Chroma key problem

[Tim Kolb]

Cigarette smoke can be pretty wispy…there are several options…and i don’t know if switching applications will gain much.

You could try layering the key source over itself, several times if necessary. That will “bulk up” some of the more transparent elements in your scene. Also you could try a very small supplemental backlight to “beef up’ the cigarette smoke as well…

TimK,
Director, Consultant
Kolb Productions,

CPO, Digieffects

[Todd Morgan]

First off all, DV is the worst way to plan to shoot and test keying. Keylight hates DV footage and you will never get a decent key. If you are shooting HD, you should have no problem with your smoke wisps. But if that is a concern, why smoke in the first place? Is it vital to the story? If not, don’t smoke.

[Clint Lemaire]

If you have to have the smoke, why don’t you just go with the flow and Key it out then add it back with a particle generator. I use Primatte on DV footage with decent results, but it wont key your smoke on DV footage either. At least not consistently.

[David Aretsky]

I have had success in keying DV footage with keylight, sometimes you need to pull more than one key. (Most of the time I should say) After you pull the key you can just go back and use a particle generator to put the smoke back in.

dangerd

[Chris Wright]

i would garbage matte a separate key for smoke and use primatte transparency setting to see smoke. If you have green smudging, use a difference key precomped and add at top of new comp to re-introduce detail. This works excellent for spill problems too. Post a picture on picbucket so we can see it.

[Tom Scott]

[Todd Morgan] “Keylight hates DV footage and you will never get a decent key.”

Rubbish!!

I’ve managed to get some pretty decent keys, using Keylight on DV footage. Anybody who says it can’t be done either hasn’t made the effort or has a dislike for DV (probably due to jumping on the anti-DV bandwagon).

[Todd Morgan]

[Tom Scott] “Rubbish!!”

I have pulled many a fine key using client supplied DV footage. The point I was trying to make is that why use DV as a test when you are planning HD shoot. Two different worlds and Keylight will handle HD footage a gazillion times better than DV. I also wonder how much you charge a client for the “effort” of spending hours dealing with crappy green screen shots on DV…

I found this online:

You should give DVMatte from DV Garage a try. I’m working on a show right now with a lot of greenscreen shot on DX100’s and it gives great results for keys. DV is a different animal when it comes to keying due to the specific compression, and DV Matte was made to specifically address this problem. They have a demo you could try out.

While you can pull “decent” keys from DV footage, you will never be able to get “perfect” keys from it, even with the Keylight plugin (as good as that keyer is). It’s simply an inherent problem with the 4:1:1 sampling of DV that causes (i.e. color is sampled with 4 times less resolution than luminance) colors to appear blocky, and you’ll get awful stair-stepping on you keyed edges if you aren’t aware of this.

DV Matte Pro is a good plugin, but it’s been pretty much obsoleted by Keylight, and you can get much better results than DV Matte Pro by feeding Keylight a properly “pre-treated” source.

You can perform this “pre treatment” in AE (basically a chroma smoothing operation which a plugin like DV Matte does internally) by duplicating your clip over itself, and then precomping both layers into a single new comp. Then, on the top clip in the precomp, add a Fast Blur (or any blur filter that lets you specify the direction of the blur…you want to blur in a vertical direction only) of around 0.3 to 0.5 pixels.

Then, change the blend/transfer mode for the top layer to “Color”. You wont see a difference with the naked eye, but when you go back to your main comp, and then apply Keylight to the precomp, you’ll get *dramatically* cleaner edges on your keys than if you simply applied it to an untreated clip.

Another tip, regarding your bluescreen: DON’T USE A BLUESCREEN!

Blue is the worst backing color you can use when keying from DV sources. Most of the time, the blue channel of an image has the most noise in it (which you can clearly see by clicking the blue color chip at the bottom of your AE comp window), which makes it really difficult to key fine detail like hair, reflections, shadows or any type of transparency, without getting ugly “mosquito” artificating in these areas. Add that to the fact that the color resolution in your DV clip is inherently low to begin with, and you’ll find the whole keying process to be a huge pain in the ass. This is one of many reasons why most professional keying pipelines insist on using greenscreens.

The only time where a blue screen might be preferable is when you’re shooting talent has very light hair color, and a blue screen gives more contrast for the keyer to work with. Since you’re just shooting inanimate objects, however, it’s wise to stick with green.

Also, while creating a garbage matte is good, you also should create what’s called a “holdout” or “foreground matte”, which basically does what a garbage matte does, but on the foreground image instead. The trick to good keying is concentrating *only* on the edges of your FG subject. Don’t try to tweak the keying plugin to completely eliminate your background screen (even if the BG is garbage matted) while keeping the FG solid, because you’ll rarely get optimal results that way. The holdout matte tells the keyer what should be considered 100% foreground, and it will ignore anything within that area when keying the image.

The drawback to this method, of course, is that all the green/blue spill from the screen is retained in the areas defined by the holdout matte, but at least the keyer isnt keying out this spill, eating holes in your FG object. Remember that it’s much easier to colorcorrect/despill your keyed FG after the fact than it is to fix funky/overly-soft/choked edges caused by an overzealous keyer trying to work on the entire image.

Good luck!!!

[Tim Kolb]

regarding using blue…

Blue is the most noisy color in any image…photo or motion, analog or digital…

however…for the basic function of most keyers, it’s still easiest to spill-suppress for optimal skintones against blue. On a vectorscope, you see that the opposite of blue is yellow, which is very close to skintone color (the basic natural skin color differences between ethnic groups is largely saturation, very little hue), whereas green and its opposite-magenta are both approaching 90 degrees out from skintone.

If you have a high bitrate, uncompressed source, blue might still have advantages in certain situations.

TimK,
Director, Consultant
Kolb Productions,

CPO, Digieffects