Camera tracking
From Free net encyclopedia
Template:Compu-graphics-stub Camera tracking or matchmoving refers to the process of matching the position and angle of CGI (such as a spaceship or animated creature) to real footage shot with a film or video camera. This process can be accomplished manually, on a frame by frame basis. More common today is the use of automated matchmoving software based on computer vision techniques that can extract camera position and direction data over time from a piece of digitized footage with minimal user intervention. Done correctly, matchmoving allows the seamless compositing of CG and bluescreen elements into camera footage, even handheld camera footage.
Camera tracking takes a moving video footage, analyses the direction and the rate of movement of the pixels that make up the objects in the shot. From that analysis, it derives the distance and angle that the camera was from the subject when the shot was filmed. With this data it will reconstruct a camera in 3D that tries to match the movement of the camera in the video footage as closely as possible. Once we have the camera position and trajectory in 3D, we can insert 3D elements into the shot and it will match seamlessly with the real video footage in position and perspective.
Camera tracking usually apply to shots that involve moving cameras. If the camera in a shot to be composited is static, then the CGI artist only needs to match the position and perspective once, in the 3D animation software, and the rest of the objects placed in the scene will have the correct perspective as well. However, the difficulty comes when the shot is moving. In this case, the 3D artist would need to match the shot frame-by-frame with his human eye, so that the 3D camera has the correctly matched perspective in each frame of the composite. There are several disadvantages to this. Firstly it is tedious and time-consuming. Secondly, it is prone to errors in perception on the part of the artist that is doing the manual camera match. With the match-moving software, tracking is much faster and more accurate because there is advanced mathematical algorithm behind the tracking software.
Match-moving works by the principle of parallax. Abiding to this physical law, when we place our camera in a moving vehicle and observe static objects, the objects nearest to the camera will move across our screen the fastest, (taking the shortest time to move from one end of the frame to the other). The furthest object will travel the slowest, (taking the slowest time to get from one end of the frame to the other). When we give the match-moving software a moving footage, it will use this principle to track fast and slow moving points in the shot and try to derive the angle, distance and speed which the camera is moving. The above mentioned method is used just for solving tracking shots. More complex algorithms in the software will solve more complicated camera moves in the shot like panning, tilting and zooming. The software will also need to compensate for lens distortion in the footage, if the camera was using wide-angle lens which exaggerates perspective, or narrow-angle lens, which flattens perspective.
The matchmoving software will have difficulty with accuracy under these circumstances: 1. Poor quality of the footage shot. This could be due to compression or encoding problems for digital footage. For film it could be caused by scratches and dirt on the film, or light leaking into the camera when the film was shot, or the film not properly seated in the film sprokets when filming. 2. Movement of objects in the footage is too fast, producing motion-blurring in the shot. This makes tracking quite hard because the track point now becomes a blurred. 3. Change of focus in the camera. When the focus changes, different portions of the image becomes blurred, and the other blurred portions become sharper. This can make the software loose track of the points 4. Changes in illumination/colour of the track points. In the course of the shot, if the object suddenly goes into a darker/brighter area, or the light that illuminates the object changes colour, the software may not follow that change and loose the point. 5. Tracked points become occluded. When another object gets between the camera and the points being tracked, the software will loose the track-points.
In the case when the result of the matchmove are less than satisfactory, many software have tools for you to help it along, to tell it where the track points are. It may also allow the user to input camera settings like the type of lens, the focal distance, the height of the camera, the known distance between certain track-points to the camera, or distances between specific track-ponts. All these will help to achieve higher accuracy. With this in mind, film productions that require match-moving, many camera assistants are required to take down the camera settings and distances between the camera and certain objects for those shots. Very often, after the software has solved the match-move, a match-move artist will need to step in to refine the animation of the 3D camera, to refine its trajectory.