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Capturing 3D Surfaces Simply With a Flash Camera
MojoKid writes with this excerpt from Hot Hardware (linking to a video demonstration): "Creating 3D maps and worlds can be extremely labor intensive and time consuming. Also, the final result might not be all that accurate or realistic.
A new technique developed by scientists at The University of Manchester's School of Computer Science and Dolby Canada, however, might make capturing depth and textures for 3D surfaces as simple as shooting two pictures with a digital camera — one with flash and one without. First an image of a surface is captured without flash. The problem is that the different colors of a surface also reflect light differently, making it difficult to determine if the brightness difference is a function of depth or color.
By taking a second photo with flash, however, the accurate colors of all visible portions of the surface can be captured. The two captured images essentially become a reflectance map (albedo) and a depth map (height field)."
Slashdot (can't be bothered to find it) had a story several years ago about the (then old!) technique of capturing complicated 3D objects, such as car engines, by using two flash images, each with the flash located in slightly different locations. Threshholding the difference between the images gives very nice edge detection, along with very accurate depth information.
A project I'm working on uses the technique to capture information about arrowheads/spearheads.
This is just a way to automatically generate surface bump maps. It does not really capture depth information (like a Z-buffer).
Conceptually it seems simple enough (take a photo with shadows from a light source not in line with the camera, take another where all the shadows are in line with the camera (making them virtually invisible), tell the software which direction the light is coming from in the first photo, and let it figure out the relative height of each pixel, by analysing the difference between it and the uniform (flash-lit) version, after averaging the brightness of the two. It's similar to the technique some film scanners use to automatically remove scratches.
I can think of a lot of cases where it won't work at all (shiny objects, detached layers, photos with multiple "natural" light sources, photos with long shadows), but still, for stuff like rock or tree bark textures it should save a lot of time. As the video suggests, this should be very pretty useful for archaeologists.
No, with flash (light source coming from the camera) shows the colors without shadows; i.e. without color perspective. Without flash (light source at an angle to the model/subject) shows the deeper parts in shadow (known to us former art students as "color perspective").
You could actually fo this with two flashes, provided one was on the camera and one to the side. The fact that it flashes has nothing to do with it, it has to do with the angle of the light sources.
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