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Green Screen shots with camera movement where the background and foreground are perfectly in sync really help to sell the scene. Ideally, these shots are carefully planned with tracking points and precise camera information that you can utilize to achieve great results. But what do you do when your footage doesn't have those points, or has camera movement you don't even want? In this segment, we show you how to
track motion in your shot, sync your background with your footage, stabilize shaky footage, and keyframe motion manually.
Using these techniques can help sell your scene, and save otherwise unusable footage.
Pulling off good green screen work takes a lot of skills you might not expect, and being able to track and stabilize motion is definitely one of them. Whether your green screen shot has intentional camera movement you want to match, or unintended camera movement you need to stabilize, After Effects can make your job a lot easier with its built in tracking tools. Let's take a look at an example.
We've got our green screen footage here, and if I play it, you'll see there is some camera movement in the shot. We're going to stabilize this footage, in order to make the shot static. Double clicking on our footage will bring up the shot in our layer window. To Bring up the tracking controls, go into the window menu, and select tracker. Now click the track motion button, and use the track type dropdown menu to select stabilize motion. This will bring up the tracking selector. The cross hair in the middle of the square is the attach point. The inner square is called the feature region, and the outer square is the search region. The attach point is used to identify the exact point in your shot that subsequent graphics or background layers will sync to. The feature region defines the group of pixels you want After Effects to use for tracking purposes. Ideally, we want to pick an area that has a grouping of pixels with high contrast that remains on the screen throughout the clip. In this case, we'll use this white label against the black fabric of the lights that were left in this shot. When you see the cursor change to the 4 arrows, you're ready to move your feature region. I'll move this over to the area I've selected. You can resize the feature region by hovering over the corner points and clicking and dragging. Holding control or open apple and clicking will allow you to change the shape without regard to the center point. Once you've got your feature region surrounding the right area, you can adjust your search region. The search region is used to let After Effects know how wide to search for the featured region pixels from frame to frame. This becomes essential if your featured region moves a significant distance in one frame, and is also helpful if you are tracking a point that consistently moves in the same direction. If the movement is fairly gradual, it's best to keep this tight around your featured region. Now that we've got our selected area, we can analyze our footage. The best way to do this is start at the beginning or end of your clip. We'll move to the beginning of our clip, and click the analyze forward button in the tracker window. It looks like the track is pretty good. If your track doesn't follow well, you can click reset and pick a different area until you get better results. Once you're satisfied with your track, you can click apply. You'll now have the option to control whether the tracker uses the x and y parameters, x only,or y only. In this case, we'll use x and y, and select ok. Now we should be back in our main composition. If we play the footage, you'll see that we've successfully stabilized our actor. Depending on how much movement was in your original shot, you may need to scale your footage up, or consider placing 1080 footage in a 720 comp to avoid seeing the edges of your footage. From here, it's a matter of keying out our footage, masking out the portions of the shot we don't need, and adding in our background.
Sometimes, you may have a shot with camera motion you'd like to keep. The challenge here is to make sure your background moves in sync with your subject. Using a similar method, you can achieve this in After Effects.
Using the same clip, we'll create a null object layer by going into the layer menu, new, null object. We're going to apply our tracking results to this layer once we've tracked our shot. We'll select our footage layer and double click it to bring up the layer panel. Then go into the window menu, and select tracker. This time, we'll select track motion, and make sure the track type is set to transform. If your footage has a zoom or tilt in it, you can check the scale and rotation boxes to incorporate that data. In these cases, you would use two trackers on different points on the screen for the tracker to analyze the clip. This will add scale and rotation parameters to your tracking information. In this case, we'll uncheck the boxes, and select an area to track using the same criteria as we did for motion stabilization. First I'll move our attach point to the middle of our actor, then I'll move the feature and search region to the area we're going to track. I'll make some adjustments... and once we're satisfied with our selection, I'll click the analyze forward button to see how the track looks. Once you've got a good track, click edit target, select the null layer you created, click okay, then apply. Select x and y, click okay, and we should now be back in our original composition. If we select our null layer and hit "P" on the keyboard to bring up our position keyframes. You can see that the tracker converted it's information into keyframes to move the null layer around in sync with the shot. Also, notice that the null layer is positioned at the exact location we placed our attach point in the tracker. We can turn on our keying and saturation effects, and activate our mask to reveal our background. At this point, we can select our background layer and parent it to the null object. Parenting the background layer will cause it to move with the null layer. If you don't see the parent column you can right click near source or layer name, go into the columns menu, and select parent. Now you can select your null layer from the parent column dropdown box, or use the pickwhip by clicking and dragging your pointer to the null layer. If I play back the video... you'll see that our background is now moving in sync with our video. You can now reposition your background using scale... and position without affecting the tracking.
If your program doesn't have a tracking function, you'll have to do it manually. While this can be a time consuming process, and the results often fall short of automated tracking, you can still save footage that might otherwise need to be reshot. Let's take a look at one method using Premiere Pro CS5.
We've got our green screen footage on the timeline, and in order to track it, I'm going to create a solid video layer by right clicking in the project window and selecting new item, color matte. I'm going to change the resolution to be 200 by 200... click ok... select red for my color...25
and name it tracker... Now I'll drag my tracker layer into the timeline, place it above my footage. Now, I'll make sure my tracker layer is selected and go into the effect controls window. First, let's make sure we're at the beginning of our clip. If we toggle open our opacity settings, we can change it to 25. Now we can see the clip through our solid. Now let's drop down our motion effects and click on the stopwatch next to position to add a keyframe. You can click and drag the solid to move it around the shot, or use hover over the position values and click and drag. We'll move our box until we've got a nice point to track. The ideal spot will have a high contrast corner you can follow. I'm going to use the dropdown box in my program window and zoom in, then use the sliders to find my solid. Now that I've found a good spot, we can start tracking. Use the right arrow key to advance your video frame by frame, and move the center of your solid to match your tracking point as you go. This can be time consuming, but take your time and try to be as accurate as possible. Once you've tracked your shot, you can play it back to see if your solid follows reasonably well. If you need to adjust any points, simply go to the section on the timeline and reposition your solid. When you're satisfied, go into your timeline and go to the beginning of your tracked shot. Select the tracking layer, right click motion in the effect controls window and select copy. Now select your background layer, right click on motion in the effects control window and select paste. Now your background layer will move with your tracked point. To adjust the placement of the background layer, you can adjust the anchor point. We'll turn off our tracker layer, and play our footage back. And this looks pretty good.
Learning to track and stabilize footage is a great skill for any video project, and is particularly useful when trying to match or stabilize tricky green screen footage. In our next segment, we discuss advanced keying issues including multiple layer keys, and rotoscoping. These skills help tackle those keys that just won't be tamed by traditional keying effects.