Next week I am going to shoot some greenscreen, exterior, using the sun (if weather works) to light the greenscreen. I'm worried if my foreground actors, which I might light with HMIs, are not bright enough, I may need to put up a 20x with nets to take down the level on the greenscreen.
So will the shadow of the net's pattern on the greenscreen be a problem? I would think that seams in the net would certainly cause a problem.
Anyone done much of this?
Tilt the greenscreen. Net far away to reduce pattern(s). Use diffusion (and if needed, net) to eliminate pattern. Crank up FG light. Tent your world with black and light greenscreen with controlled light. Create and patent your own original line of "ND Greenscreens".
Good luck Jim,
As with so much of what we do, the devil is in the details:-) If, for instance, the green screen is far enough away and your stop is light enough that the greenscreen is a bit out of focus, the net pattern will be inconsequential. On the other hand, the further back the green goes, the bigger it has to be. I would check with whoever is doing the post work as to whether or not you can be a bit over on the green. On one show that I did, the digital I shot exposure wedges of the green labelled slated only with letters, and the green that the post house liked the most was 1 stop over key. We soon modified that when they started looking at the level of green spill coming back onto the set, whcih had a lot of brushed stainless bits in it.
Depending on how bright the background plate is going to be, the artifacts of an overexposed but even green might be easier to deal with than a correctly exposed but less even (shadows of the net seams, etc) green. Degree of likely motion blur of the foreground elements would impact this discussion also - blurred edges become a mix of fg and bkg elements, and the less even the green, the touchier it is to pull the key. As obvious as it sounds, most people forget that the reflected value of the green will be pretty much the same whether it is 5 feet from the subject or 20 feet from the subject, but the effect of the green on the subject will differ greatly. If you are getting a lot of green contamination on the fg from the sun-lit green, you can try covering whatever portion of the greenscreen that you do not need to show you the edge of your fg element with black douvetyne - people usually leave the screens all exposed even if they only need the bottom half, and the top half is pumping green light back into your set.
By the way, you may want to use the older, darker tempo cloth green fabric instead of the new digital green stuff that is more lemony. The older stuff is less reflective and is less likely to overexpose than the newer stuff. As to which is better, I can line up a collection of ditgital folk who will swear on a stack of SGI's that the digital color is better and another set that will swear the exact opposite. For your purposes, I generally prefer the tempo cloth (fuzzy stuff) over the spandex stuff because I believe that with less specular reflection, I get less chrominance contamination with the fuzzy stuff. On the other hand, in the lighter, yellower "digital green" color, I have seen some fuzzy fabrics that were terrible - they reflected a lot of red. If you are painting flats, check with your post house whether to use the Ultimatte green or chroma key green - I've heard strong arguements for both.
Mark H. Weingartner
Lighting and VFX for Motion Pictures
I've done plenty of blue and green exterior. I've always tried to shoot these elements at the beginning and end of the day. Place the sun behind the screen (low enough as a back/edge light) Expose the screen using the skylight ambience - read no shadows. Use HMI and or Flekkies as modeling for the foreground elements. Alternatively having an oversized screen may help if you are going for a mid day look, this will allow you to possibly angle the screen to avoid direct light onto it or even allow the cutting down of light levels slightly.
As always shoot a reference clear b/g screen - difference mattes are sometimes a saviour.
Director of Photography
If uneven illumination/colour on the greenscreen is unavoidable, then consider shooting a clean plate of the "empty" greenscreen lit exactly as it will be lit for the main "hero" pass. This will enable the person compositing the shot (depending on which system he/she's using, anyway) to better determine what is foreground and what is background. Think of it as having not only the "chromakey" information, but also some "difference matte" information too, all with relatively little extra effort by you or your crew.
Double Negative, London
You have gotten some good advice, to which I would like to add the following:
Be careful in leaving the green screen up and exposed for long periods of time to full direct sunlight. Two or three days of this will bleach at least some of the color out of it.