Automatic focussing system

• US 4,814,889 A
• Filed: 10/07/1987
• Issued: 03/21/1989
• Est. Priority Date: 10/07/1987
• Status: Expired due to Fees

• Current Assignee: General Electric Company
• Original Assignee: General Electric Company
• Inventors: Covey, Robert L.
• Primary Examiner(s): ADAMS JR, RUSSELL E
• Application Number:
  US07/105,411
• Time in Patent Office: 531 Days
• Field of Search: 354/400, 354/402, 354/403, 354/404, 354/405, 354/406, 354/407, 354/408, 358/227
• US Class Current: 348/351
• CPC Class Codes: H04N 23/673 based on contrast or high frequency components of image signals, e.g. hill climbing method

Abstract

A video camera output signal is digitized by an analog-to-digital converter (ADC) and the digitized video frame stored in memory. The stored frame is high-pass spatially filtered and the filtering result is subjected to a non-linear point transform to suppress to zero less bright pixel samples. The grey scale values of the remaining pixels are summed to produce a summation value. Different focussed states have different summation values. The maximum summation value represents the point of optimum focus, and the system reaches the maximum value by changing the focus state of the optics, recomputing the grey scale sum and repeating the process until a global maximum grey scale summation value is reached, regardless of the initial focus state.

Claims

1. An automatic focussing system comprising:

imaging means for producing a signal representative of an image of an object spaced from the imaging means, said imaging means having a given focus state relative to the object;

signal processing means for generating a grey scale summation value of at least a portion of the image, said processing means distorting the grey scale values of said image so that said summation value tends to be a function of said focus state;

means for periodically changing the focus state of said imaging means and for causing said processing means to produce successive grey scale summation values representing successive different corresponding focussing states of said imaging means; and

comparison means for comparing said successive summation values and for causing said imaging means to be placed in a focus state in which the grey scale summation values of said successive focussing states increase to a maximum;

said signal processing means including means for digitizing said image a frame at a time and for dividing each said image frame into a plurality of pixels, each pixel having a given grey scale value, said grey scale distorting including non linear transformation of each said image frame to change the grey scale values of certain of said pixels having a grey scale value in a certain threshold range and means for convolving each said frame with a high pass spatial filter prior or said distorting.

2. The system of claim 1 wherein said high-pass spatial filter is a two-dimensional high-pass spatial filter.

3. The system of claim 1 wherein said imaging means includes a video camera for generating a video signal manifesting said given focus state, said signal processing means including analog-to-digital converter means for digitizing said video signal and means for storing said digitized signal one frame at a time.

4. The system of claim 3 wherein said processing means includes means for convolving each digitized frame, means for grey scale non-linear transforming said convoluted digitized frame in a monotonically non-decreasing relationship of input grey scale values to output grey scale values, and means for summing the grey scale values of said non-linear transformed frame.

5. The system of claim 1 wherein said comparison means includes means for changing the focus states of said imaging means in increments of such value to avoid false summation peaks.

6. The system of claim 5 wherein said comparison means includes means for storing the values of three successive summation values and for curve fitting said these values to a parabola to determine the maximum summation value.

7. In an optical apparatus including optical means for projecting an image of an object from an object plane to an image plane, image pickup means adapted to provide an image signal representative of the projected image at the image plane, and means responsive to a control signal for changing the focus state of the optical means, an automatic focus control system comprising:

means responsive to said image signal applied as an input thereto for generating a digitized image representation of at least a portion of the projected image, said digitized image representation comprising a plurality of separate image pixels each having a given grey scale value in a given grey scale range;

high-pass filtering means for filtering the digitized image representation;

non-linear grey scale transform means for changing the grey scale value of those pixels whose grey scale value is in a predetermined range of values;

means for summing the grey scale values of all pixels in said digitized image representation to produce a signal representing the grey scale summation value; and

means responsive to said grey scale summation signal applied thereto for generating said control signal to incrementally change the focus state of said optical means until the grey scale summation value is a maximum.

8. The system of claim 7 wherein said predetermined range is in the lowermost end of said grey scale values.

9. The system of claim 7 wherein said means for generating said image representation include means for generating said image representation subsequent to the incremental change of said optical means focus state for each of a plurality of successive states.

10. The system of claim 7 wherein said means for generating a digitized image representation includes means for digitizing a plurality of successive single video frames, each frame corresponding to and resulting in the generation of a given grey scale summation value, said means for generating said control signal including means for comparing the grey scale summation values of said successive frames, and for changing the focus state of said optical means in a direction to increase the grey scale summation value of said successive frames.

11. The system of claim 7 wherein said apparatus includes a video camera and means for varying the relative position of the optical means, said latter means including drive means for displacing the camera, said means for generating said control signal including means for causing the camera to incrementally displace in a direction to cause successive grey scale summation values for each incremental displacement to increase in value, said system including means for automatically generating said digitized image representation after each said incremental displacement.

12. The system of claim 7 wherein said high-pass filtering means includes means for generating a Laplacian cross convolution of said digitized image representation.

13. The system of claim 12 wherein said convolution generating means includes means for replacing each pixel in each image representation with a linear summation over a region surrounding that pixel according to the equation:

##EQU2## for all (i,j), and all (h,k), in an n×

n region surrounding (i,j), where i and h are pixel row numbers and j and k are pixel column numbers, P is the original image pixel, n is an odd integer, Q is the image pixel resulting from the convolution, and C is the coefficient matrix defining the convolution, the region surrounding the pixel P being centered on P, and may be any region which includes the same odd number of pixels in both the x and y directions.

14. The system of claim 7 wherein said transform means includes means for replacing each pixel in the image representation with an arbitrary function of that pixel according to the equation:

space="preserve" listing-type="equation">Q(i,j)=F[P(i,j))]

for all (i,j), where i is the pixel row number and j is the pixel column number, P is the original pixel and Q is the resultant pixel.

15. An automatic focussing system comprising:

imaging means for projecting an object image at an object plane onto an image plane;

image storing means for digitizing and storing at least a portion said image at said image plane, said digitizing forming said stored image into a plurality of pixels;

filtering means for high-pass filtering said stored digitized image;

non-linear grey scale transform means for changing the grey scale value of those pixels of the filtered image whose grey scale pixel values are in a given grey scale range of values;

grey scale summation means for summing the grey scale values for all said pixels of said filtered and transformed image;

focal adjustment means for periodically changing the focus state of said imaging means; and

means for comparing successive summed grey scale values and for causing said focal altering means to change the focus state of the imaging means in a direction such that the summed grey scale values tend to increase in value with successive displacements.

16. The system of claim 15 wherein said summed grey scale values tend to have false peaks intermediate the minimum and maximum values of said grey scale summed values, said means for comparing including means for ignoring said false peaks.

17. The system of claim 15 wherein said imaging means comprises a video camera, said focal adjustment means including means for displacing said camera relative to said object plane.

18. The system of claim 15 wherein said filtering means comprises Laplacian cross convolution means for convolving said stored image and wherein said transform means includes means for reducing to a value of substantially zero the grey scale values of those pixels which are below a given threshold value.

19. The system of claim 15 wherein said focal adjustment means includes first means for coarsely incrementing said imaging means in first displacement increments until said summed grey scale values approach a given maximum range and for finely incrementing said imaging means in second displacement increments smaller than the first displacement increments until a maximum value is reached.

20. A system for automatically focussing a video camera on an object comprising:

a video camera movably secured to a support and adapted to produce a video signal having a given focus state according to the relative position of the camera to an object;

means for digitizing said video signal into a plurality of pixels;

means for successively storing said digitized signal one frame at a time;

means for high-pass spatial filtering each said stored digitized frame;

means for grey scale distorting each said high-pass filtered frame;

means for summing the grey scale values of all pixels in at least a portion of each said convoluted grey scale distorted frame and for storing the summation value;

means for periodically displacing the camera to cause the camera to be placed in different focussed states, each frame corresponding to and representing a different state;

said means for storing the summation value including means for storing a plurality of successive summation values corresponding to the different successive focussed states;

means for determining when the grey scale summation value of a frame is above a given threshold; and

means for comparing said successive stored summation values when said grey scale summation value is above said given threshold for displacing said camera in a direction to maximize said grey scale summation values.