Image-Based Rendering - The Magic of Computer Graphics - page 122

Film plane Film plane

(a) (b)

Figure 5.1

A camera directs light rays onto a film plane. (a) In a pinhole camera, all rays go through
the pinhole aperture. (b) A lens camera focuses light so that rays from a focal plane in the
scene converge on the film plane; rays from elsewhere do not converge, which results in
blurring. (Courtesy of Paul Debevec.)

is related to the distance from the lens to the film plane; “focusing” the camera
amounts to changing this distance. Light from points away from the focal plane is
spread out on the film, so objects off the focal plane appear blurred. The blurring
increases with distance from the focal plane. The distance at which objects start
to appear noticeably out of focus is known as the depth of field and is controlled
by the size of a circular aperture inside the lens. As the aperture size approaches
zero, the lens camera approaches a pinhole camera in which everything appears
in focus ( Figure 5.1(b) ) .
A pinhole camera projects straight lines in the scene to straight lines on the
film plane, as does the basic perspective projection. In an ideal lens camera
model, straight lines on the focal plane are imaged as straight lines on the film
plane. However, real cameras do not have ideal lenses. The ideal lens is typically
approximated most closely near the center of the film plane. The image becomes
more distorted further away from the center. The effects of distortion can be
significant enough to affect the results of stereo reconstruction. Most camera dis-
tortion depends only on the distance from the center, and is therefore known as
radial distortion . The distortion of a particular camera is typically measured by
capturing an image of a known target such as a checkerboard. The distortion can
be determined by comparing the image to the original target ( Figure 5.2 (left) ) . A
polynomial approximation to the radial distortion is often used in practice. Once
its coefficients are determined, the distortion in each image captured by the cam-
era can be corrected by applying the polynomial function to the image ( Figure 5.2
(right) ) . The remaining intrinsic camera parameters can be determined from im-
ages corrected for distortion.