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Old July 6th, 2006, 09:17 PM   #1
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Interesting Chroma key Idea

Hi, I am in a place where I can't do as much with video as I want. I'm deployed to Iraq for the moment. I bought a camera I am learning with, and I am trying to do what I can i a very small area.

I was playing with a blue sheet on the wall, and trying to do a chroma key shot, but I just don't have the lights, and space needed. Especially lighting the wall evenly is a problem.

A buddy of mine suggested something, and I want to see what you all think of it. It seems so obvious there must be some reason people don't do it, but I can't think of one.

Here is his idea, instead of lighting a sheet (or a painted wall) use a projector to project blue light on a dark, white wall. Tada, even blue light. I can borrow a projector, and a wall easily enough. Separating the object I am shooting might be hard, as we won't be able to get it much more than abut 3 feet from the wall. Anyway, your thoughts? I hope to try it out in the next week, and see what it looks like on camera. I'll try to post up some stills to show you how it looks. I'm trying to shoot a lego model of the Death Star, and put a starfield behind it in post. So far my chroma key attempts have been pretty terrible.

Your thoughts?
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Old July 6th, 2006, 10:35 PM   #2
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I sincerely want to thank you for serving your country.

You don't mention which application you are using to do your chroma key.

I've used lots of different ways to get a chroma key background. I've never tried the specific one you mention but am pretty sure that you can make it work. Your greatest problem may be lighting the wall without lighting your subject with the same light (in a tight area).

Reflection onto the subject may be another problem you experience. If your blue light (on the wall) reflects onto the edges of your subject, it will spoil your efforts to chroma key as the areas of reflected light (on your subject) will go transparent along with your background. You might be able to overcome the reflection with one or two "kicker" lights. These would need to be red or yellow and pointed at the subject from approximately 30 to 45 degrees from the rear of the subject. Doing all of this in a confined area might prove difficult.

I have often used the sky as a "blue screen". The first time I used the sky, I did it accidently when I was shooting a video of the American flag. It might work for what you are trying to do.

You might also try a black background if you subject doesn't have harsh shadows or doesn't contain black.

Depending on the NLE you are using, you can use almost any color of background, as long as it is basically one color and doesn't share a color (like a purple object with a blue background) of your background. Vegas does a great job of chroma key, even when the background is full of gradients.

Good luck,
- Jesse
XL2 - Vegas - DVDA - Sound Forge - Acid Pro - Cubase SX 3
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Old July 7th, 2006, 10:19 AM   #3
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Thanks for the ideas Jesse

I am using Vegas 6. I tried shooting this with a black background, but the problem is the shadows go transparent, and it makes parts of my "Death Star" show stars through it. I considered putting a mask over the starfield the size of the death star so that they don't show through. I need to learn how to make a mask with the outline of my death star.

I was thinking that using a digital projector might show pixels, but then I realised that if I make the projection out of focus, the pixels should blend nicely.

My lighting options are very limited. All I can get here are regular incandescent lamps. I bought some truck window shade things. One set is reflective, the other is a silver cloth. The silver cloth one does a good job of softening the light, but I have a hard time getting enough light from 60 watt bulbs.

I went to buy a few more lamps to shine off it, and the PX (the only store here) stopped selling lamps! I need to go try again, maybe they started again, but there are no gels to be had here!

Hehe, it's frustrasting, that I am so limited, and it's funny that the area I live in has been nicknamed Deming's Movie Studio.

I will go back to the states in 3 months, and find a better place to set up. I am just doing this now to learn what I can during my free time here.

I love your idea of using the sky! It won't work here, because the sun is so strong, I would have no shadows. Also the only way to get a clean looking sky would be to shoot straight up at my model, and I don't think it is strong enough to be tilted like that. The sky here turns orange toward the horizon because of the sand in the air.
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Old July 7th, 2006, 03:12 PM   #4
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I recently shot on a green screen background that was large, not well lit and the background was muslin and it was wrinkeled and had lots of waves in the fabric.

I thought it was going to be a nightmare but it turned out just fine in Vegas and from what I have read, not many other programs have the same ability Vegas does without buying plug ins.

You might just try to do what I did:

When you do the chroma key in Vegas 6, put the "Chroma Blur" effects plug in before "Chroma Keyer" effects plug in, on the effects line.

In the Chroma Blur, I set both the Horizontal and Vertical Blur to 1.571.

The on the Chroma Keyer effects plug in I set R to 0, G to 255 and B to 0. Then I set low to .860, high to .890 and blur to .030.

It worked out great but I was sweating it until the render. You might use these settings as a place to start then modify them to your specific conditions.

You can also use the bezier capabilities to mask your subject from any background (looks the same as chroma key). If you are interested, I can explain how you can do this also.

Another approach would be to take a photograph and in Photoshop, use the pen tool (works the same as the bezier capabilities in Vegas) to delete the background, then save it as a xxx.png file with no (or transparent) background, then import it into Vegas and use the crop tool to move it where you want it and size it how you want it.

Let me know how it goes.
- Jesse
XL2 - Vegas - DVDA - Sound Forge - Acid Pro - Cubase SX 3
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Old July 9th, 2006, 01:31 AM   #5
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Try using green. In film the blue had the finest grain ideal for keying. DV carries the least amount of information along the blue channel and the most on the green which is why film bluescreens have become video greenscreens.
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Old July 9th, 2006, 04:19 AM   #6
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The blue projector idea might work ok. The only requirement is that it be a unique color and as even as possible around the subject. Whether it's from a backdrop or emmisive light source doesn't matter much. Anything beyond the subject can be "garbage matted". What matters most during the trickly keying process is where the details are.

The brightness should be adjusted so that it's not too bright in relation to the foreground and, hopefully, not spilling too much on your model -- whether that model's made out of plastic or flesh and blood.

Some software can tolerate spill. really depends.

DV doesn't do keying all that well. But that said, some awesome work has been done with DV (405, the Movie for example).

Something you might want to check on is the sharpening setting of your camera. Turning it down will prevent nasty edges from forming on your composite.

If you need advice on other special effects like muzzle flashes or explosions you should talk to the guys in the armory... :-)

Take care in Iraq!
Dean Sensui
Exec Producer, Hawaii Goes Fishing
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Old July 9th, 2006, 07:12 AM   #7
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Actually with Vegas you can use ANY background so long as
a) It's static
b) The camera doesn't move
c) You don't get shadows on the background.
d) You need a clean background plate of the background, just record say a minute of the background minus the subject(s)

The technique is to use a difference mask, it's not as easy to setup as a conventional chroma key however Vegas will do it as well as a CK and a difference mask has several advantages over a CK in terms of flexibility.

There's a tutorial on the VASST site and several posts and explainations on the Sony vegas forum. If you want to see a very complex use of difference mask there's a sample Vegas project on the Sony site called "Two Cats" along with an explaination of how it's done. It could not have been done using CK.

For a good blue screen here's a simple one that's real easy to find, the sky, minus any clouds of course, should be easy enough to find in Iraq I guess.
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