Picking the Main Frame

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Picking the Main Frame

Before you create a planar tracker, you have two important decisions to make: on what frame to create it, and what type it will be.

The frame where you create the planar tracker is called its main frame. The image contained in the four corners will be used as the image reference for the rest of the shot, to exactly re-locate the tracker in the other frames. (You can set additional reference images, but main frame is the first and most important!)

For a 3-D planar tracker, the corner placements on the main frame determine the camera field of view for the entire shot (unless there's a zooming lens). And they determine the physical aspect ratio of the 3-D planar tracker rectangle (in the real on-set world, not the image).

Here are factors to consider when picking the main frame:

 You do want the planar tracker bigger and closer to the camera, so there is good interior resolution. (If it is far away, it may only be 10 or 20 pixels across, and not sharp.)

 You do not want it so close that parts are off-screen.

 There is a performance cost to processing a lot of pixels, so you may not want to pick the exactly largest frame if it is unreasonably larger than most.

 You do want the camera to be looking generally perpendicularly at the plane, not edge on—edge on will produce a blurry low resolution reference image in that direction.

 You do not want the camera looking exactly head on, perpendicular to a 3-D planar tracker. If so, there is no perspective and therefore no ability to determine field of view and aspect ratio.

 You do want the plane in a position where there is some perspective present, to help determine field of view and aspect ratio.

 You do not want the main frame to be occluded by anything, if at all possible.

So it's a helpful list, for each piece of advice the opposite piece as well. This is why you are a tracking artist, not a robot.

You should scrub through the shot quickly to determine a suitable frame to use as the main frame, stop on that frame, and proceed to the next step.

Note that later after you have created the tracker and perhaps several key frames, you'll be able to distinguish the main frame on the main time bar by the taller key marker on the main frame.

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