Green Stripe Reveal

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Caterpillar with Green Stripe is made with stop-motion. It took two months of planning, thirteen precision jigs, over 20,000 photos, and a week of walking back and forth in a room with unchanging light.

When I first started working on the caterpillar in 1998 my goal was to figure out the math and show the beauty of the caterpillar’s movement. I ended up diving deep into mechanics trying to achieve that goal, but strangely, the Green Stripe satisfies my original instincts in a way that a purely mechanical piece never could.

Since introducing the Green Stripe as a puzzle two weeks ago I’ve received over a hundred comments on Instagram and YouTube and emailed to me proposing thoroughly ingenious solutions. Enough good ideas to make a fleet of caterpillars! They included using magnets, electromagnets, subterranean conveyor belts, superconductors, springs, invisible strings, gravity, cams, and plain old magic. Two friends, Lorca and Lola Rossman, got it right (thanks for not spilling the beans!), but other then that, it remained a mystery. For me, at the heart of the caterpillar has always been a puzzle: how does the math work? How to make it? And so it made sense to introduce the Green Stripe as a puzzle. But now it’s time to share its inner workings.

I started making the Green Stripe when I was halfway though the Chariot. At the time I was doubtful that I’d ever get the Chariot to work, and so was casting about for another solution. Since I felt I had successfully worked the math out it occurred to me I could make a series of jigs to shape a little caterpillar and stitch together a series of stills. I’ve never done a stop-motion before. It turns out it’s a lot harder then it sounds.

To make the caterpillar I turned aluminum on my lathe and machined the green steel links to .001” tolerance. It’s assembled with spring washers so that it is somewhat malleable at the same time as it holds its shape. Since I knew that I’d have to move the caterpillar between photos, I milled precise grooves in a piece of Mahogany to index its location. I machined the jigs out of plywood on the mill that would give incremental movement to the caterpillar. I designed the jigs to give 3744 discrete positions as the caterpillar walked along the track. Since video is put out at 24 frames per second, I calculated it would take about two minutes to walk from one end of the wood to the other. After experimenting with what color to paint the background, and blocking all the windows with cardboard, I was ready to take pictures.

Chris Potter, who has helped me with video for years, came over and he played around with lighting. We chose LEDs because they would be cooler, and also less likely to burn out over the course of the shoot. Chris brought over an old Mac installed with Windows and program that allows you to trigger multiple cameras, plus change their settings and download images though USB cords, so we wouldn’t have to take the cameras off the tripods. I rented six cameras to have different angles to cut between and Chris got me all set up so when I pressed the space bar on the keyboard all six cameras would take a picture. Actually the space bar caused everything to crash, and so I ended up pressing the “page down” button to trigger the cameras. He also showed me how to download the images and reformat the cards as I went along. Since each camera would record 3744 images, I ended up with a total of more then 20,000 photos.

It would have taken much too long to take the images in the same sequence as they are stitched together in the video. So instead my strategy was to shape the caterpillar by hammering it over a jig, and then position it in the first groove. After taking a picture I would keep the caterpillar in the same shape and advance it along the plank to the next groove and take another photo. While quicker, this method had a couple disadvantages. One was that I’d have to shuffle all 20,000 images into the correct order later, and two, until I had taken the last photos I wouldn’t have even enough to make a six second video and wouldn’t know if the whole system was working or not. There was no way to preview anything.

Taking the photos took longer then expected and I ended up working 14 hours a day for five days trying to finish it before I had to return the gear. It involved walking back and forth in a small room by myself in completely consistent light. Having the light stay exactly the same all day long for a week is a mind-altering experience and it took me a few days to get back to normal afterwards.

I was also terrified that something would break right at the end, rendering all the work useless. And in fact when I was only a couple hours from finishing one camera did break. Chris came over and determined the camera was unsalvageable and so we took it off and put in a new one, doing our best to set it up exactly the same. When I had taken the last image, we also took some footage of me placing the caterpillar on the wood and looking at it. Chris overlaid this onto the stop-motion to cement the illusion in the opening sequence.

For shuffling the images I knew who to turn to. My friend Dan Torop, who has helped on lots of projects, is a photographer, but is also really good at coding and math. In this case he taught me how to use Terminal and sent me a shell script I could run that would shuffle a directory of images in a particular way. It worked perfectly, and for the first time I could click through the images and get a sense of the motion.

Chris took all the ordered images and rendered out videos and began experimenting with how to edit them. We found it most pleasing to transition between cameras at the same place of the cycle to give a fluid motion. The camera that broke proved immensely challenging. The new camera was slightly off in both alignment and exposure, and Chris had to work hard to adjust the photos so they didn’t clash when the new and old camera images were placed next to each other. We looked at other more tricky effects, such as adding motion blur to each frame, but ultimately kept each frame as a pure image. It has a sort of unearthly clarity this way, even if it makes it more obvious it’s stop-motion. Ultimately I was less interested in deception, and more interested in trying to make the most beautiful caterpillar I could.

I hope that you have enjoyed the Green Stripe! Not everyone wants to know how it is made, so please share it by sharing the Green Stripe puzzle page, and letting people go to the hint or full reveal only if they wish.

For more caterpillars see the Chariot and the Early Caterpillars.