Spatial standard observer

• US 20060165311A1
• Filed: 01/24/2005
• Published: 07/27/2006
• Est. Priority Date: 01/24/2005
• Status: Active Grant

• Current Assignee: USA As Represented By The Administrator Of The NASA
• Original Assignee: U.S.A. as represented by the Administrator of the National Aeronautics and Space Administration
• Inventors: Watson, Andrew B.
• Granted Patent: US 7,783,130 B2
• Time in Patent Office: Days
• US Class Current: 382/286
• CPC Class Codes: G06T 2207/30168 Image quality inspection
• CPC Class Codes: G06T 7/0002 Inspection of images, e.g. flaw detection
• CPC Class Codes: H04N 17/04 for receivers
• CPC Class Codes: H04N 19/60 using transform coding
• CPC Class Codes: H04N 19/86 involving reduction of coding artifacts, e.g. of blockiness

Abstract

The present invention relates to devices and methods for the measurement and/or for the specification of the perceptual intensity of a visual image, or the perceptual distance between a pair of images. Grayscale test and reference images are processed to produce test and reference luminance images. A luminance filter function is convolved with the reference luminance image to produce a local mean luminance reference image. Test and reference contrast images are produced from the local mean luminance reference image and the test and reference luminance images respectively, followed by application of a contrast sensitivity filter. The resulting images are combined according to mathematical prescriptions to produce a Just Noticeable Difference, JND value, indicative of a Spatial Standard Observer, SSO. Some embodiments include masking functions, window functions, special treatment for images lying on or near borders and pre-processing of test images.

Claims

1. A method of producing a visibility metric from a digital reference image and a digital test image, the method comprising:

a) producing a reference luminance image from the reference image and a test luminance image from the test image;

b) producing a local mean luminance reference image as a convolution of the reference luminance image and a luminance filter function;

c) producing a test contrast image as at least one of the following;

(c1) a mathematical combination of the test luminance image and the local mean luminance reference image and (c2) a mathematical combination of the test luminance image, the local mean luminance reference image and a border aperture function and (c3) a mathematical combination of the test luminance image, the local mean luminance reference image and an image of a border surrounding the reference image;

d) producing a reference contrast image as at least one of the following;

(d1) a mathematical combination of the reference luminance image and the local mean luminance reference image and (d2) a mathematical combination of the reference luminance image, the local mean luminance reference image and the border aperture function;

e) applying a contrast sensitivity filter to the test contrast image to produce a filtered test image;

f) applying the contrast sensitivity filter to the reference contrast image to produce a filtered reference image;

g) providing a difference image by at least one of the following two processes;

(g1) subtracting the filtered reference image from the filtered test image to produce a difference image, and (g2) producing a mask image as a mathematical combination of the filtered reference image with a masking filter, and producing a difference image as a ratio of the difference image and the mask image;

h) producing a just noticeable difference image as a mathematical combination of the difference image with a window function; and

i) pooling the just noticeable difference image to produce a visibility metric.

4. A method as in claim 1, further comprising:

providing an enlarged reference luminance image by including with the reference image a selected portion of said border surrounding said reference luminance image.

5. A method as in claim 1 further comprising pre-processing at least one of said digital reference image and said digital test image immediately preceding performance of said process (a).

6. A method as in claim 5, wherein said pre-processing is performed by pre-filtering.

7. A method as in claim 6, wherein said pre-filtering is performed by convolution with a pre-filtering function.

8. A method as in claim 7 wherein said pre-filtering function is a Gaussian function.

9. A method as in claim 5 wherein said pre-processing is performed by downsampling.

10. A method as in claim 5 wherein said pre-processing is performed by cropping.

2. (canceled)

3. (canceled)

11. An article of manufacture comprising a computer usable medium embodying one or more instructions executable by a computer for performing a method of producing a visibility metric, the instructions comprising:

a) producing a reference luminance image from the reference image and a test luminance image from the test image;

b) producing a local mean luminance reference image as a convolution of the reference luminance image and a luminance filter function;

c) producing a test contrast image as at least one of the following;

(c1) a mathematical combination of the test luminance image and the local mean luminance reference image and (c2) a mathematical combination of the test luminance image, the local mean luminance reference image and a border aperture function and (c3) a mathematical combination of the test luminance image, the local mean luminance reference image and an image of a border surrounding said reference image;

d) producing a reference contrast image as at least one of the following;

(d1) a mathematical combination of the reference luminance image and the local mean luminance reference image and (d2a mathematical combination of the reference luminance image, the local mean luminance reference image and the border aperture function;

e) applying a contrast sensitivity filter to the test contrast image to produce a filtered test image;

f) applying the contrast sensitivity filter to the reference contrast image to produce a filtered reference image;

g) providing a difference image by at least one of the following two processes;

(g1) subtracting the filtered reference image from the filtered test image to produce a difference image, and (g2) producing a mask image as a mathematical combination of the filtered reference image with a masking filter, and producing a difference image as a ratio of the difference image and the mask image;

h) producing a just noticeable difference image as a mathematical combination of the difference image with a window function; and

i) pooling the just noticeable difference image to produce a visibility metric.

14. An article of manufacture as in claim 11, wherein said instructions further comprise:

providing an enlarged reference luminance image by including with the reference image a selected portion of said border surrounding said reference luminance image.

15. An article of manufacture as claim 11, wherein said instructions further comprise pre-processing at least one of said digital reference image and said digital test image immediately preceding performance of said st

16. An article of manufacture as in claim 15 wherein said pre-processing is performed by pre-filtering.

17. An article of manufacture as in claim 16 wherein said pre-filtering is performed by convolution with a pre-filtering function.

18. An article of manufacture as in claim 17 wherein said pre-filtering function is a Gaussian function.

19. An article of manufacture as in claim 15 wherein said pre-processing is performed by downsampling.

20. An article of manufacture as in claim 15 wherein said pre-processing is performed by cropping.

12. (canceled)

13. (canceled)

21. A spatial standard observer apparatus for producing a visibility metric, the apparatus comprising:

a computer that is programmed for;

i) receiving a digital reference image and a digital test image;

ii) producing a reference luminance image from the reference image and a test luminance image from the test image;

iii) producing a local mean luminance reference image as a convolution of the reference luminance image and a luminance filter function;

iv) producing a test contrast image as at least one of the following;

(iv-1) a mathematical combination of the test luminance image and the local mean luminance reference image and (iv-2) a mathematical combination of the test luminance image, the local mean luminance reference image and a border aperture function and (iv-3) a mathematical combination of the test luminance image, the local mean luminance reference image and an image of a border surrounding the reference image;

v) producing a reference contrast image as at least one of the following;

(v-1) a mathematical combination of the reference luminance image and the local mean luminance reference image and (v-2) a mathematical combination of the reference luminance image, the local mean luminance reference image and the border aperture function;

vi) applying a contrast sensitivity filter to the test contrast image to produce a filtered test image;

vii) applying said contrast sensitivity filter to the reference contrast image to produce a filtered reference image; and

,

viii) providing a difference image by at least one of the following two processes;

(viii-1) subtracting the filtered reference image from the filtered test image to produce a difference image, and (viii-2) producing mask image as a mathematical combination of the filtered reference image with a masking filter, and producing a difference image as a ratio of the difference image and the mask image; and

,

ix) producing a just noticeable difference image as a mathematical combination of the difference image with a window function; and

,

x) pooling the just noticeable difference image to produce a visibility metric.

24. Apparatus as in claim 21, wherein said computer is further programmed for

providing an enlarged reference luminance image by including with the reference image a selected portion of said border surrounding said reference luminance image;

and,

25. Apparatus as in claim 21, wherein said computer is further programmed for said pre-processing of said digital reference image and said digital test image immediately preceding performance of said process (a).

26. Apparatus as in claim 25 wherein said pre-processing is performance of pre-filtering.

27. Apparatus as in claim 26 wherein said pre-filtering is performed by convolution with a pre-filtering function.

28. Apparatus as in claim 27 wherein said pre-filtering function is a Gaussian function.

29. Apparatus as in claim 25 wherein said pre-processing is performed by downsampling.

30. Apparatus as in claim 25 wherein said pre-processing is performed by cropping.

22. (canceled)

23. (canceled)

31. A method of forming a digital reference image from a digital test image, the method comprising:

a) performing a convolution of the test image with a reference filter, to thereby form the digital reference image.

32. A method as in claim 31, further comprising choosing said reference filter to be a smoothing filter.

33. A method as in claim 32 further comprising choosing said smoothing filter to be a Gaussian function.

34. A method as in claim 31 further comprising choosing said convolution to be confined convolution.

35. A method of reducing wrap-around in a digital image processing process that includes one or more convolution steps, the method comprising:

a) producing a reference luminance image from the reference image and a test luminance image from the test image;

b) producing a local mean luminance reference image as convolution of the reference luminance image and a luminance filter function;

c) producing a test contrast image as at least one of the following;

(c1) a mathematical combination of the test luminance image and the local mean luminance reference image and (c2) a mathematical combination of the test luminance image, the local mean luminance reference image and a border aperture function and (c3) a mathematical combination of the test luminance image, the local mean luminance reference image and an image of a border surrounding the reference image;

d) producing a reference contrast image as at least one of the following;

(d1) a mathematical combination of the reference luminance image and the local mean luminance reference image and (d2) a mathematical combination of the reference luminance image, the local mean luminance reference image and the border aperture function;

e) applying a contrast sensitivity filter to the test contrast image to produce a filtered test image;

f) applying the contrast sensitivity filter to the reference contrast image to produce a filtered reference image;

g) providing a difference image by at least one of the following two processes;

(g1) subtracting the filtered reference image from the filtered test image to produce a difference image, and (g2) producing a ask image as a mathematical combination of the filtered reference image with a masking filter, and producing a difference image as a ratio of the difference image and the mask image;

h) producing a just noticeable difference image as a mathematical combination of the difference image with a window function; and

i) pooling the just noticeable difference image to produce a visibility metric,

wherein the convolution operation in at least one of process (b) and process (h) is performed using confined convolution.

36. A method of reducing wrap-around in a digital imaging process that includes at least one image convolution step, the method comprising:

a) producing a reference luminance image from a reference image and a test luminance image from the test image;

b) producing a local mean luminance reference image as a convolution of the reference luminance image and a luminance filter function;

c) producing a test contrast image from the test luminance image and the local mean luminance reference image;

d) producing a reference contrast image from the reference luminance image and the local mean luminance reference image;

e) applying a contrast sensitivity filter to the test contrast image to produce a filtered test image;

f) applying the contrast sensitivity filter to the reference contrast image to produce a filtered reference image;

g) subtracting the filtered reference image from the filtered test image to produce a difference image;

h) producing a just noticeable difference image as a mathematical combination of the difference image with a window function, wherein the convolution process in at leat5 one of processes (b) and (h) includes further convolution with a reference filter function RF(x,y) that is substantially zero except for coordinates x and y lying in respective small coordinate ranges;

i) pooling said just noticeable difference image to produce a visibility metric.

37. (canceled)

38. (canceled)