View Full Version : Non-visible spectrum matting?

07 July 2005, 07:41 PM
For harry potter they used this crazy little cloak which acted as a greenscreen, and was illuminated by green led's so that the camera saw very few shadows.
And then for Sin City they used glow in the dark gaffers tape for bandages to pop the bandages out for easier manipulation.

So what I'm wondering is if this could be taken a step further, and simply use a seperate spectrum to isolate objects. Say spray on an invisible fixative which turns a normal wall into a uniform shade that can essentially be used as an automatic alpha channel.

Are there any production level cameras that record both visible and non-visible spectrums? Has this ever been used or is anyone working on it? Would this even be in any way practical?

07 July 2005, 12:31 PM
I seriously think you might be on to something here. I'm sure there are some big issues to overcome, but its feasible.

I'll ask around and see if anyone has tried it.

07 July 2005, 01:36 PM
I don't know if it can be done, although I thought of something similar a while back. The problems that I saw with it were:

If you're not using visible light, you could use UV or IR. Both can be recorded by cameras, so the technology to see them exists. But if you use UV, you have to light your actors with UV, which will give them sunburn, considering how many lights are used in filming, and if you go further down that end of the spectrum, you get higher energy radiation.

And if you go to IR, it's safe, but you'd have to prep a lot more than just the backdrop, because a lot of things reflect near-IR light to varying degrees. Your actors won't show up as either black or white, but in varying shades just like in the visible spectrum, but in shades that your prop/makeup people wont be able to easily predict, and may end up being the same shade as the backdrop. And if you go further into the far-IR spectrum, you start to see body heat, and it blurs. So you'd have to be able to coat your actors with something that's clear in the regular spectrum, but doesn't reflect any near-IR light, which I don't know if it exists, and might end up being very toxic.

However, maybe someone could set it up so that the actors are being lit by spotlights that emit little or no near-IR light (if that's even possible), and the backdrop is lit by regular lights. You would need to somehow find a filter that cuts near-IR without cutting visible light (too much), but as far as I know, most IR filters still let a lot of it through. If someone could make an efficient filter, they could probably do this.


07 July 2005, 06:26 PM
interesting idea... as mentioned, UV becomes a burn/cancer hazard, and IR is very close to the visible spectrum. However, there are notch filters that can pass or block very specific frequencies. What you'd be looking at then is a prism that splits the video signal in two - one for visible, and one for IR to use as a mask.

Green is commonly used because it's easy to see (human eyes are sensitive to it), easy to 'notch' filter, and can be masked pretty thoroughly. Current digital technology also uses twice the number of green pixels over red or blue (when using a Bayer mask). Anyway, current software is built to look for greens in a particular range when working with masks and screens.

I'm not sure, though, why you'd want to go through the trouble of applying a coating or other treatment to ordinary materials unless you could more cheaply and easily do the work. One possible advantage, though, would be partial masking, like spraying dots of mask on a moving object (say a cloak with lots of holes...?).

On a side note, digital cameras are automatically sensitive to IR, so much so that they typically come with an IR filter in front of the sensor. There are a handful of mods that can be done to remove the filter, but it's a hack process. UV requires more work to record and process.

07 July 2005, 06:36 PM
Good points chris.

I'm afraid that I have difficulty thinking in terms of the non-visible. I don't see it, so while I know that its fairly similar, I just assume that its blank.

Its quite possible that this might not be the way to go, for the reasons you just mentioned. Plus, since you can't see any of this without special equipment, it'd be a little hard to work with.

Maybe some sort of special material/coat that returns an unnatural, specifically manufactured color, which the camera automatically interprets as an alpha would be more feasible. Say something that only returned specific, narrow portions of the spectrum.

Lokki: I was thinking that having to have a green screen studio is a bit cumbersome. Being able to easily mask out anything could be a huge advantage in terms of flexibility. Plus, if this was done in something other than what we can see, the original image would be preserved as well. So, say you wanted to modify a background in post, but didn't want to have to matte it out by hand, or green screen the actor in, viola, just turn the bg into a matted object.

07 July 2005, 06:41 PM
Maybe some sort of special material/coat that returns an unnatural, specifically manufactured color, which the camera automatically interprets as an alpha would be more feasible. Say something that only returned specific, narrow portions of the spectrum.

Wouldn't it be more cost effective to just use a bright green?

07 July 2005, 06:56 PM
Why, you ask?

Mainly because I'm attracted to the idea of not having key out a color. An automatic, or easily usable alpha channel seems quite attractive. I don't know crap about compositing, so this thread is just the musings of an ignorant man.

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