Figure 4.1: The cart in its calibration posture before the calibration pattern. A program automatically locates the cross and the spots, and deduces the camera's focal length and distortion.
The cart camera, like most vidicons, has peculiar geometric properties. Its precision has been enhanced by an automatic focal length and distortion determining program.
The cart is parked a precise distance in front of a wall of many spots and one cross (Figure 4.1). The program digitizes an image of the spot array, locates the spots and the cross, and constructs a a two dimensional polynomial that relates the position of the spots in the image to their position in an ideal unity focal length camera, and another polynomial that converts points from the ideal camera to points in the image. These polynomials are used to correct the positions of perceived objects in later scenes.
Figure 4.2: The spot array, as digitized by the cart camera
The program tolerates a wide range of spot parameters (about 3 to 12 spots across), arbitrary image rotation, and is very robust. After being intensely fiddled with to work successfully on an initial set of 20 widely varying images, it has worked without error on 50 successive images. The test pattern for the cart is a 3 meter square painted on a wall, with 5 cm spots at 30 cm intervals. The program has also been used successfully with a small array (22 x 28 cm) to calibrate cameras other than the cart's \ref(W1).
Figure 4.3: Power spectrum of Figure 4.2, and folded transform
Figure 4.4: Results of the calibration program. The distortion polynomial it produced has been used to map an undistorted grid of ideal spot positions into the calculated real world ones. The result is superimposed on the original digitized spot image, making any discrepancies obvious.
Figure 4.5: Another instance of the distortion corrector at work; a longer focal length lens
Figure 4.6: Yet another example; a rotation
Figure 4.7: And yet another example
The algorithm reads in an image of such an array, and begins by determining its approximate spacing and orientation. It trims the picture to make it square, reduces it by averaging to 64 by 64, calculates the Fourier transform of the reduced image and takes its power spectrum, arriving at a 2D transform symmetric about the origin, and having strong peaks at frequencies corresponding to the horizontal and vertical and half-diagonal spacings, with weaker peaks at the harmonics. It multiplies each point $[i,j]$ in this transform by point $[-j,i]$ and points $[j-i,j+i]$ and $[i+j,j-i]$, effectively folding the primary peaks onto one another. The strongest peak in the 90° wedge around the $Y$ axis gives the spacing and orientation information needed by the next part of the process.
The directional variance interest operator described later (Chapter 5) is applied to roughly locate a spot near the center of the image. A special operator examines a window surrounding this position, generates a histogram of intensity values within the window, decides a threshold for separating the black spot from the white background, and calculates the centroid and first and second moment of the spot. This operator is again applied at a displacement from the first centroid indicated by the orientation and spacing of the grid, and so on, the region of found spots growing outward from the seed.
A binary template for the expected appearance of the cross in the middle of the array is constructed from the orientation/spacing determined determined by the Fourier transform step. The area around each of the found spots is thresholded on the basis of the expected cross area, and the resulting two valued pattern is convolved with the cross template. The closest match in the central portion of the picture is declared to be the origin.
Two least-squares polynomials (one for $X$ and one for $Y$) of third (or sometimes fourth) degree in two variables, relating the actual positions of the spots to the ideal positions in a unity focal length camera, are then generated and written into a file.
The polynomials are used in the obstacle avoider to correct for camera roll, tilt, focal length and long term variations in the vidicon geometry.<-- Previous  Next -->