An exploration of motion blur techniques for automotive photography and CG integration.
Backplates for automotive photography paired with CG vehicles are typically static shots, taken with a camera locked off on a tripod. However the finished style that is popular is to have a pin-sharp car on a blurred background, suggesting speed and neatly separating the subject from its environment.
The style of blurred shots used today is a result of the way car photography has evolved over the years by physically rigging a camera at the end of a long boom and exposing with the vehicle in motion. The following video, part of a longer blog post by experienced automotive photographer Nigel Harniman, shows in great details the procedure. A somewhat surprising observation is the extremely slow motion while keeping the shutter open for as long as 30 seconds, even though the result suggests speed:
Now, this sounds easy enough to arrive at in post, but things are never that simple. The complexity comes from the type of blur that is required to be realistic: what needs to be simulated is not the camera panning around, as the simple Photoshop motion blur filter does, but rather the camera being locked in relation to the car (to keep it sharp) and following its motion. The result is blur lines that are proportional to the distance to the camera, closer objects or ground being increasingly blurred as they approach the lens, typically at the bottom of the frame, while distant landscape features very subtly or not blurred at all.
Additionally, since the front of the vehicle is typically shown, which means that the view is looking back compared to the direction of camera motion, there's a strong sense of perspective given by the converging blur lines, which is another difference with the simple blur tool available in Photoshop where the blur direction is constant in the entire image. Finally, if the camera is static relative to the car, and the road is curved, then — you guessed it — the blur lines will be curved as well following the road, and cause the background to be blurred in the direction of rotation.
Update July 2014: Check out a new technique.
In the rarefied field of vector blurring software, two leaders seem to have cornered the market (there are always 2 leaders in esoteric image processing software it seems): Virtual Rig and Bleex. They both do exactly the same thing, they're both German, and they're both expensive for such a single-purpose application. Using them means defining blur vectors over the backplate, which will tell the software how to blur the image, by how much and in which direction. It's not particularly difficult to do so, and the starter tutorials do a good job of pacing the prospective motion “blurrer” through the various steps necessary. Perhaps the greater difficulty, as is the case with many digital tools, is to not get carried away and keep an eye on the realism and overall art direction of the resulting image.