Most of us who've used Photosynth have tried to synth a car at one time or another. Indeed, there are about 1,500 synths with the "car" tag already.

But if you've tried, you've probably found that car synths often don't work out as well as room synths, art synths, landscape synths, or even people synths. Let's discuss why that is, and what can be done about it.

One of the first steps in the Photosynth processing pipeline is "feature extraction". This is where we analyze each photo and look for areas of interesting texture that we hope can later be matched to textures in other photos. Notice that I said "texture", because Photosynth doesn't look for corners or edges; it looks for areas of visual "roughness". Our feature detection algorithm was inspired by the popular SIFT Algorithm created by David Lowe at UBC.

The problem with cars is that they have very little "visual roughness". Most cars are smooth and shiny, and that leads to very few matchable features. In fact, because if this, we've always said that dirty cars synth better than clean ones. :)

What often happens when synthing a car is that the background gets matched much more than the car, resulting in a very poor point cloud, and a frustrating navigation experience:

car
It looks like the car got matched, but when you turn off the photos and examine the point cloud...

points
... only the bushes in the background and the ground on the lower left got matched. Apart from a small patch on the rear door, the car itself was almost invisible to Photosynth. This makes for very poor connectivity and very poor navigation.

Nonetheless, if you pay attention to the following you CAN get some pretty good results. In increasing order of difficulty, Here's what you can try:

How to Shoot a Car Interior (easy)

  1. Set your camera to wide angle
  2. If you can get in the car, sit in the back and do a panorama of the front area. Get at least 65% overlap on all shots.
  3. Take some zoom-ed shots of the instrument cluster, the gear shifter, etc.
  4. Here's a good example.

How to Shoot a Car Exterior (hard)

  1. Cars with lots of detailing will work better than very "clean" cars.
  2. Walk around it at an even distance, and shoot at least 40 photos each showing the whole car. If you can shoot 60 photos, then all the better.
  3. If your angle is from slightly above the car, then you have the opportunity to get lots of ground in your shots. This should anchor the photos, although this will have the side effect that transitions will look a little jerky.
  4. Move in to get closer shots of the wheels, grill, etc. As you move in, take a shot every few feet.
  5. Critical: Add highlights after synthing to make sure that people can get to all the good bits, even if they didn't connect very well.
  6. Here's a good example.

How to Shoot a Booth at an Auto Show (ambitious!)

  1. Find a spot where you have a clear view of the cars you want to include.
  2. Take a wide-angle panoramic series of shots from this position, including all your target cars.  Overlap successive shots by 65%.
  3. Walk from the panorama spot in a straight line to the first car, taking shots every 5-10 paces.
  4. Go back to the panorama spot and repeat for each car.
  5. Avoid cars on moving turntables. This confuses Photosynth!
  6. Make one highlight per car once your synth is complete.
  7. If you've got even more energy, combine this technique with the interior and exterior techniques, but limit the total number of photos to ~300.
  8. Here's a good example.

Good luck!
David.