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I have restarted Nuke and reloaded the Camera Track training clip so we can take a closer look at the first step in the process, setting the track features. So we will add a Tracker node, select the Read node, go to 3D tab and select CameraTracker. The first thing we want to do, before setting the tracking, is to set the Analysis Range. By default, it's going to track the entire length of the source clip and it really means it. If you have a 500 frame clip with a 100 frame timeline, it's going to track all 500 frames.
So if you don't want to track the entire clip, select Analysis Range, and then enter the start frame and the end frame which you want to analyze. We'll put that back to default. Now, let's take a look at the Tracking tab. The Tracking tab contains two parts; the Features and the Tracking. The Features control how many trackers are placed on the screen and their distribution. The Tracking section controls the mathematics used to analyze the tracks themselves.
So let's start with the number of features. In order to see what we're doing, we'll turn on the Preview Features. To see these better, I will turn the Viewer Gain down. This allows you to see where the Feature Selections are going to place Trackers. As we adjust the Features values, you will see these trackers changing. Starting with the number of features, 150. When you finish, you want to have about 100 good tracks, and a lot of tracks get rejected over the process. So if you're going to lose a lot of tracks, you might want to increase the number of features to, let's say, 300. And now you see we have a lot more trackers on the screen.
I will put that back to default. The Detection Threshold set the distribution of features over the input image. If you enter a low Detection Threshold value, features will be tracked evenly of all parts of the image; a higher number, they will clump. Next, the Feature Separation. This forces the features to spread out, and cover the screen more uniformly. For example, I'll put in a low value here, 3, and you see how they tend to clump up over tracking features in the picture. I will set a high value like 24 and it forces them to spread out and cover more of the picture.
But, I am going to put it back to the default of 12. To show you the refined feature locations here, we will need to zoom into the picture. When you turn that on, it causes the trackers to snap to the nearest corner. If you have a lot of corners in your picture like mine, you might want to leave that on. I am going to turn that off and re-home the viewer. Sometimes you want to place trackers in particular locations. You can do that by planting what they call a seed track. With the cursor over the part of the picture you want to track, right mouse pop-up, select seed track.
We don't see anything, because Preview Features are on. So if we turn that off, then and only then can you see your seed tracks. So I will come over here, right mouse pop-up, add a seed track, over here, right mouse pop-up, add a seed track. Then I come back and turn on Preview Features, my seed tracks disappear. So don't forget, when you want to plant your seed tracks, turn off Preview Features first. Next, we'll take a look at the Tracking settings here. The Minimum Length rejects all tracks that are shorter than this value.
So if you have tracking data that have lots of real short tracks, raise the Minimum Length. The Track Threshold controls how similar features look over a number of frames. If the features are changing their appearance, you might have to lower this Threshold, so you can keep more tracks. This parameter rejects tracks that jitter too much. So if you have a lot of jitter in your tracking data, and you want to keep them, you would raise this value, so it retains more of the jittery tracks. Track Consistency will reject tracks that do not have local consistency.
If you raise this value up, you're making sure that your tracks are locally consistent. And last is Track Validation. This pertains the kind of camera move in your scene. By default, it's a Free Camera, but if the camera was mounted on a fixed tripod, and not moving, it might be panning left and right or tilting up and down, then you would choose the Rotating Camera option. If the camera is moving through the scene like this one, it's on a dolly, or a boom or maybe it's handheld, then you want to choose the Free Camera option. Okay.
Once the Tracking Tab settings are done, we go back to the Camera Tracker Tab, and select Track Features to track the shot. Next, we'll see how to refine the tracking data when doing the camera solve.
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