Television – Camera – system and detail – Camera connected to computer
Reexamination Certificate
1996-06-06
2001-07-03
Faile, Andrew I. (Department: 2712)
Television
Camera, system and detail
Camera connected to computer
C348S036000
Reexamination Certificate
active
06256058
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to image processing, and more particularly to compositing images made by an uncalibrated camera.
BACKGROUND OF THE INVENTION
A panoramic scene spans the entire 360 degree horizontal field of view. Panoramic images of scenes have important applications in computerized visualization. Panoramic images give the viewer an impression of being immersed in the scene.
A panoramic image can be generated by first taking a set of overlapping images of the scene while rotating a camera about a vertical axis which passes through the optical center of the camera. The set of images, if not already in digital form, can be digitized and processed to improve quality. The processed images can be projected onto a cylindrical surface for viewing. Alternatively, the images can be viewed using, for example, a head-mounted viewing device which simulates the cylindrical surface as the head is rotated. This virtually gives the viewer a sense of observing a real scene. In order to properly process and project the set of images, the focal length of the viewing system, e.g., the cross-sectional radius of the surface of the viewing cylinder, must substantially correspond to the focal length of the camera.
In the prior art, the focal length (ƒ) of the camera is usually known. The focal length of the camera can be determined by calculating the distance between the image plane and the optical center of the camera. This can be done through the process of camera calibration.
Some calibration methods take images of calibration patterns such as regularly spaced dots or lines having known dimensions. Other methods take images of structures having constrained and known shapes, for example spheres or cubes. These methods have the disadvantage of requiring explicit feature extraction. In addition, these methods are not practical if the focal length of the camera is known to drift over time, or change with zooming or focussing.
In methods that involve a set of images taken while rotating the camera 360 degrees, feature tracking can be used on the set of images to recover the focal length. In some methods, translation estimates are made of a local area in the vicinity of the center of the images. This method requires small incremental steps while panning the camera. Another method requires explicit feature extraction, such as corners and edges, and tracking the features from one image to the next.
It is desired to automatically determine the focal length of a camera using a set of images without using special calibration set-ups and without explicit feature detection and tracking. Then, using the correct focal length, it becomes possible to create a panoramic view of the images that is both optically correct and seamlessly blended.
SUMMARY OF THE INVENTION
Disclosed is a method for calibrating a camera while extracting and compositing a panoramic image. A conventional commercially available camera, either analog or digital, is coupled to a computerized image acquisition and processing system. The camera acquires a set of images of a scene while rotating the camera about a vertical axis passing through the optical center of the camera. The images of the set are taken such that successively captured images overlap each other. The axis of rotation of the camera is substantially perpendicular to the focal axis of the camera to acquire images of a full 360 panoramic scene.
An initial estimate of the focal length of the camera is made. This estimate can be substantially smaller or larger than the true focal length. For example, the initial focal length can be estimated to be infinity. The set of images are composited by the image processing system to determine an initial composited length using the initial estimate of the focal length. The initial composited length is used to derive a next estimate of the focal length.
The images are recomposited using the next estimate of the focal length to determine a next composited length. This process is iterated until the absolute difference between a next and current estimates of the focal length is less than a predetermined threshold to substantially approximate the accurate focal length of the camera.
Prior to compositing, pixels of planar images are first projected onto pixels of a cylindrical surface having a cross-sectional radius that is the initial focal length estimate. If the initial focal length estimate is different from the actual focal length, then the length of the composite image will not be equal to the length of the circumference of the cross-sectional cylindrical surface. The net angular difference between the two lengths can be used to iteratively improve the estimate of the focal length.
The panoramic image may be blurred as a result of camera noise, digitization, image interpolation, and incorrect focal length. While generating the panoramic image, a weighted interpolation scheme can be used to significantly reduce the blurring effects in the panoramic image. In this scheme, the contribution of pixels of an image to the final composited panoramic image is reduced in proportion to the distance that the pixels are away from a central column of the image.
The method disclosed can be used either as a stand-alone technique to calibrate a camera, or as a technique to composite a panoramic image from a set of images with no prior knowledge of the camera focal length.
REFERENCES:
patent: 3757040 (1973-09-01), Bennett et al.
patent: 4549208 (1985-10-01), Kamejima et al.
patent: 5200818 (1993-04-01), Neta et al.
patent: 5262867 (1993-11-01), Kojima
Kang Sing Bing
Weiss Richard S.
Compaq Computer Corporation
Faile Andrew I.
Hamilton Brook Smith & Reynolds P.C.
Ho Tuan V.
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