This is my final post about Mocha for a while, I promise. I feel that after honing these skills I’ll have a decent enough grasp on Mocha and Planar tracking to use it freely within my own projects.
Anyways, the following explains my methods for countering two more situations that can make Planar tracking within a shot difficult – both are based around the tracked plane leaving and entering the shot.
Fortunately, Mocha includes several visual aids that can help allign a surface even when it out of shot. (Either partially or even fully.)
Situation 1: Our Tracked Plane Partially Leaves Frame
1.) First, I moved to a point in the footage where the entire plane I wished to track is in Shot. In the case of the first example this was fortunately the first frame.
2.) Next I drew out a spline and carried out the initial track. On the frame where I began the track I then switched on “Surface” and positioned corner pins over corners of the plane. Under “Layer Properties” in Mocha I selected the checker surface pattern. This is a great way to check for distortion.
3.) Next, I switched on Mocha’s “Grid” overlay. (Via the “View Options” panel). The pink overlay grid’s position is relative to the blue “Surface” corner pins.
4.) I positioned the Blue corner pins so that the pink grid was lined up with the edges of my plane. The surface corner points no longer matched the corners of my surface as a result but that’s okay. The pink grid lines form an excellent reference for perspective within the shot.
5.) After playing back the sequence I identified areas where distortion was occurring and found the last frame prior to this where the grid was still alligned with my plane.
6.) I switched on “Adjust Track”, and position my 4 red corner markers on top of the 4 corners of my plane. This created a master frame for my following Adjust Track keyframes.
7.) Next I selected the point in my footage where movement out of frame was at it greatest. Using the pink grid lines and checkered surface patterns as a guide, I positioned the 4 red “adjust Track” markers so that the grid lined up with my surface plane, and that the squares on the checkered surface remained constant with my original reference frame. An adjust track keyframe was then set.
8.) I moved forward in my footage until all 4 corners of my plane are back in shot. Here I re-aligned once again, with all four red markers placed once again on top of the corners of my plane. I set another “Adjust Track” key.
9.) I repeated steps 5-9 every time some of my corner pins left my shot. (Only twice in this example.)
Situation 2: Our Tracked Plane Begins outside the shot, or completely leaves the frame.
This was simple enough to work around:
1.) Instead of tracking the plane from the beginning of the footage, I rather moved along the timeline until I found a point when the plane I wished to track was completely in shot.
2.) I drew a spline around my plane, lined up my surface corners, then tracked first backwards, then forwards from this point.
3.) As my plane was to move in and out of my shot multiple times, I decided to set keyframes for the shape of my spline, each time at a position where my plane was nicely centred in my shot. This was done by moving the spline points into place and hitting “Add Key” under mocha’s “Keyframe Control” panel. Thus my spline had a shape to return to after leaving shot.
4.) With Splines and tracking in place, I enable my checkered surface pattern and my pink overlay grid and alligned them as I did in the previous example.
5.) Finally, I used “Adjust Track” to minimize drift, by adding keyframes just before, during, and shortly after my plane exited the shot.
Check out the following video for the two workflows highlighted above:
Continuing on with my learning up on Planar tracking and Mocha, I’ve come across two factors thta can adversely effect a track. Namely reflective surfaces and drifting. Good news though, for I quickly learnt some techniques to minize such things! Please… read on…
Surface Reflections Throwing of a Track
For a reflective sufrace, use a Cutout exclude refletcive area from the Planar Track. Check out the following images for a brief explaination.
Fixing Drifting Throughout Your Track Using “Adjust Track” Mode
Often times a track will drift off target as it moves through the shot. In this case you can use Mocha’s “Adjust Track” feature to nudge the track back on target.
My workflow for doing this is as follows:
1.) Identify the last frame in which your tracked surface is undistorted. (Without any drifting)
2.) Make sure that the “Surface” cornder pins are in place to specify the surface you wish to insert/replace.
3.) Click to enable Mocha’s “Adjust Track” feature. Mocha will then generate 4 red ‘X’s, one at each Surface corner pin. Position these new markers over 4 easily identifyable features on you footage (High contrast areas work best). These 4 red X’s will form the reference points from which mocha will then base your future adjustment tweaks/keyframes to. The offset between the reference markers and the blue Surface corner pins will be maintained – the general procedure is to keyframe animate the position of the red markers, to bump the track in the right direction.
There should also be a keyframe marked on your timeline , this is referred to as the “master frame”.
4.) On your timeline move to where distortion / drift is at it’s worst, and reposition the 4 red markers so they are accurately positioned ontop of what you placed them on top of in step #3. (On top of the same object that they were in the “Master Frame”.
5.) “In-between animate the sections where drift still remains – eventually drift will be minimized!
Check out the following video for a summary of the two planar tracking techniques I’ve described above.
Stay tuned for more!