A few days ago, I wrote about the snowpack we get on the roof during snowstorms:
It doesn't just sit there until it gets warm enough to melt and run off as water. Instead, the whole mass of snow moves together, gradually, down the metal roof, like a glacier.
When it gets to the edge, it still doesn't fall; it somehow stays intact, curling over and inward, until the mass is too great and it loses cohesion and a clump falls with a Clunk!
The day after I posted that, I had a chance to see what happens as the
snow sheet slides off a roof if it doesn't have a long distance
to fall. It folds gracefully and gradually, like a sheet.
But does it really move like a glacier? I decided to set up a camera and film it on the move. I set the Rebel on a tripod with an AC power adaptor, pointed it out the window at a section of roof with a good snow load, plugged in the intervalometer I bought last summer, located the manual to re-learn how to program it, and set it for a 30-second interval. I ran that way for a bit over an hour -- long enough that one section of ice had detached and fallen and a new section was starting to slide down. Then I moved to another window and shot a series of the same section of snow from underneath, with a 40-second interval.
I uploaded the photos to my workstation and verified that they'd captured what I wanted. But when I stitched them into a movie, the way I'd used for my time-lapse clouds last summer, it went way too fast -- the movie was over in just a few seconds and you couldn't see what it was doing. Evidently a 30-second interval is far too slow for the motion of a roof glacier on a day in the mid-thirties.
But surely that's solvable in software? There must be a way to get avconv to make duplicates of each frame, if I don't mind that the movie come out slightly jump. I read through the avconv manual, but it wasn't very clear about this. After a lot of fiddling and googling and help from a more expert friend, I ended up with this:
avconv -r 3 -start_number 8252 -i 'img_%04d.jpg' -vcodec libx264 -r 30 timelapse.mp4
In avconv, -r specifies a frame rate for the next file, input or
output, that will be specified. So
-r 3 specifies the
frame rate for the set of input images,
and then the later
-r 30 overrides that 3 and sets a new
frame rate for the output file,
-timelapse.mp4. The start
number is because the first file in my sequence is named img_8252.jpg.
30, I'm told, is a reasonable frame rate for movies intended to be watched
on typical 60FPS monitors; 3 is a number I adjusted until the glacier in
the movie moved at what seemed like a good speed.
The movies came out quite interesting! The main movie, from the top, is the most interesting; the one from the underside is shorter.
|Roof Glacier from underneath.|
I wish I had a time-lapse of that folded sheet I showed above ... but that happened overnight on the night after I made the movies. By the next morning there wasn't enough left to be worth setting up another time-lapse. But maybe one of these years I'll have a chance to catch a sheet-folding roof glacier.
[ 19:46 Feb 03, 2015 More photo | permalink to this entry | ]