A nice trick I learned to remove the green "rim" or "spill" around a subject is to use an overhead light shining down own the back of the subject. For instance, if you stand 6 feet from the screen, then put a light on the ceiling 3 feet behind you and make sure it covers the top-rear part of your head and shoulders. With the proper light and placement, you wont have the residual green spill from the screen behind you.
Now somewhere once long ago, in a galaxy far away, I read different colored gels on that top light will work even better to counter the green. I BELIEVE, but don't quote me... that AMBER is the gel to use. I guess for 1 dollar for an amber gel it might be worth a try? But I've found that one of those $5 clamp on silver bowls from home depot with a 100 watt flood works great.
P.S. Adobe Ultra kicks Ass for keying. Handles high def and quicktime and also exports to still image sequences so you can add special effects in stuff like particle illusion. And it's a breeze to use, very user friendly.
Keep it up, there needs to be more posts on this board for chromakeying!!!
here is my latest green screen shoot as a final package. http://www.welcomebackspring.com/ (note: not shot on hv20) I got to use the same camera as Lucas on Star Wars
Last edited by jmorton; 2008 May 8th at 22:52.
Here is the official Adobe tutorial webpage with about 27 Ultra CS3 tutorials!
I've never heard of the dark curtain trick before. Can you elaborate on that?
Like I said before, the flood from behind with no gel has worked for me. One of the problems I've encountered is if I'm using greenscreen on the ground, the green reflects up onto the front of the subjects legs and feet. That's much harder to key than the spill from behind. But I noticed it much less on bluescreen than green. I haven't done too much full body shots to worry much about it, but I plan on getting more into it. Any tips on that?
Here are some more chroma key green tips with sample clips:
You want as good a RAW video as you can get so strive for: no spill at all so control your lighting; you want to record as much detail / data as possible so get good exposure and sharp focus; with the HV20 if the subject moves too fast you will not get a sharp image and noise will be recorded and this noise is impossible to key; you want smooth rather than fuzzy / frizzy edges; white detail can be impossible to key and dark detail is easiest to key; you want good contrast; white colors readily picks up the green spill.
Here are some suggestions and thoughts to solve these problems: For sharp images I'm thinking using Tv60 as a good initial compromise and even a faster shutter if necessary. Then maybe after you've gotten your key and output your transparency video with alpha channel then use a Blur effect to make it look like Tv48.
I have my greenscreen spread out flat on a spare wall with a white wall on one side perpendicular to it. So I went to the fabric store and bought some $1.99 / yd. black cotton cloth and have begun to cover up the walls and ceiling.
I have a dark carpet on the floor. I plan to place my greenscreen lights high centered above and behind the talent with the ones on the right aimed left and the ones on the left aimed right. So the angles will be down and acute so the reflections will be down into the rug and more directly to the back of the talent rather than up and off the walls. The dark fabric walls will also absorb spill.
I mentioned I now have a crewcut. I wish I would have at least wet my hair and brushed it back when I made that chroma key video.
I'm sure there are other tricks.
Now I made another chroma key greenscreen video from scratch using the same video and still image as before. And I made another with my crewcut. These show some of the problems I mention above and some of the solutions.
This first clip is with the same greenscreen video but I did much better keying it. You can see that with just a little practice you can get pretty good in a short time. The problem areas in this clip have been narrowed down to the white sideburns, particularly the talent's right sideburn: there was significant spill and lack of detail. I just couldn't get the key in this area adjusted properly. The HV20 just didn't get the detail that is required to get a good key there. Look at the edges and color of the rest of the keyed video. I also slightly blurred the background image.
http://hv20.info/yopu/1080p_Ultra2aa.mov (16,604 KB)
http://hv20.info/yopu/720p_Ultra2a.mov (11,159 KB)
http://hv20.info/yopu/960p_Ultra2a.flv (2,164 KB)
And this reminds me that HDV compression is less than optimal for excellent chroma key. If you could use a RAID and shoot true uncompressed 24p 1920 x 1080 video your chorma key would be studio quality or nearly so.
Speaking of greenscreen chroma key, I just watched "Ghost" today and they use alot of chroma key. You can actually see lots of green spill in some of their shots. In particular, check out the subway scene where Patrick Swayze meets that other "dead guy" who shows him how to move objects. When this guy gets angry he leaps into a passing subway car. It is here where you can see a bright green halo completely around his image. There are others. Swayze shows up with a dark colored halo around him a few times. It seems the director chose to keep the detail with the halos rather than lose the detail with the halos. Check it out.
The second clip here is with my crewcut. You can see the excellent results when you don't have frizzy edges. And if you look closely you can see the problems in the hands when they move too quickly. Check out the last frame. The hands simply cannot be keyed at Tv48 when they move this quickly.
http://hv20.info/yopu/1080p_Ultra3a.mov (14,536 KB)
http://hv20.info/yopu/720p_Ultra3a.mov (9,795 KB)
So keep these things in mind.
I'm throwing this last clip in where I simply composite a video clip for the background if you're interested: a video on top of a video. You can see that the exposure and lighting does not match at all between these two clips. This is something that must be considered.
http://hv20.info/yopu/720p_Ultra2a_2.mov (11,483 KB)
Looks like fun. I liked the web pages.
I try to get the talent as far away from the green as far as possible by having an entire wall painted flat green. It doesn't have to be that bright green either. A little darker green reflects less light. Also a little DOF blur on the green screen allows you to get a tighter tolerance.
They do make (light peach) gels called 1/4 minus green, 1/2 minus green, etc but they are really for correcting color temperatures. For instance if you have HMI or florescent lights that are 5200 (slight greenish tint) instead of 5400 (sunlight) and you throw a 1/4 minus green over the 5200 fixture it will match pretty well with a 5400 one.