Author: Terry Stetler
How to move an object like a ball around or behind portions of the background, say the columns or windows in a building, without using 3D software?
Basically this effect only requires compositing multiple layers consisting of the ball, the building and the parts of the building you want the ball to pass behind.
You start out with the picture of your building. In this case I chose Benicia, the old capital building in California;
In an image editor cut out its columns and door (I used PhotoImpact 11) and paste them as a new image. Next cut out the door between them and select the background color. Now invert the selection before saving it as a *.tga, *.png etc. with transparancy. This gives you an image + alpha channel overlay of just the columns;
I also exported a *.tga w/alpha channel of the globe object from Cool3D Production Studio;
Now these were all assembled on the MSP8 timeline, or MSP7 with appropriate changes. I used a 2D moving path to both align & scale the columns with the building. The ball was placed on a track between them;
and the globe animated using a moving path;
The globe starts out of frame to the left and large, then shrinks as it moves to a point just to the left of the left column. It then bounds back & forth twice before moving out of frame to the right and large again.
Note the repeating keyframes in the middle that are used to make the ball bound back & forth. I only keyframed the first and last of these, copy/pasting their settings to the other alternately placed ones (Lt/Rt/Lt/Rt) for consistancy.
As long as the globe is in front of the building or in the columns alpha channel (between them) it's visible, but as soon as it exits the alpha channel and into the columns image data it's \"behind\" them.
When exported you get this (right-click + Save As please);
If you export an animated object from C3DPS or whatever using a 32 bit *.avi or *.tga image sequence the effect would be even cooler.
If the background were a video file that doesn't pan etc. just save a single frame and cut out only that portion of the background that the animated object would pass behind for use as the overlay. Now you can have people walking by while it \"invades\" their space
Another alternative is to do the compositing in Video Paint. This has a steeper learning curve, but is more flexible in that it lets you create video mattes for those times when the background object isn't stationary in the shot.
More on that in another tutorial
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