Time-lapse photography: stitching movies together on Linux (Shallow Thoughts)

Akkana's Musings on Open Source Computing and Technology, Science, and Nature.

Fri, 15 Aug 2014

Time-lapse photography: stitching movies together on Linux

[Time-lapse clouds movie on youtube] A few weeks ago I wrote about building a simple Arduino-driven camera intervalometer to take repeat photos with my DSLR. I'd been entertained by watching the clouds build and gather and dissipate again while I stepped through all the false positives in my crittercam, and I wanted to try capturing them intentionally so I could make cloud movies.

Of course, you don't have to build an Arduino device. A search for timer remote control or intervalometer will find lots of good options around $20-30. I bought one so I'll have a nice LCD interface rather than having to program an Arduino every time I want to make movies.

Setting the image size

Okay, so you've set up your camera on a tripod with the intervalometer hooked to it. (Depending on how long your movie is, you may also want an external power supply for your camera.)

Now think about what size images you want. If you're targeting YouTube, you probably want to use one of YouTube's preferred settings, bitrates and resolutions, perhaps 1280x720 or 1920x1080. But you may have some other reason to shoot at higher resolution: perhaps you want to use some of the still images as well as making video.

For my first test, I shot at the full resolution of the camera. So I had a directory full of big ten-megapixel photos with filenames ranging from img_6624.jpg to img_6715.jpg. I copied these into a new directory, so I didn't overwrite the originals. You can use ImageMagick's mogrify to scale them all:

mogrify -scale 1280x720 *.jpg

I had an additional issue, though: rain was threatening and I didn't want to leave my camera at risk of getting wet while I went dinner shopping, so I moved the camera back under the patio roof. But with my fisheye lens, that meant I had a lot of extra house showing and I wanted to crop that off. I used GIMP on one image to determine the x, y, width and height for the crop rectangle I wanted. You can even crop to a different aspect ratio from your target, and then fill the extra space with black:

mogrify img_6624.jpg -crop 2720x1450+135+315 -scale 1280 -gravity center -background black -extent 1280x720 *.jpg

If you decide to rescale your images to an unusual size, make sure both dimensions are even, otherwise avconv will complain that they're not divisible by two.

Finally: Making your movie

I found lots of pages explaining how to stitch together time-lapse movies using mencoder, and a few using ffmpeg. Unfortunately, in Debian, both are deprecated. Mplayer has been removed entirely. The ffmpeg-vs-avconv issue is apparently a big political war, and I have no position on the matter, except that Debian has come down strongly on the side of avconv and I get tired of getting nagged at every time I run a program. So I needed to figure out how to use avconv.

I found some pages on avconv, but most of them didn't actually work. Here's what worked for me:

avconv -f image2 -r 15 -start_number 6624 -i 'img_%04d.jpg' -vcodec libx264 time-lapse.mp4
Update: I don't know where that -f image2 came from -- ignore it. And avconv can take an input and an output frame rate; they're both specified with -r, and the only way input and output are distinguished is their position in the command line. So a more appropriate command might be something like this:
avconv -r 15 -start_number 6624 -i 'img_%04d.jpg' -vcodec libx264 -r 30 time-lapse.mp4
using 30 as a good output frame rate for people viewing on 60fps monitors. Adjust the input frame rate, the -r 15, as needed to control the speed of your time-lapse video.

Adjust the start_number and filename appropriately for the files you have.

Avconv produces an mp4 file suitable for uploading to youtube. So here is my little test movie: Time Lapse Clouds.

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[ 12:05 Aug 15, 2014    More photo | permalink to this entry | comments ]
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