Thursday, 17 March 2011

Motion tracking Richard's run

The following shot that I will be talking about involves our runner, Richard running away before being cornered against a graffiti wall.  In order to make this shot believable, I used motion tracking, as well as masking and green screen.  Here is the original footage that we recorded in one of the studios at uni.
The green screen is where I will place the CG environment.  The pieces of sellotape that we have place on the green screen are what we will use to track the movement and speed of the camera when we key out the green screen.  They will also be used by Match Mover, a program that will track and reproduce the camera movements, so I can export them to a camera in Maya, meaning that the cameras in both the live action and CGI will move in sync with each other.

The next stage was to load the sequence in Match Mover.  In this program, I applied Automatic Tracking, which tracks the camera movement in each shot.  After this tracking was applied, I used the clean assistant to reduce the number of tracks, so that the camera movement was not too erratic.  Here is how the scene looks within Match Mover after an Automatic Track.
The green lines in this shot display the movements that have been tracked.  Not only does the Automatic Track capture the camera movement, but also Richard's movements.  At first, I didn't think this was a problem, but when I exported the camera movements into Maya, I found that the camera moved really erratically and even moved out of the scene.  I could not figure out why the camera was moving like this.

After showing this problem to my tutor, he advised that I delete all of the trackers the are near or on Richard's figure, because the camera was tracing his movements, as well as the movements of the camera.  I went through the whole sequence within Match Mover and deleted each tracker that appeared on Richard.  Here is the same shot, after I had gone through the sequence.
I exported these camera movements to Maya.  I then applied them to the relevant scene, where I made a few manual arrangements, such as deleting a few unnecessary key frames and making the movement smoother in the Graph Editor.  Here is an unrendered Playblast of the camera movement in the chase scene.
I now needed to composite this with Richard's running sequence, and the desired outcome would be that Richard's run would move in sync with the camera movements in Maya.  In After Effects, I uploaded both the camera sequence in Maya, fully rendered, and the green screen footage of Richard running.  I overlaid the live action footage over the green screen footage and then I keyed out the green screen, with a Keylight filter.
The white of the walls around the green screen was still present.  I didn't key out the white, because keying out any colour, other than green or blue would have caused problems, as it would start keying out things like Richard's skin tone and clothing.  In order to get rid of the white walls, I place Masks around them, which I then keyframed as they moved around.
In order for Richard to look as though he is running towards the wall, I keyfamed the position and scale of the video footage, to create the illusion of Richard running away from the camera.  As a finishing touch, I applied a Brightness and Contrast filter, which I then keyframed to make Richard's figure become darker as he ran into the darkness, which would highlight how he is running into peril.

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