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It Took Me 30 Years to Solve This VFX Problem – Green Screen Problem [video]

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Re: It Took Me 30 Years to Solve This VFX Problem – Green Screen Problem [video]

#71

Post by somat »

diacritical wrote: post #7

From ~04:10 till 05:00 they talk about sodium-vapor lights and how Disney has the exclusive rights to use it. From what I read the knowledge on how to make them is a trade secret, so it's not patented. Seems weird that it would be hard to recreate something from the 1950's. I also wonder how many hours were wasted by people who had to use inferior technology because Disney kept it secret. Cutting out animals and obje…

It is a well known process, Not a lot of general use so costs are not low, but not nearly as high as the original disney prism, I would guess around a 1000 USD for one. As far as I can tell any well equipped optics laboratory could make a beam splitter with whatever frequency gate they want.

https://accucoatinc.com/technical-notes/beamsplitter-coating...

I have no idea about that specific company I just picked it after a search for "beam splitter"

After I saw that video on the sodium vapor illumination process I was curious as to what if you could instead use near-IR light as the mask illumination. In theory you would have a perfect mask(as in the disney process) and no color interference. I found that frequency gated beam splitters are a fairly common scientific instrument.

Re: It Took Me 30 Years to Solve This VFX Problem – Green Screen Problem [video]

#72

Post by aidenn0 »

toast0 wrote: post #67
aidenn0 wrote: post #26

Earlier quoted context omitted.

It would seem to me to be relatively easy to build something like that if you're okay shooting with effectively a full stop less light (just split the image with a half-silvered reflector and use a dichroic filter to pass the sodium-vapor light one one side. The splitter would have to be behind the lens, so it would require a custom camera setup (probably a longer lens-to-sensor distance than most lenses are designed…

At the end of this video they link to another video from a year ago [1] (this is the same link as the comment you were commenting on, whoops), where they recreate the sodium vapor process with a rig with a beam splitter, one side had a filter to reject sodium vapor light and the other has one to reject everything but sodium vapor light, and then a camera on each side. The Disney process had the filter essentially bui…

We know how to make dichroic prisms (Technicolor used them when filming, as did "3 CCD" digital cameras), but I imagine that to have a sufficiently narrow rejection band for the sodium-vapor prociess, you would need to be smart about where you place the prism, since the stop-band of a dichroic filter changes with angle of incidence.

Re: It Took Me 30 Years to Solve This VFX Problem – Green Screen Problem [video]

#73

Post by diacritical »

somat wrote: post #71
diacritical wrote: post #7

From ~04:10 till 05:00 they talk about sodium-vapor lights and how Disney has the exclusive rights to use it. From what I read the knowledge on how to make them is a trade secret, so it's not patented. Seems weird that it would be hard to recreate something from the 1950's. I also wonder how many hours were wasted by people who had to use inferior technology because Disney kept it secret. Cutting out animals and obje…

It is a well known process, Not a lot of general use so costs are not low, but not nearly as high as the original disney prism, I would guess around a 1000 USD for one. As far as I can tell any well equipped optics laboratory could make a beam splitter with whatever frequency gate they want. https://accucoatinc.com/technical-notes/beamsplitter-coating... I have no idea about that specific company I just picked it aft…

Thanks for the info.

As for the IR idea, I wonder if there's something like a crowdfunding/crowdsourcing site for ideas where the person who had the idea doesn't really want to do it, but leaves it open to others to try. You said you "don't really have the budget to try it out", but let's say even if you had the money, it wouldn't be a priority for you, as you're not an expert or you have better things to do or whatever. Is there a place to just shout ideas into and see if any market-oriented entity would take it upon themselves to try doing it? Besides forums full of ideas like "tinder but for X" and such crap? Because, imagine if your idea really is a great one. A couple hours from now it would be buried in HN.

Re: It Took Me 30 Years to Solve This VFX Problem – Green Screen Problem [video]

#74

Post by shdudns »

actionfromafar wrote: post #16
somat wrote: post #10

Earlier quoted context omitted.

With regards to the sodium vapor process, an idea has been percolating in the back of my head ever since I saw that video. But I don't really have the budget to try it out. theory: make the mask out of non-visable light illuminate the backing screen in near Infra-Red light. (after a bit of thought I chose near-IR as opposed to near-UV for hopefully obvious reasons) point two cameras at a splitting prism with a near I…

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…

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.

Re: It Took Me 30 Years to Solve This VFX Problem – Green Screen Problem [video]

#75

Post by SoftTalker »

amluto wrote: post #61
kibibu wrote: post #32

Earlier quoted context omitted.

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

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 fr…

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

Re: It Took Me 30 Years to Solve This VFX Problem – Green Screen Problem [video]

#77

Post by toss1 »

amluto wrote: post #61
kibibu wrote: post #32

Earlier quoted context omitted.

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

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 fr…

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).

Re: It Took Me 30 Years to Solve This VFX Problem – Green Screen Problem [video]

#78

Post by huflungdung »

actionfromafar wrote: post #16
somat wrote: post #10

Earlier quoted context omitted.

With regards to the sodium vapor process, an idea has been percolating in the back of my head ever since I saw that video. But I don't really have the budget to try it out. theory: make the mask out of non-visable light illuminate the backing screen in near Infra-Red light. (after a bit of thought I chose near-IR as opposed to near-UV for hopefully obvious reasons) point two cameras at a splitting prism with a near I…

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…

[dead]

Re: It Took Me 30 Years to Solve This VFX Problem – Green Screen Problem [video]

#79

Post by wlesieutre »

actionfromafar wrote: post #70
amluto wrote: post #58

Earlier quoted context omitted.

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 discuss…

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

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