View Full Version : Chroma keying - all the rules don't seem to apply here?
January 1st, 2007, 02:20 PM
I was watching this little demo :
When I greenscreen, I have been told to light it evenly and have the actor stand about 8-10 feet away from the screen to avoid spillage. In this video, this is clearly not case - and yet, the key comes out great! Hair looks good and it is nice and clearly defined around the body, hands and legs. I am not sure what program they are using to do the keying, but even with AE 6.5, even with the best greenscreening I can do-using tutorials, it never comes out that good. Am I missing something here? Am I right in thinking this is a great key, even though a really bad setup?
January 1st, 2007, 03:11 PM
obviously this a short demo to show from original take to final result, hiding a lot of processing. So it is not simple green screen.
the proof is that the mask showed in black and white is impossible to obtain with simple green screen as showed , since the right leg of the girl is not in the green part.
So if you are ready to heavily post-process your shoot, i see no problem to obtain such quality.
January 1st, 2007, 03:14 PM
What did they do to obtain such a great key then, considering they didn't really have the 'proper' setup for greenscreening?
January 2nd, 2007, 05:04 PM
1- Their key may be a combination of both a difference key and a chroma key. You can combine the best parts of both techniques.
A difference key can be done if the camera does not move. You need to take a clean shot of the background (without the talent in it), and then your footage with the talent in it. The difference keyer will compare the two shots. Where they are different, then obviously that part shouldn't be keyed out. A difference key doesn't work that well when the talent wears colors that are in the background.
Alex J Ferrari I believe occasionally posts on this forum, you might try asking him (and I believe their DVD has a making of section).
2- (I believe) Spill is not a problem in this case because the talent is lit in a way that the lighting overpowers the green spill. It may also be that if you light the greenscreen a little dark, it will have less spill (but you can't light the greenscreen too dark). I'm not sure what they put the greenscreen so close though.
3- In my opinion, you don't really have to light a greenscreen evenly. What you do need to do is to avoid dark pockets of shadows. Shadows never key well. Medium and overlit greenscreen will key fine, other than the extra spill.
4- The legs would be easy to rotoscope if they don't move.
5- The clips is a low-resolution clip, so it's hard to tell what's going on. It may be that the spill needed some work in getting the spill suppression right. Some of the better keyers can cheat the spill so that you don't notice it.
January 2nd, 2007, 05:48 PM
When I look at this clip I fail to see why a green screen was used (unless there was something behind the talent that needed replacing) as it looks like the backplate is the actual location that the talent was performing in. They wouldn't have had to do anything with the legs as this portion could be simply masked to get rid of the cable and fan. Also, we don't know what medium the clip was filmed on (DV, DIGI-Beta etc) which would make a difference.
I have achieved some great results using combustion. The key to keying is to split it up into portions. What is good for keying the hair may not work best for other parts of the image. Good book on the subject is Digital Compositing for film and video by Steve Wright.
January 2nd, 2007, 09:36 PM
I do understand that the shot is with a mobile greenscreen and takes place exactly in the same place/angle with a clean plate. I was just amazed at how well the hair looked and with no spillage that I could detect. Again, I was taught that the subject is to be farther away from the greenscreen and this was not the case. This further stumps me because in feature films, they have the actors walks on a green floor - therefore, the actor cannot be far away from the greenfloor - so how do they get rid of the obvious spill onto the legs? Always wear black for footwear ;) ?
January 3rd, 2007, 12:54 PM
Quality greenscreening is a combination of at least three factors.
Proper lighting - resolution of the camera sensor - and the resolution capacity of the recording media.
Quality lighting, a high rez camera, and media that records and preserves the resolution of the shot make greenscreen work pretty easy.
Where it's NOT as easy is if ANY of the elements are compromised.
If you're shooting standard 4:1:1 encoded 3.5Mbps DV, you simply don't have as much signal resolution as you would if you shot in 4:4:4 or another higher resolution digital video format. So pulling a clean key is "harder" but not impossible.
Many tools like FCP and other modern NLE's have evolved robust keying processes that can pull better keys from less robust signals than any time in the past.
But if your goal is real hollywood quality, you need to optimize all three of the areas I mentioned at the top of this post.
I suspect your example had the benefit of an excellent camera and a quality recording chain and that overcame any lighting issues better than a standard prosumer quality DV camera would.
Hope that helps.
January 3rd, 2007, 01:16 PM
Not everyone makes it easy on themselves during the shoot. Scheduling, staffing, budget and physical layout of a location can make it difficult or impossible to shoot an "ideal" plate. Compositors work to make up for the difficulties of real-life shooting conditions.
My understanding was that they used the green screen here in order to have flexibility to apply interactive lighting effects on the background independant from effects applied to the character.
As has been mentioned, there are many techniques for compositing.
True chromakeying isn't used too often. Vector keying (like Primatte) and color difference keying (ex.: Keylight) are very common. As is plane and simple luma keying and rotoscoping. (All the backgrounds for the cannable island in Pirates 2 were replaced using roto work to composit around all the spears, head-dresses, hair, foliage, ropes, etc. .. no greenscreen at all .... talk about not ideal! )
In this example, the compositor most likely used some loose roto masks to isloate problem areas and cleaned them up using keying parameters optimized for the foreground and background colors in each area.
Color spill should rarely be an issue. (bad edges due to over sharpening in camera or color compression can DEFINITELY be an issue).
Edge spill is genuine background color blending around the edges of a subject and is useful for accurately calculating how much of the new background should be mixed into these pixels. Despill and techniques like edge blending and light wrap are generally used to eliminate the "look" of color spill.
Yes, the ideal situation is to light evenly, avoid shadows and reflected spill. Doing these things on set makes the job of the compositor MUCH easier and faster. The reality is that these things don't always happens, and it's cool for the compositor's demo real when they manage to successfully overcome all the problems in a "messy" plate. :)
January 3rd, 2007, 01:24 PM
I do understand that the shot is with a mobile greenscreen and takes place exactly in the same place/angle with a clean plate. I was just amazed at how well the hair looked and with no spillage that I could detect.
One way to do this is to pull a luma key for whispy hair, then use this for a mask of a color solid that matches the hair color of the subject. Ta dah ... no spill.
Again, I was taught that the subject is to be farther away from the greenscreen and this was not the case. This further stumps me because in feature films, they have the actors walks on a green floor - therefore, the actor cannot be far away from the greenfloor - so how do they get rid of the obvious spill onto the legs? Always wear black for footwear ;) ?
Dark footwear helps, but it's never perfect.
Here's shot where the people were in fact laying on the greenscreen floor and you can see a lot of green reflection on downward facing areas of skin (the girl's arms and under the neck for the guy). The solution was to use despill calculations to lower the green color in relation to the other color channels for the comp.