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Eliminating motion artifacts using Detailed Analysis

From: After Effects CS5.5 New Features

Video: Eliminating motion artifacts using Detailed Analysis

Now let's take a look at a Warp Stabilizer shot in which the Subspace Warp option is a necessity, but it also causes too much distortion. This shot contains heavy parallax and strong rolling shutter artifacts. Rolling shutter is a particularly nasty artifact. It comes from cameras that have a CMOS chip and that chip scans from the top to the bottom frame instead of taking the entire image simultaneously. So camera such as the RED camera, any DSLR, or even the iPhone, will show this artifact.

Eliminating motion artifacts using Detailed Analysis

Now let's take a look at a Warp Stabilizer shot in which the Subspace Warp option is a necessity, but it also causes too much distortion. This shot contains heavy parallax and strong rolling shutter artifacts. Rolling shutter is a particularly nasty artifact. It comes from cameras that have a CMOS chip and that chip scans from the top to the bottom frame instead of taking the entire image simultaneously. So camera such as the RED camera, any DSLR, or even the iPhone, will show this artifact.

Let me just preview this shot so we can see what we're dealing with. So we have a couple of different challenges here. Any shot with heavy left to right motion taken with a CMOS sensor is going to have rolling shutter problems of some degree. This particular shot was taken with a Canon 7D and so we know that's a factor here. But there are other issues with this shot as well. Notice that the cameramen did a pretty good job of keeping the horizon steady in the shot, despite bouncing around on the back of a jeep.

But in the foreground the figure is moving around a lot. There is heavy jitter on him. So in other words, we have different perceived amounts of motion on him and the far horizon. I'll go ahead and run a stabilization on this shot. This time I am going to right-click and choose Stabilize Motion from right here. The process is the same. We have a blue banner and progress is shown in the effect controls. Because this is a long shot and quite a bit more complicated than the previous one, we are going to skip ahead to the final stabilized shot.

Now that we have this shot stabilized, the foreground is remarkably good. There is a strange wobbliness around the edges of the guy at certain points, but my real problem is with the horizon. It's now bouncing around in a really distracting rubbery kind of way. When you see these kind of rubbery weird distortions, your first thought should be to blame Subspace Warp, because that is where Warp Stabilizer is actually changing the pixel data. In this case, bumping this down to the next option down, Perspective, which doesn't warp the pixels, causes a different problem.

Sure, the cyclist is still looking good, but now the horizon is going crazy. This is the problem of parallax. There's a fight going on between the amount of motion in the foreground, which was more pronounced and is now more stable, and the background, which now has to have more pronounced motion to move along with the foreground. That's why this is a case where you really need Subspace Warp. But what to do about all the wobbliness? Under the Advanced section is this little Detailed Analysis toggle.

Now, as soon as I activate this, it's going to trigger a reanalysis of the entire clip. Before I do that I want to show you the option right below it, Rolling Shutter Ripple. You can use Automatic or Enhanced Reduction. Now, this is a case where we know that we're going to have significant Rolling Shutter Ripple. So let's start by just bumping this up to Enhanced. That just triggers a new stabilization pass, which is much quicker than the reanalysis.

We will preview that. Well, he is looking a bit better, but I've still got major problems with that horizon. It looks like it has got a sort of weird rubber band motion going through it. Now I am going to check Detailed Analysis and again we are going to skip ahead because of the time required to reanalyze this long clip. So this result, while not perfect, has certainly done an effective job of dealing with the issues that we saw.

We no longer have a distracting amount of motion either on the foreground figure or on that horizon. Could the shot be made better? Certainly. One thing to look at is the amount of Auto-scaling happening here. I think it might be appropriate in this case to loosen the shot up a little bit using some of the methods already discussed. For example, lowering the amount of Smoothness and experimenting with that versus the amount of scaling happening. Overall and particularly in cases with heavy parallax, which is this big difference between the foreground and background motion, and any shot taken on the CMOS camera, such as the DSLR, RED camera, or the iPhone, the Advanced Detailed Analysis and Rolling Shutter Ripple controls can make a huge difference.

Why not leave them on all the time? Well, Detailed Analysis is certainly much slower and for many shots these additions will actually overcompensate and add artifacts that don't even need to be there. Keep Detailed Analysis as a kind of secret weapon to use when things just aren't lining up properly regarding parallax. For cases where you know you are going to have rolling shutter, like this one, Enhanced Reduction can help reduce the amount of wobble throughout the shot.

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After Effects CS5.5 New Features

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Mark Christiansen
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