Determining sharpness predictors for a digital image

1. A method for determining-sharpness predictors for input digital images that are not test pattern images, captured by a digital camera, comprising:

a) capturing a single input digital image by photographing a natural scene using the digital camera;

b) dividing the single input digital image into image blocks;

c) using a processor to compute blocks of discrete cosine transform coefficients by applying discrete cosine transforms to the image blocks;

d) using a processor to compute one or more image metrics by analyzing only the single input digital image captured by the digital camera, wherein at least one of the image metrics is an image sharpness metric related to the spatial frequency content of the single input digital image which is determined by combining the discrete cosine transform coefficients computed for a plurality of image blocks;

e) using a processor to compute a single sharpness predictor for the single input digital image from the one or more image metrics, wherein the sharpness predictor represents an indication of the level of sharpness degradations introduced by the digital camera; and

f) displaying a message on a user interface of the digital camera in response to the computed sharpness predictor.

2. The method of claim 1 wherein the input digital image is a color input digital image and one or more of the image metrics are computed from a luminance signal computed from the color input digital image.

3. The method of claim 1 wherein the computation of the block metrics includes the step of computing image statistics for the blocks of discrete cosine transform coefficients.

4. The method of claim 3 wherein the computation of the image statistics includes computing a centroid for the blocks of discrete cosine transform coefficients.

5. The method of claim 3 wherein the computation of the image statistics includes computing sums for the blocks of discrete cosine transform coefficients.

6. The method of claim 3 wherein blocks of discrete cosine transform coefficients failing to meet a threshold criterion are excluded from the computation of the image metric.

7. The method of claim 1 wherein the step of applying discrete cosine transforms to blocks of the input digital image is applied as part of an image compression operation.

8. The method of claim 1 wherein the image sharpness metric is computed from compressed digital image data determined using an image compression operation.

9. The method of claim 1 wherein the computation of one or more of the image metrics includes the step of computing the number of image pixels darker than a specified threshold.

10. The method of claim 1 wherein the computation of one or more of the image metrics includes the step of computing the number of image pixels lighter than a specified threshold.

11. The method of claim 1 wherein the image metrics are computed from a subset of the input digital image.

12. The method of claim 11 wherein the subset of the input digital image is extracted from the center of the input digital image.

13. The method of claim 11 wherein the subset of the input digital image corresponds to a main subject region of the input digital image.

14. The method of claim 13 wherein an automatic main subject detection algorithm is used to identify the main subject region of the input digital image.

15. The method of claim 11 wherein the subset of the input digital image is interactively specified by a user.

16. The method of claim 1 wherein the sharpness predictor is computed by computing a weighted summation of the two or more image metrics.

17. The method of claim 1 wherein the sharpness predictor is computed by computing a weighted sum of functions of the one or more image metrics.

18. The method of claim 1 further including using the sharpness predictor to estimate whether the quality of the input digital image is sufficient for a particular usage.

19. The method of claim 18 wherein the usage is producing a reflection print.

20. The method of claim 1 wherein the displayed message presents a recommended image usage to a user.

21. The method of claim 1 further including using the sharpness predictor as a control parameter for a subsequent image processing operation.

22. The method of claim 1 wherein the step of computing the sharpness predictor further includes using one or more additional sharpness metrics computed from image capture parameters.

23. The method of claim 22 wherein the image capture parameters include an exposure time parameter, a lens aperture parameter, a flash-fired parameter, or combinations thereof.

24. A computer program product, comprising a computer readable storage medium for practicing the method according to claim 1 .

25. The method of claim 1 wherein the displayed message is a warning message is displayed on the user interface when the computed sharpness predictor indicates that unacceptable sharpness degradations have been introduced by the digital camera.