Wednesday, September 06, 2006

tracing a specific path using camera automation

For my Paris video, I wanted to simulate our walk of the city by panning an old map of Paris highlighted with the streets we traveled. To do this, I imported a series of large (2800x2100) still images created in Gimp and used camera automation to simulate movement. The camera automation needed to be very specific to trace our path. Thus, I had to get the X,Y coordinate pairs of each point I wanted the camera to center upon. The hurdle I encountered is that the origin of the X,Y coordinate system used in Gimp is at the upper-left hand corner of the image, whereas the origin of an image or video imported into Cinelerra is at the center of the image. So I had to do a conversion.

For an image that is 2800x2100:
(0,0) in Gimp is (-1400,-1050) in Cinelerra
(1400,1050) in Gimp is (0,0) in Cinelerra
(2800,2100) in Gimp is (1400,1050) in Cinelerra

Hence, in Gimp, all X/Y values are positive, but in Cinelerra, X and Y can have negative values.

Most of the camera automations to simulate our walk required about four coordinate pairs; hence, four conversions:
1) origin
2) first leg of walk
3) second leg of walk
4) destination

As you can see, when I wanted to change the speed or direction of the pan, the conversion of coordinates got a bit tedious. Also, I varied the movement of some of the camera automations so that not all were bezier curves, but more linear. As noted in an earlier post, this can be accomplished by displaying the X and Y camera positions and using CTRL-click on the vertices to "pull" the curves straight. This takes a little getting used to and is very time consuming. Note that there will be two sets of "pull" vertices on either side of the actual X/Y positions that you can adjust to straighten the curve slope.

But the result is quite pleasing:

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