- The "Picture" clip
The key to mokey is understanding the implications of the following sentence:
The tracker in mokey is a 2D plane tracker
Until you really understand what this means you will most likely not get very good results when you use the remove, matte and stabilize module. If you don't after reading this chapter, please give us a call. We want you to understand.
Please refer to the manual for a full explanation of the differences between a 2D point tracker and a 2D planar tracker, how to make selections and choose the right tracking parameters.
Now let's start with the Picture shot:
We want to track the picture frame, but as you will notice, in the first frames there is a strong reflection that moves across the glass and it moves differently to the plane we want to track. We therefore want to avoid including the reflection as part of the layer that selects the frame we want to track.
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Image 1 - First frame in the Picture shot |
Image 2 - A later frame. Note how the highlight is moving across the glass |
- Select the picture with one selection and make a selection of the reflection on top of it. Remember that later selections, or selections that are on top of other selections, subtract from the garbage mattes of earlier selections where they overlap.
- Make the selection of the background such that it does not include the reflection. We will use this approach, as it is an opportunity to introduce you to the 'add contour to selection' feature:
- We start by making a selection
that covers the upper left-hand corner of the picture frame. Then, instead
of adding a new selection, we add a contour to the existing selection
by clicking on the Add Contour tool
in the toolbar.
Image 3 - To avoid including the highlight that moves differently to the frame in the selection, multiple contours were added to the selection. Note that all these contours belong to the same layer and are treated regarded as being on the same plane.
The new contour is place over the upper right-hand corner of the frame. This is repeated for the last two corners. Note how all the contours of a selection appear active when the selection is selected, whereas other selections will appear inactive.
Having made the selections, we now go to the Track tab and choose the Perspective motion model from the Motion Model group as we see that the picture undergoes perspective changes through the shot. We can now start tracking!
Once tracked, the best way to evaluate the tracking of a layer is to fully stabilize the layer. If it is rock solid we know that the tracking went well, if not we may have to go back and make adjustments to the selection to track a slightly different area, or change parameters such as Max % Pixels Used to improve the accuracy of the tracker.
One last word of warning: Remember that moving the control points of the selection does not change the tracking data for the layer!!
Exporting Tracking Data
You can export tracking data in a variety of formats from mokey for use elsewhere. You may not have noticed yet, but each selection has a set of tracking reference points. If you are using the affine motion model you will find three crosses at the edges of the frame, two in the bottom corners and one at the centre of the upper edge of the frame, if you are using the perspective motion model you will find four crosses at each corner of the frame.
These reference points can be use as a guide to evaluate tracking, and can be positioned anywhere on the frame. Apart from giving you a way to evaluate the tracking, the also define which points are actually exported when you export the tracking data as a four-point track.