Motion Tracking & Object Tracking inside Cinema 4D: The Importance of a Survey Shot

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In this video, I will explain what a "Survey Shot" is, and why it's so important to the "Matchmoving" process.

In this video, I will explain what a "Survey Shot" is, and why it's so important to the "Matchmoving" process.



In this video, I will explain what a survey shot is, and why it's so important to the match moving process. In order for the motion tracker to do its job better, and provide us with a more accurate 3D solve, it allows us to feed the 3D solving algorithm with the camera lens focal length. If you have your lens spec sheet, you can probably find the focal length, but there's a problem. The named focal length of the lens itself is not the actual focal length of the lens-camera combination. The actual focal length is calculated by multiplying the lens focal length by the focal length multiplier which is derived from the sensor size or something like that. Just to get an idea of how large the discrepancy is between named and actual focal lengths for different senses. Just search for focal length calculator on the magnificent web, and you'll see what I mean. What is the solution, then? Well, simply let the tracker calculate it for you, but immediately you would ask me, "Why did we go through all of the above if we could calculate it in the first place?".Well here's why, this is a shot we are going to track, and let's play forward and you will see that although it's a pretty stable shot, we have certain things which don't help the tracking process. For example, we have this girl in the foreground. We have all sorts of things in the background which don't provide us with great accuracy and because these are trees maybe there's a bit of a breeze and they're moving around and so forth. We have a background that hasn't had any handmade trackers added to it. So we have to rely on whatever was there to do the tracking. So what I'm trying to say here is that when you're shooting your movie, you cannot adapt your artistic flavor or your storyline to cater to the algorithms of a motion tracker, that's not the way to do it. What you can do though is the following. With the same equipment and when I say equipment, I mean the same camera and the same lens. Go and shoot something that doesn't have any of the problems that this shot may have. Let's go here, and I'll show you the survey shot that was shot for this particular shot. So, the same person that shot the previous movie, he actually just did this, he put the prop on the floor. He added a few bricks. He made sure there are some tracks here some features that will help the camera. They have good contrast and so forth. And what's going to happen is that when we track this scene which is a very easy scene for a camera tracker to track. We can then extract that information and then go and feed it in the solving part of reconstruction. We're going to talk about this later on. But that is what a survey shot is. It's a very sterile scene that is catered for motion tracking that will allow us to extract some information providing you use exactly the same material. So please don't go and shoot your survey shot with a different lens, a different camera or a different focal length and then put that number in a scene that's been shot with different equipment and expect anything good to come out of that. In some cases, you may even use a survey shot to extract the 3D shape of your prop. As you can see, this prop here is exactly the same one that's been used over here and because this is moving, it will be more difficult to extract its shape and dimensions from this scene, so you can go and do it here. So the two main reasons for a survey shot is to extract some information about your equipment and possibly even making a 3D model from your prop, and use it to add it to your main shot.
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