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MillimeterThe Art of Tiling in Public Enemies
Jul 8, 2009 12:00 PM, By Robert Stadd, visual-effects supervisor on Public Enemies
Tiling, or “stitching,” is the process in which multiple photographed images are placed together in alignment in order to create a smooth background. This illusion should be distortion- and parallax-free, and when used in composites, it allows the foreground element a wide range of movement.
The technique was put to good use in Public Enemies, a movie that needed visual effects to be as invisible as possible. In an opening scene, for instance, John Dillinger breaks into a prison in order to bust his friends out. For the part of the scene inside the prison, we shot the backgrounds first, which is what you would normally do in terms of helping to light the complementary shot. But in this case, I had no idea what the camera angle would be nor how far away Michael Mann would want the prison block house in the background. The solution to both these problems was to tile the background with images from my digital still camera (a Nikon D300), and later give those tiled images to a visual-effects facility (in this case, Illusion Arts) to put them together as a background.
In the distance, you can see the block house that’s featured as an exterior in an earlier section of the movie. This exterior needed to tie into the interior. Note that it wasn’t practical to film in the laundry room, as this was the inside of a working prison (Stateville Correctional Center in Illinois). Nevertheless, it was good reference to have.
Obviously, we could only shoot in the Stateville prison for a limited period, since prisoners had to be locked down while we shot. I had the morning after the full company wrapped to shoot my tiles and get out before the prisoners took to the yard. I shot several tiles from several positions once the sun was overhead, giving me adequate light on the front of the “big house.”
Note that these tiles pictured were only a portion of the actual tiling that took place, from only one position. Tiles were done from many positions to allow for maximum flexibility during final compositing.
In the final composite, the tiled background has been inserted into the shot. Note the size of the background is larger than in the reference photo, and it was in a different position. Only by using a high-resolution, tiled background could we ensure that it would work in the shot, from a grain size and position standpoint. Approximately eight pictures were used to tile the background, creating a file that was roughly 200MB.
In case you ever find yourself doing work that would benefit from such a technique, here are a few tips on how to create a background tile. First, for shooting your tile photos:
- Use a nodal head. For this project, I used the Manfrotto 303SPH nodal head.
- Control exposure, white balance, and focus manually. Determine the best exposure for the entire range of pan, and don’t change it shot to shot.
- Keep your shot-to-shot overlap at roughly 30 percent. Depending on the focal length, this could mean a 12-to-16-photo 360.
- Mount your camera vertically to shoot on the long axis of the film and minimize distortion.
You want to find the nodal point of the lens by aligning right-angle objects near and far. Mount the camera onto a nodal head, and while looking through the lens, pan the camera from side to side, sliding the camera back and forth on the slider plate until the two objects line up. Note that each focal length (particularly true in a zoom lens), will have a different nodal point. Don’t assume once you find the nodal point, you’ve found it for all focal lengths.
A background tile won’t tie to the foreground from every position. Tiled backgrounds are most successful when the shot only pans or tilts from one position. You also should use tiling software, such as The Panorama Factory or Autodesk Stitcher, to check your work.
One more suggestion: Practice stitches before you get in the field and find yourself having to do it under extreme pressure.
Continue the discussion on Crosstalk the Millimeter Forum.