithm. And now I have a new desktop background each day. Today it's something prosaic: mud cracks from Death Valley. Who knows what I'll see tomorrow?
Update, years later:
I've written a script for the whole job,
because on my laptop I want to choose from a different set of
backgrounds depending on whether I'm plugged in to an external monitor
or using the lower resolution laptop display.
But meanwhile, I've just been pointed to the shuf command, which does pretty much what my randomline script did. So you don't actually need any scripts, just
hsetroot -fill `find ~/Images/Backgrounds/1680x1050/ -name '*.jpg' | shuf -n 1`
[ 14:02 Feb 28, 2007 More programming | permalink to this entry | ]