Join Steve Wright for an in-depth discussion in this video Step 1: Analyze the clip, part of VFX Keying: Master Course.
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- [Instructor] This chapter is about pre-processing the greenscreen or bluescreen to sweeten the clip for the keyer to get a better key. The first step is to analyze the clip to plan your keying strategy, select the type of keyer to use, and identify any special issues. You want to find these issues before you start keying, not discover them halfway through. Work smarter, not harder. Okay, we're going to study the clip and we're looking for things that change over the length of the clip.
So as we step through here we notice, well, here I got a whole bunch of motion blur. While the key that worked for the body may not work so well for the motion blur so I got to be thinking about that. As we cruise through the rest of the clip we see that the head and the hand go up above this line so I'm going to need some Roto work. So that's the idea, cruise the clip looking for changes. Next, let's take a look at the backing screen characteristics.
Uniformity, this is not a uniform backing color, the keyer is not going to like this. This is also not uniform. The steps have different shades of green on the top, the sides, and the front. And of course, we have this one. The keyer is not going to like this one at all. So these un-uniform backing regions might require the IBK keyer, or perhaps some Roto, or even heroic rescue efforts.
Next, let's take a look at even lighting. Now, in this clip you can see that the lighting is not even, you get a falloff over here in the dark area which will be a problem if the character moves in that general direction. But what about this clip? Has this got even lighting? Well, we're going to want to measure and the scopes are not terribly helpful here so I've set up a little thing. I'm going to hook my Viewer up to this ToggleSwitch, open it up, and then open the Sampler.
You'll see what that toggle is for in just a second. So the Sampler is going to sample a horizontal line across to here, from edge to edge, wherever I have this button. Now, it doesn't care where I put it left and right, it's simply going to sample wherever it is vertically, right straight across to horizontal line. Now because of the grain the lines look a little choppy so I like to put in a Blur Node which cleans things up very nicely. So, very uniform left to right but what about top to bottom? Okay, for that I put in a little toggle to flip the image over using the Reformat Node and we'll sample it down here.
So it's going to cross this away which is top to bottom in our sideways picture and there we see we have a very uniform lighting so this is very nice and we put that back. Now, let's take a look at the plate from "Clockstoppers." Well, we'll sample up here and we can see very uneven lighting. It gets noticeably brighter on the right that would be up here in this part of the picture. So let's flip it on the side and take a look at it from top to bottom.
Whoa, it's really hot at the top and gets a lot darker towards the bottom. So the keyer is going to have to take this into account. This might need some heroic rescue. We'll see how later, in our pre-processing chapter. Okay, let's take a look at the question of purity. I'll hook my Viewer up to this guy and open up this Sampler Node, closing the other one. If I sample on the blue here's what I'm looking for in the way of purity.
The primary color is up here and the two secondary colors, the green and the red, are down below. There's a good gap between the primary and the secondary, that's the color difference that the keyers are looking for and that the two secondaries are fairly close together. If we take a look at a green screen, again, I've carefully chosen very pure colors to give you an idea of the ideal state. So there's our green primary color, our two secondaries, a good gap, and the two secondaries are close together so that's what we love to see.
Will we see that here? I think not. Eww, not only do we have a falloff in brightness to the right but you can see how high the red channel is. So the color difference between the green and the red is not very good so this plate is going to need some heroic rescue to make a good key, I can tell you that. So let's come over here to this picture and let me get my Sampler Point back into frame as this is a smaller image.
Okay, the green is way up here, it's very uneven. The red and blue are very low, in fact, there's something funny going on with the blue. Let's take a look up here in the Viewer at the red channel, the green channel, and the blue channel. It's gone, there is no blue data. Keyer lost data, this is going to be a problem child. we'll take a look at this one here. Okay, and I'll move my Sampler up here to the top, let's push in a little bit to get a little better thing going on here.
Okay, well, my primary color does not have much of a difference between it and the two secondaries so the keyer only has this little color difference to work with. This is not going to be fun to key. Exposure, let's take a look at exposure. So this is Will Smith from "Wild Wild West" and you can see that this thing is very, very dark so let me move my Sampler Point back into frame and see what we got here.
Oh dear, look at this, very little color difference so this is going to be a very difficult key. However, Meg Ryan is way overblown, I mean, look at this, okay. I'll adjust my Sampler Point here, in fact, the secondary colors are low and together which is nice but where the heck is the primary color. I mean this is code value one, the others where down in here or lower. Well, it is so hot that it is way off the top of the screen here.
This is going to be a very serious problem key, it is so wildly overblown. Next, we'll take a look at the grain. I'll close my Sampler window here and let's push in on this clip. And let's take a look at the red channel. Okay, that looks pretty good, then we'll take a look at green channel, not bad. And of course, as we learned in the previous section the blue channel has all the noise and the chop so we're going to want to degrain this guy for sure.
So once we set that back here, next, let's take a look at the content issues. Ha, you thought we were done. We're just getting warmed up. Shadows, shadows are a problem. Usually the key setting that gets you a good character will not get you a good shadow. Worse, the shadow is over hot spots over here and darker spots over there so the keyer setting that works in this region may be not so good in that region. And these shadows even go off the blue screen, now this happens all the time.
They shoot stuff on a blue or green screen and stuff goes off the edge, we still have to fix it. Another content issue to be aware of, depth of field. A keyer setting that gets to be a nice character map is probably going to chew in, choke in, and cut off all my depth of field here so we might have to isolate this character and give him a separate key. Same exact issue with motion blur. Okay, the keyer is not going to like motion blur if it's set for the sharp characters so very often this is going to require separate processing.
Also take a look for transparency. Is there any transparency in the shot? Now most keyers do a great job with transparency. Here's some green screen transparency. The thing you got to watch out for is the green screen reflecting off the glass, punching a hole in your matte. We'll see that in just a minute. Semi-transparency is another issue and again, most of the keyers will do a wonderful job with semi-transparency including fine hair detail which is also a transparency problem.
Now here's a transparency problem with some smoke. Now if there was another object in the frame, the keyer setting that got you the object well keyed may not work on the smoke so we might need two separate keys here. And then there's this. This is an Indian visual effects shot and they turned on the rainmakers over the blue screen so now we have no purity and no evenness. This is going to be a very difficult key.
Next, backing kicks. The backing color hits shiny, reflective objects and creates a hole in the matte. Now this green screen plate from "Sphere," it's a little miniature, we have green kicks right here on the glass and they're going to punch a hole in the matte right there. There they are, green kick, hole. Here's another example.
This guy has buttons that are reflecting the green screen so when I switch to the Alpha Channel you can see I've got holes where the buttons are. Of course this will be an easy fix we can just hand track a Garbage Matte over that and fix that, no problem at all. Now here's an interesting one, this guy, he's a vampire character, they decided to light his eyes with a spooky vampire color which of course the keyer saw and punched out his eyes.
So again, we'll just have to do a Garbage Matte over that and track it in. And then there are just plain dumb things. Here's Harrison Ford in "Air Force One" and they smoked the set. And this, of course, destroys the purity of our color. This is going to make a terrible key. Video issues, interlaced video, you absolutely do not want to try to key an interlaced video.
The reason is, you cannot use any of your neighborhood operators like a blur, or a dilate, or an erode. There are, however, much worse video artifact out there. There's a lot of people shooting visual effects that don't know what they're doing and they're using low-grade, prosumer cameras like DVC-Pro. The problem is these cameras undersample the color and they compress the heck out of the picture so this may not look too bad and if we look at the green channel, why, that's not too bad.
But if we look at the blue channel, look at this rim. The keyer is going to see that and go ballistic. And then wait till you see the red channel, this is disastrous. This black and light edge comes from edge ringing artifacts because the camera operator left the camera sharpening on during the shoot and that introduced these edge artifacts. These are notoriously difficult to fix but we'll take a look at some techniques later in the course for trying to cope with that.
And then there's additional work. Do I have to do any Roto? In this case, we're going to have to Roto everything above this line or tracking markers. The problem with tracking markers is everybody does them differently. In fact, we have a video coming up later in this chapter on dealing with tracking markers. And here is tracking markers from that Indian movie. Now this is an anamorphic plate so our tracking markers are squeezed but that's okay.
My objection here is, due to depth of field, the tracking markers are out of focus. So a Point Tracker will have a very difficult time locking on. Perhaps a Planar Tracker could lock onto this plate. We'll have a whole video later in this chapter on dealing with marker removal. So analyzing the clip is always the first step in keying a green screen or a blue screen in order to plan your keying strategy and identify what pre-processing you might need. The most common pre-processing operation is to degrain the clip which is the subject of the next video.
Note: This course was created by Steve Wright, author of the seminal book, Digital Compositing for Film and Video. We are proud to host this course in our library.
- Creating an uber key
- Keying green-screen vs. blue-screen footage
- Preprocessing footage
- Building a clean plate
- Making luma keys
- Keying on hue and saturation
- Pasting keys together
- Grain management
- Saving time with garbage mattes
- Using spill suppression
- Improving edges
- Color correcting keys
- Sweetening the comp
- Alternative compositing workflows
- Fixing edge problems
- Using KEYLIGHT, Primatte, Ultimatte, and other tools