Method and apparatus for automatically focusing an image-acquisition device

• US 5,040,228 A
• Filed: 08/28/1989
• Issued: 08/13/1991
• Est. Priority Date: 08/28/1989
• Status: Expired due to Fees

• Current Assignee: American Telephone & Telegraph Company (AT&T, Inc.), Bell Telephone Laboratories, Inc. (Nokia Corporation)
• Original Assignee: AT&T, Inc.
• Inventors: Kim, Ilyoung, Bose, Chinmoy B.
• Primary Examiner(s): Razavi, Michael
• Application Number:
  US07/398,876
• Time in Patent Office: 715 Days
• Field of Search: 382/8, 382/1, 382/51, 382/52, 382/53, 358/227, 358/448, 358/458
• US Class Current: 382/141
• CPC Class Codes: H04N 23/673 based on contrast or high frequency components of image signals, e.g. hill climbing method

Abstract

Autofocusing of a lens (18) on a television camera (12) is achieved by first processing the image captured by the camera to establish a histogram of the intensity gradients for each of a plurality of small areas (pixels) in the captured image. From the histogram of the pixel intensity gradients, a focus index value, indicative of the sharpness of focus of the captured image, is established. The just-computed focus index value is then compared to the previously established value which is initially set to zero. The difference between the just- established and previously established focus index values establishes the direction in which the lens is incrementally stepped to more sharply focus the image. The steps of (a) capturing the image, (b) establishing the intensity gradient histogram, (c) comparing focus index value to the previously established value, and (d) incrementally stepping the lens are repeated as long as the just-established focus index values exceeds the previous value. Once the focus index value no longer exceeds the previously established value, the lens is stepped by an amount dependent on the just-established and previously established focus index values to sharply focus the image.

Claims

1. A method for adjusting the focus of an image-acquisition device so that the image-acquisition device captures the image of an article in sharp focus, comprising the steps of:

(a) establishing a current focus index value as having an initial value of zero;

(b) capturing the image of at least a portion of an article with an image-acquisition device having adjustable focus;

(c) processing the image captured by the image-acquisition device to determine an intensity gradient for all of the small areas within the image;

(d) establishing a histogram of the gradient intensities of all the pixels in the image;

(e) establishing a new focus index value from the histogram of the pixel intensity gradients;

(f) comparing the current focus index value to the new focus index value to determine in which direction the focus of the image-acquisition device should be adjusted so that the captured image is more sharply focused, thereby increasing the focus-index value;

(g) substituting the new focus index value for the current focus index value;

(h) incrementally stepping the focus of the image-acquisition device in the direction expected to increase the focus index value;

(i) repeating the steps of (b)-(h) until the new focus index value no longer exceeds the current focus index value;

(j) adjusting the focus of the image-acquisition device in the opposite direction by a step amount established in accordance with the difference between the new and current focus index values so that the captured image is sharply focused.

2. The method according to claim 1 wherein the new focus index value is established in with the relationship ##EQU4## where A is a constant, g_i represents a successive one of N (N being an integer ≦

35) pixel gradient intensity values, each above a noise threshold n₀ and each lying in the range between g_max, the maximum pixel intensity gradient, and g_max -h, h being an integer greater than N.

3. The method according to claim 1 wherein the new focus index value is established in accordance with the relationship:

##EQU5## where B is a constant, g_i represents a successive one of N (N being an integer ≦

35) pixel gradient intensity values, each above a noise threshold n₀ and each lying in the range between g_max, the maximum pixel intensity gradient, and g_max -h, h being an integer greater than N.

4. The method according to claim 1 wherein the step amount by which the focus on the optics system is finally adjusted is given by the term S-Δ

S where S is the incremental amount which the focus adjustment is normally stepped, and Δ

S is a signed quantity given by ##EQU6## where M_new is the most recently computed focus index value, M₀ is the second most recently computed focus index value, and M_-1 is the third most recently computed focus index value.