Since no two camera lenses are the same, a large black-and-white grid should be shot for every lens that was used to shoot the film. So if the main and 2nd unit both used the same range of lenses, it is still necessary to shoot a unique grid for every lens in both kits. By doing so, the unique lens distortion created by each lens will be obvious based on how many lines of the grid get bowed and distorted toward the edges of the frame.
For best results, the grid should have the same aspect ratio as the film or digital cameras that were used. For example, a grid that is 4 feet by 3 feet can be easily filmed to fill the frame of a full aperture film frame of 2048 Å~ 1556 pixels as it shares a similar aspect ratio. When filming the grid, it should be placed against a flat surface, such as a wall, and the camera should be positioned along a dolly track until the grid fills the camera frame edge to edge.
Then in post, if a plate has a lot of camera distortion, use this data to create a version of the plate with the camera lens distortion removed for camera tracking and 3D element creation. This is done by matching the serial number of the lens used to shoot the plate with the corresponding grid that was shot with that same lens.
Using the compositing software tool set, undistort the grid until the grid lines look completely straight and absent of any distortion or bowing.
Now when the CG elements are added into the composite, simply apply the inverse of the numbers used to undistort the plate to actually distort the CG elements by the amount needed to match them back into the original plate. Voilà!
A lens chart can be as simple as a series of grid lines or a checkerboard.
HDRI AND CHROME BALLS
Matching to the practical outdoor and indoor lighting used to shoot the film is one of the more difficult tasks required for the visual effects teams to make their elements photoreal and fit them seamlessly into a shot.
One relatively quick method is to shoot a chrome ball on set for each camera setup. It is a quick way of seeing where the light sources are coming from and how the object reacts to light. It is also a good idea to paint half of the ball with a matte gray to see how dull surfaces react to the same light. When shooting the ball, the data wrangler can simply hold the chrome side of the ball up to the camera for a few seconds and then rotate the ball to reveal the matte gray side for a few seconds.
The advantage of using the chrome ball is that it can be done quickly for each setup without holding anyone up. It can be done any time but most often during the slating of the shot or at the very end as the last take. The disadvantage is that the chrome ball simply provides visual reference of where the lights were on set and how shiny and matte objects respond to that light.
Another technique, which provides a lot more information when re-creating a digital version of the position and intensity of the set or location lighting, is to use HDRI (high dynamic range imaging).
By photographing the same scene with a wide range of exposure settings and then combining those different exposures into one HDR image, an image is created that represents a very high dynamic range from the darkest shadows all the way up to the brightest lights.
Many visual effects houses have created their own proprietary software that can use these HDR images to calculate where the lights need to be placed in a scene and how bright they need to be. This technique has great advantages over the simple chrome ball because it greatly improves the ability to re-create photorealism and accurately light visual effects elements in a scene.
The disadvantage of taking HDR images on set, however, is that it can take a few minutes to set up and take all the bracketed photos needed and crew members should not be walking through the scene during this time.
If shooting on a set in which the lighting does not change, simply take the HDR photos during lunch hour without disrupting anyone.
However, if HDR images are shot for an outdoor shoot with a constantly moving sun position, it can be quite difficult to get the set cleared after every camera setup.
So it is a good idea to still use the old-fashioned chrome ball during the slating of each scene as a backup, and grab those HDRs whenever there is a window of opportunity.
ON-SET ANIMATION CAPTURE: WITNESS CAM (IBMC)
A witness camera setup generally refers to the use of one or more video, high-definition, or motion digital cameras on a live-action set for the purpose of capturing the motion of one or more of the actors while the production cameras are shooting. This technique is also known as Image Based Motion Capture (IBMC). The preferred method is to use two to four cameras to capture true 3D information. The action from the various camera perspectives is tracked and converged into one set of 3D motion data after the shoot.
Capturing animation by tracking the view only from the primary production camera is possible, of course, but it is not as accurate, and an animation with conflicting information may result. For instance, because it is not possible to capture distance from only one point of view, the lower part of a body may resolve as being farther away than the upper part of the body when in fact it is the same distance.
It is appropriate to use one or more witness cameras when there is a need to capture more motion information than can be obtained from the production’s primary or secondary taking cameras. Using one witness camera will only work if it is used in conjunction with the production camera. Using four witness cameras will aid in the motion solve and prevent interruption of data if one or more of the camera views becomes obstructed.
Generally, witness cameras are used to gather 3D information.
However, they can also be used as an aid to 2D tracking when the view of the subject from the taking camera is partially obscured or in and out of focus. In these cases a sharp unobstructed view of the subject from a nearby witness camera can provide the track (this assumes that either the taking camera is static or that it can be stabilized).
SHOOTING VIDEO AS A REFERENCE
Shooting video for reference sounds like a great idea for tying loose threads together, putting a helpful perspective on the production shot, or recording some detail in real time as the camera rolls.
However, video reference can quickly transform into hours and hours of unusable and incomprehensible material. This is not to say that video is without its use, but many factors need to be kept in mind in order to make video valuable:
1. First and foremost, get a slate shot for the video – preferably at the head. If this video pertains to a particular scene and/ or sequence make sure that information is on the slate. Verbally reading the slate as the video is slated (since the video camera also captures audio) is a great idea. It might make a hastily written slate easier to understand. Slates should always have the current date, time of day (to coordinate with other time-of-day information), and production shot number matching the production slate used to film/record the scene. Additionally, this slate should contain unique identifying information such as the set or location name. If the video is shot for a particular purpose, it is always good to put this information on the slate as well as recording it verbally.
2. The second priority is to use video precisely. This suggestion cannot be stressed enough. The most common reason video becomes useless is that by the end of the production there are hundreds of hours of video that someone must search, log, copy, edit, organize, and/or perform other arduous and repetitive tasks on to make it usable. Also keep in mind that the person organizing the video may not be the one who shot the video, therefore the purpose and end usage may be outright impossible to understand later. As a result, the time spent shooting video has no benefit because it lies unused, misfiled, or thrown away due to the time-consuming, frustrating, and often impossible task of organizing and deciphering it.
Another issue is the willingness and ability of later, post-production artists to watch the video. In many cases an alternate to video, such as two or three reference still photographs, photogrammetry, or other standard data gathered on set, would provide a more precise method of capturing the same information. Standard methods are likely to prove to be more useful and easier to access, and usually mean data gets to the proper artists instead of some general reference that becomes unused because it is not clear who should see it. In this sense shooting precisely is the key to successful video reference.
Important to remember: Shoot just enough to get the job done, and keep it organized and logged to insure it goes to the proper users.