Guerilla chroma key

April 26, 2006 12:30 PM

DV Film Effects: Techniques & Tips

I've recently been shooting some short instructional video clips. I've got a GL1, some basic flourescent photo lights and a handful of shielded photographic bulbs. I've rigged up a lazy susan with a circular top on an old dust collection barrel and covered it in a blue photo background paper. This paper also hangs behind that assembly for good measure. I center an item on the circle, filming it while I spin it around slowly to produce a very quick intro view of the item. I've backlit the subject and covered it from three other angles to reduce shadowplay, in general lighting is not a problem.

However, most of these items I am attempting to shoot are constructed of very reflective materials: black phenolic, aluminum rails and colored HDPE to name a few. I've got my bag of tricks for shooting print resolution catalog photos of these materials, but pulling a good key off this video material seems to be very difficult. I'm using AE 7 with the Keylight plugin. I've been generating garbage mattes to reduce the amount of calculations that need to be done but I still think I could get better results on the final key - it's not horrible, but it's not perfect.

So I call on all MeFi video mavens, what say ye? How have you overcome technical issues in your video projects and still accomplished the task on the cheap? I don't think expensive muslin or special chroma material will help, as the subjects are inherently reflective in varying degrees and will always pick up a slight color cast from the nearest object.

Anything else you'd like to mention about shooting DV material at a sub-professional level is certainly welcome, thanks for listening.

posted by prostyle to media & arts (8 comments total)

Would it make sense to reduce the reflectivity by giving the object a spray of hairspray or underarm deoderant?
posted by plinth at 1:46 PM on April 26, 2006

DIY film equipment

My question is: if you live in NY and work fulltime, how do you find a DP, actors, etc.?
posted by kensanway at 3:10 PM on April 26, 2006

I hadn't thought of anything along those lines, plinth. Sounds like a good idea, I'll have to try it out.

Thanks for that excellent link kensanway, lots of information there.
posted by prostyle at 3:40 PM on April 26, 2006

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.
posted by phirleh at 5:06 PM on April 26, 2006

You could also try difference matting, shoot your chroma key with and without your foreground object using a tripod and you can usually use it as an extra input in most keyers (AE has a built in Difference keyer and I think Keylight will accept it too).
posted by phirleh at 6:01 PM on April 26, 2006

christmas lights are good at creating a diffused even lighting for screening
posted by psychobum at 11:54 PM on April 26, 2006

Good separation between your subject and your background is helpful for a good key. By having your subject rest directly on your blue photo paper, you're 1) casting a shadow on your photo paper and 2) reflecting blue light back onto your subject, both of which result in an inferior key.

Your setup is great for its simplicity and speed, but you might want to experiment with other ways to display (suspend?) your subjects to improve the key.
posted by zanni at 2:03 AM on April 27, 2006

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.

Do a Google search for "procedural matte"/holdout creation, and you'll find many great tutorials. This is one of the better ones
posted by melorama at 4:40 AM on April 28, 2006 [1 favorite has favorites]