[actionfromafar]

I'll do you one better, which requires no special cameras (most have IR filters) nor double cameras or prisms.

Shoot the scene in 48 or 96 fps. Sync the set lighting to odd frames. Every odd frame, the set lights are on. Every even frame, set lights are off.

For the backing screen, do the reverse. Even frames, the backing screen is on. Odd frames, backing screen is off.

There you go. Mask / normal shot / Mask normal shot / Mask ... you get the idea.

Of course, motion will cause normal image and mask go out of sync, but I bet that can be remedied by interpolating a new frame between every mask frame. Plus, when you mix it down to 24fps you can introduce as much motion blur and shutter angle "emulation" as you want.

[ryandamm]

This is called “ghost frame” and already exists in Red cameras and virtual production wall tools like Disguise.

[actionfromafar]

You need to basically timecode/genlock the greenscreen "illumination LEDs" to the camera so the greenscreen lights up only exactly at every other frame. Not sure if there exists any off the shelf solution which can do that but if not it can't be super hard to cobble together.

[cma]

https://evermorestud.io/retroreflective-chromakey-experiment...

[eichin]

Somebody recently used a variation of this to get good video of welding - basically a camera synced with a very bright (strobe-ish) light, brighter than the weld itself, so you adjust the camera to the ludicrous-but-consistent brightness level and get details of the weld and the surroundings. https://www.youtube.com/watch?v=wSUxK8q4D0Q (Chronos "Helios", from early 2025)

[amluto]

Surely this makes your actors feel sick? And wouldn’t it make your motion blur look dashed and also cause artifacts at the edge of the mask if there’s a lot of motion?

[throwway120385]

You could strobe at some multiple of the sensor frame rate as long as your strobes are continuous through the integration period of the sensor and the lighting fades very quickly. This probably wouldn't work with incandescents but people strobe LEDs a lot to boost the instantaneous illumination without going past the continuous power rating in the datasheet.

[amluto]

You mean do strobe, strobe, strobe, strobe, pause, pause, pause, pause? I bet that's at least as bad as holding the source on for the first four intervals and then off for the latter four intervals.

In any case, if you actually have a scene bright for 1/24th of a second and then dark for 1/24th of a second, repeating, you're well within photosensitive epilepsy range. Don't do that to your actors unless you've discussed it with them and with your insurance company first.

[actionfromafar]

So, shoot at 240 fps and strobe set lights for 1/240s and backdrop for 1/240s.

[wlesieutre]

And if you want a slower than 1/240th second shutter speed, no you don't

[actionfromafar]

Or... you frame blend in Fusion or go full hog in Nuke.

( https://www.nukepedia.com/tools/gizmos/time/vectorframeblend... )

[actionfromafar]

Feel sick? Possibly. People are more or less sensitive to imperceptable flicker.

Artifacts?

I bet that can be remedied by interpolating a new frame between every mask frame. Plus, when you mix it down to 24fps you can introduce as much motion blur and shutter angle "emulation" as you want.

Motion blur can also be very forgiving. You are more likely to notice artifacts in still or slow moving scenes and then the problem goes away.

[kibibu]

Incandescent and fluorescent lights already flicker at your AC power frequency. Just gotta be higher than that

[amluto]

No.

Incandescent lights flicker at twice your AC power frequency -- to a decent approximation, their power is proportional to V^2. But this is input power -- the cooling of the filament is slowish and the modulation depth is low. Most people aren't bothered by this.

Fluorescent lights with old or very crappy "magnetic" ballasts flicker at twice the mains frequency, with deep modulation. The effect on people varies from moderate to extremely unpleasant, and it's extra bad if anything is moving quickly (gyms, etc). There are even studies showing that office workers perform worse under such lighting even if they don't experience personally perceptible symptoms. The effect is so severe that people invented the "electronic ballast", which flickers at much, much higher frequency and avoids low-frequency components. Phew. (The light might still be a nasty color, but the temporal output is okay.)

"Driverless LEDs" are deeply modulated at twice the mains frequency. These are very nasty.

If you actually have a light that flickers at the AC power frequency (certain LED sources in a two-brightness diode-dimmed kitchen appliance fixture will do this, as will driverless LEDs with certain types of failures), then it's extra nasty.

There are plenty of people around who find (depending on the actual waveform) 60Hz flicker intolerable and 120Hz flicker extremely unpleasant. And there are plenty of people who can often perceive flicker under appropriate circumstances up to at least several hundred Hz and even into the low kHz with certain shapes of light sources. You can read up on IEEE 1789 to find a standard based on actual research on what lighting waveforms should look like.

The effect of 120 Hz flicker is bad enough that energy codes in some places (e.g. California) have started to require that LED sources minimize this flicker, but, sadly, it's poorly enforced.

[toss1]

Also, the human eye sees flicker much better at the periphery than in the central area. The Rod receptor cells respond more rapidly than the Cone color-sensitive cells, and the peripheral vision is also more tuned to quick motions (much advantage in having faster detection of peripheral motion, so positive selection evolutionary pressure).

[SoftTalker]

The fluorescent light strobing is why you often see fluorescent tubes in pairs. They will be wired in opposite phase to cancel the strobing.

[tlb]

I think the total light output of each bulb in the pair is the same at all points in time, but the orange-blue gradient is reversed. So when one is orange at one end, the bulb beside it is blue at that end.

IIRC, the end that's negative looks orange, because the electrons emitted from the filament haven't gotten up to speed yet and can't ionize the mercury atoms at that end to the highest states.

If you didn't do this, you'd see 60 Hz strobing when you looked at one end.

[kibibu]

Hey thanks for clearing this up. I had no idea that CFLs and fluorescent lights with electronic ballasts now flicker at ~ 20kHz.

[joecool1029]

phosphors and capacitors are a thing that mask that, so is high frequency switching way above this rate…

Anyway, an old HN submission I still use when buying light bulbs: https://news.ycombinator.com/item?id=14023196

[shdudns]

Two problems:

- It'll bleed on fast motion. Hair in the wind would just not work.

- Incandescent lights are out.

You could solve both by having two ghost frames shot very close to the real frame (no need to evenly space the frames, after-all) and using strobing a high powered laser.

You'd need very fast sensor or another one optically on the same position.

[actionfromafar]

At some point higher fps solves it. Is 240 fps enough?

[shdudns]

I dunno, I'm just an amateur photographer.

The calculation isn't too hard though. The width of a pixel divided by the velocity of the subject on the sensor is the maximum delta(T) between real and ghost frames.

But, again, you dont have to shoot faster. You just have to drop the 180-180 degree phase between a real and ghost frame to be 10-350 degrees. Then your 24 fps is capturing the background as if it were 870 fps

[ErroneousBosh]

Corridor Crew cover this in one of their VFX breakdowns where I can't remember the film but it was supposed to be filmed on a rapidly rotating platfom.

There were a large number of lights around it and each one was blinked on for an instant while the camera shot at an insanely high frame rate - something like 288 frames per second with twelve lights.

This meant that after the fact you could pick any one of the twelve frames for that 1/24th of a second, to choose the angle the light was hitting at.

[dgently7]

this is the approach that stop motion uses, except they get to keep the camera in the same place. its still not perfect because of spill from the background onto the foreground and requires additional masking and cleanup.