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Title: There oughta be a bullet time video booth
Site: there.oughta.be

 
 There oughta be a bullet time video booth. 
 
 
 26 May 2023 
 For my cousin’s wedding I did not make a photo booth but a video booth. - With an array of DSLRs to create a bullet time effect. 
 
 
 
 
 Click the image to see the video on youtube.com. 
 
 Since this is a very visual project, you should really watch the video. This blog post contains the same info with more details, but those details are probably only relevant if you really want to recreate this project, which is not an easy thing to do. 
 What is a bullet time video booth? 
 The story of this project actually begins in 2017 when I created a video booth for my own wedding. At many wedding receptions you can find photo booths, which are simple camera setups with a remote trigger to allow guests to create some memories of the special day. Usually this involves silly props like hats, wigs and giant glasses and it is as much about creating photos as a memory as it is about the fun of being creative while doing so. …and for my wedding I mixed it up a bit by putting my Sony NEX-5T on a tripod and allowing guests to take short 5 second clips  instead of static photos. Later I cut all the clips into one long video with upbeat music and we still enjoy watching this memory today. 
 My family enjoyed it so much, that one of my cousins used the same setup a few years later at her wedding and when another cousin planned his wedding for this year, he also asked for a video booth. But this time I wanted to try something new by adding a bullet time effect. 
 
 
 
 
 A very low-res demonstration of what the bullet time video booth creates. Please check the Youtube video above for a proper quality demo. 
 
 In case you never heard of it: The bullet time effect was famously introduced with the movie “The Matrix” from 1999. It really became a signature style for these movies and has been copied and reused so many times that it pretty much became a movie cliché. But while it can easily be recreated in purely rendered form like animated movies or video games, it is still quite involved and costly to achieve with real cameras: You set up an array of photo cameras, trigger them all simultaneously  and play back the individual photos as frames of a video. The result is that the scene seems to be frozen in time while the camera moves around freely. 
 And here you see the problem for hobbyists. If you are not rendering the entire scene but need to take photos of real people, this can quickly become expensive. Bullet time-time costs one camera per frame. For someone shooting with the typical “European” 25 fps  that’s 25 cameras per second. And that of course also explains why my bullet time video booth only uses this effect for the transitions instead of recording several seconds of bullet time footage. I simply wanted to keep the costs for this project in a reasonable range by only using twelve cameras. 
 
 
 This is the video booth as seen from the guest's perspective. (Well, if the guests were dwarfs.) You can clearly see that the cameras are placed at greater distances towards the wall to the left to achieve the impression of it accelerating to/from the wall when played back in sequence. 
 
 This means that I only have a bit less than half a second of bullet time footage per guest. But as you could see in the little example clip above (and more in the Youtube video) the effect is still nice and appears longer. To achieve this I used two tricks: 
 
 I trigger the cameras at the beginning and the end of each video recording and they are arranged in an arch towards the wall behind the guests. By mirroring every second guest clip and playing the bullet time photos in different order I can combine the two twelve photo sequences from the end of the previous guest clip and the beginning of the next guest clip to make it look like the camera spins through the wall to the next clip. An additional blurred frame at the location where the camera would be inside the wall and a non-equidistant placement of the cameras for an accelerating motion towards the wall help to sell the effect. This adds up to 25 frames and therefore one second of bullet time footage. 
 I created the final video of all guests (and the example above) in DaVinci Resolve and used its Optical Flow Estimation to generate artifical additional frames. This allows to smoothly slow down the bullet time effect further and also gradually slow down the video recording after leaving a bullet time transition and before entering the next bullet time transition. 
 
 So, I was able to pull this of with twelve cameras plus the one for the video recording. But this setup requires more than just the cameras… 
 Setup 
 In this section I will go through the important components of the bullet time video booth. Keep in mind that pretty much everything was selected to keep the costs as low as possible. Unfortunately, this also means that some components were a bad decision in hind sight and some components are simply more expensive or better equipment which I happened to already have. So, if you want to build something similar, be prepared to pick your own components and adapt the software to your needs. 
 Bullet time camera array 
 Let’s start by looking at the part that obviously is the most expensive because that purchase comes with a factor of twelve: The cameras used for the bullet time effect. 
 When I said that the bullet time effect is still expensive, I have to admit that it has become much cheaper since 1999. Cameras have become so readily available and good today that a comparison to 1999 equipment would be ridiculous. Still, I set myself a limit of 50€ per camera, which means that the twelve cameras alone would cost 600€, which I find quite heavy for a fun gimmick to a video booth . And now think about what cameras you can get for 50€… There are some possibilities like webcams, used smartphones, used action cams, etc. But used ones for 50€ will have poor image quality and their small sensors are not ideal for this effect as they require more light for short shutter speeds or you will get a lot of motion blur. Raspberry Pi cams are also an interesting alternative in the maker world, but the cheap ones have very poor image quality and the better ones cost 50€ alone without a lens - and you need a few actual Raspberry Pis which are even more expensive. 
 Instead, I looked for old used DSLRs. They have large (albeit old ) sensors, they support means to remotely trigger the shutter, they have plenty of resolution, their settings can be controlled precisely and nobody except me is interested in them anymore. The trick is that we are using the photo mode of these old DSLRs to create a video, which means that we are comparing old photo specs with modern video specs. So, I looked for the oldest mainstream beginner DSLR that has a decent resolution and picked the Canon EOS 400D  from 2006. 
 
 
 Now I am the proud owner of twelve Canon EOS 400d... 
 
 That camera is now almost 17 years old, which means that craigslist  is full of those cameras. There are so many people trying to sell their old camera with almost nobody being interested in such outdated technology  that I could simply work through the offers starting with the oldest listings. I offered my 50 bucks for the 400D and its mediocre kit lens, reminded the seller that nobody has even looked at the listing in six months, and either he accepted or I contacted the next one in the list. 
 But is it good enough? Well, that’s where we can now compare the photo specs to the video specs. The old 400D from 2006 has a photo resolution (it cannot even shoot video) of 3888x2592 pixels, which is more than our modern 4k (or actually UltraHD) video with 3840x2160. 
 Video camera 
 But since the Canons cannot record video  I need one more camera as the main camera that shoots the video. Initially, when I selected the 400Ds, I aimed for 4k video and planned to use my trusted Sony a6400 for this. Unfortunately, it turned out to be rather difficult to trigger the video recording precisely and transfer the resulting video file. Both can be done via USB, but one requires the camera to be in control mode and the other one requires it to be in mass storage mode. Video files apparently cannot be transferred in PTP mode, which seems to be an oversight by Sony. 
 
 
 While the bullet time shots were done with Canon EOS 400d, the video was recorded on the Sony a5000 that some of you might know from an older post. 
 
 I either needed to use a Wifi SD card like I did in my old original video booth or record externally with an HDMI grabber. Since transferring 4k videos with a Wifi card (or maybe even recording to that old Wifi SD card) seemed like a bad idea, I dropped the 4k goal and went with one of my cheap 1080p HDMI grabbers instead. And since I did not need to tie up my good camera in this project if I only record 1080p footage through a mediocre HDMI grabber, I used my Sony a5000 instead. Some of you might remember it from an older post, because it requires  a hack to get a clean HDMI signal . 
 
 
 Only the a5000 and the HDMI grabber limit the video booth's resolution to 1080p. And a rather low quality 1080p if I am honest. 
 
 So, we are down to FullHD, but as a plus using an old cheap camera gives me some valuable peace of mind when leaving the bullet time rig unattended at the wedding venue. 
 Quarter circle stand 
 At that point I thought that I had bought the most expensive part of my bullet time rig - until I faced the question of how to mount the cameras. Whichever solution comes to your mind right now: Remember to multiply its cost by twelve and think again. 
 Using twelve individual super-cheap tripods is impractical as someone will easily hit one of the tripod feet and misalign a camera from the other ones. So, I was sure that I need a common stand to which I mount all cameras. I first thought of traverse material as found on stage, but that is super expensive. The next idea was to build something myself out of plywood when I stumbled upon a B-stock e-drum rack. Specifically, it was a rack for the Alesis Strike Pro SE. On its own this is not ideal because its components are not enough to form a proper quarter circle and the horizontal bars are not mounted at the same heights. But I got that B-stock rack and a new one for a total of 93€ and combined them in a different arrangement. This even left a few spare parts that I could use to mount a screen to the setup. 
 
 
 This is a rather bad mounting solution. You can also see the crack in one of the threaded plates. 
 
 Unfortunately, that was not the end of the problem, because I still needed to mount the cameras to the rack. That’s where I went for a solution that was too cheap. I got cheap clamps from aliexpress (which were fine) and attached them to the cameras with simple threaded rods and cheap threaded plates. My thought was that twelve cheap ballhead mounts are quite expensive in total and that instead I could rotate the clamp around the rack bar and adjust the second axis by tightening the threaded plates. Unfortunately, I ended up just tightening the clamps to the cameras until the threaded plates cracked, which worked ok that one time, but if I want to use the setup again, I need to replace all plates - and probably buy twelve of those cheap ballhead mounts after all. 
 Power supplies 
 Another bad solution that I cannot recommend is the power supplies. You obviously don’t want to rely on batteries, so a set of dummy batteries from aliexpress had to be purchased (again, factor twelve - you see how this adds up?). For some reason I thought that getting ones that plug into USB ports instead of AC outlets would be a good idea, although both need to convert the voltage to the camera’s 7.4V. The tempting idea was that I could just plug all cameras into two 8-port multi chargers. 
 That was until I did my first tests and the cameras reset instead of taking a picture. At least when I triggered all twelve cameras simultaneously. Because in that case the chargers could not keep up with the sudden surge current and were unable to provide the proper voltage for the cameras. I think the problem here is that these old DSLRs need next to no power in standby as they do not show a live view. But when triggered, they have to move the mirror as well as the shutter while suddenly reading and processing the sensor - and they were designed with a Li-Ion battery in mind and not a cheap USB-based converter. The chargers on the other hand expect a more or less constant high current draw from a device that actually buffers sudden power demands with its own<truncated>