Multiscale gradation processing method
View Patent ↗A method of generating a contrast enhanced version of a grey value image by applying contrast amplification to a multiscale representation of the grey value image wherein density in the contrast enhanced version as a function of grey value and contrast amplification are specified independently.
1. A method of generating a contrast enhanced version of a gray value image comprising applying contrast amplification to a multiscale representation of said image, said multiscale representation representing image detail at different scales, wherein the density in the contrast enhanced version as a function of grey value and the contrast amplification are specified independently, wherein said contrast amplification is specified at successive scales by a series of gradient functions that represent the amount of contrast amplification as a function of gray value at the corresponding scale, and wherein a gradient function for a predefined large scale among said scales is derived from the specified density as a function of gray value.
2. The method according to claim 1 wherein a gradient function for a predefined large scale among said scales is the derivative of a predefined gradation function that specifies density as function of grey value.
3. The method according to claim 2 wherein said large-scale gradation function has a predefined ordinate value and a predefined slope in an anchor point, the abscissa value of the anchor point being deduced from a digital image representation of said grey value image or of a large-scale image obtained by applying partial reconstruction to said multiscale representation.
4. The method according to claim 2 wherein said large-scale gradation function has a predefined shape, and is stretched and shifted along the abscissa axis in order to match a relevant subrange of pixel values of said gray value image or of a large-scale image obtained by applying partial reconstruction to said multiscale representation.
5. The method according to claim 2 wherein said large-scale gradation function is derived from the histogram of pixel values of said gray value image or of a large-scale image obtained by applying partial reconstruction to said multiscale representation.
6. The method according to claim 5 in which said large-scale gradation function is further adjusted so that it has a predefined ordinate value in at least one anchor point, the abscissa of which is determined as a characteristic point of the histogram of pixel values of said gray value image or of a large-scale image obtained by applying partial reconstruction to said multiscale representation.
7. The method according to claim 1 wherein said contrast amplification is specified at successive scales by a series of gradient functions that represent the amount of contrast amplification as a function of value at the corresponding scale, wherein a gradient function for a predefined large scale among said scales is derived from the histogram of the pixel values of said gray value image or from the histogram of pixel values of a large scale image obtained by applying partial reconstruction to said multiscale representation.
8. The method according to claim 1 wherein said contrast amplification is specified at successive scales by a series of gradient functions that represent the amount of contrast amplification as a function of gray value at the corresponding scale, wherein the gradient functions at the smaller scales larger than a predefined large scale are derived from the specified contrast amplification.
9. The method according to claim 8 wherein the contrast amplification is specified by a predefined gradient function at a predefined small scale.
10. The method according to claim 9 wherein said gradient function at said predefined small scale has a predefined value in each of at least two overlapping gray value bands.
11. The method according to claim 10 wherein said gradient function at said predefined small scale depends on a gradient function at a predefined large scale.
12. The method according to claim 9 wherein said gradient function at said predefined small scale depends on a gradient function at a predefined large scale.
13. The method according to claim 9 wherein said predefined gradient function for said small scale is expressed as a function of density.
14. The method according to claim 9 modified so that the small-scale gradient function is adjusted as a function of the signal-to-noise ratio of the original digital image.
15. The method according to claim 14 wherein gradient functions at the scales smaller than said small-scale are identical to the gradient function for said small scale.
16. The method according to claim 9 wherein gradient functions at the scales smaller than said small-scale are identical to the gradient function for said small scale.
17. The method according to claim 1 in which gradient functions for intermediate scales in between said large scale and a predefined small scale have a shape that evolves gradually from the shape of the gradient function for said large scale to the shape of a gradient function for said predefined small scale.
18. The method according to claim 17 in which the gradient functions gm k ( ) at intermediate scales k are defined by:
gm
k
(
)
=
gm
S
(
)
·
(
gm
L
(
)
gm
S
(
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)
k
-
S
L
-
S
,
wherein gm L ( ) is said large-scale gradient function at scale L, gm S ( ) is said small-scale gradient function at scale S, and S<k<L.
19. The method according to claim 1 in which one or more of the scale-specific gradient functions or scale-specific conversion functions are stored as lookup tables.
20. The method according to claim 1 in which said multiscale representation is a Burt pyramid, a multiresolution subband representation or a wavelet representation.
21. The method according to claim 1 in which said gray value image is a medical image.
22. The method according to claim 1 in which said medical image is a digital X-ray image.
23. A computer program embodied in a computer readable medium for facilitating performing the step of applying contrast amplification to a multiscale representation of said image, said multiscale representation representing image detail at different scales, wherein the density in the contrast enhanced version as a function of grey value and the contrast amplification are specified independently, wherein said contrast amplification is specified at successive scales by a series of gradient functions that represent the amount of contrast amplification as a function of gray value at the corresponding scale, and wherein a gradient function for a predefined large scale among said scales is derived from the specified density as a function of gray value.
24. A computer readable medium comprising computer executable instructions for facilitating performing the step of applying contrast amplification to a multiscale representation of said image, said multiscale representation representing image detail at different scales, wherein the density in the contrast enhanced version as a function of grey value and the contrast amplification are specified independently, wherein said contrast amplification is specified at successive scales by a series of gradient functions that represent the amount of contrast amplification as a function of gray value at the corresponding scale, and wherein a gradient function for a predefined large scale among said scales is derived from the specified density as a function of gray value.
25. A method of generating a contrast enhanced version of a grey value image by applying contrast amplification to a multiscale representation of said image, said multiscale representation representing image detail at different scales, wherein the density in the contrast enhanced version as a function of grey value and the contrast amplification are specified independently, said contrast amplification is specified at successive scales by a series of gradient functions that represent the amount of contrast amplification as a function of value at the corresponding scale, and wherein a gradient function for a predefined large scale among said scales is derived from the histogram of the pixel values of said gray value image or from the histogram of pixel values of a large scale image obtained by applying partial reconstruction to said multiscale representation.
26. A method of generating a contrast enhanced version of a grey value image by applying contrast amplification to a multiscale representation of said image, said multiscale representation representing image detail at different scales, wherein the density in the contrast enhanced version as a function of grey value and the contrast amplification are specified independently, wherein said contrast amplification is specified at successive scales by a series of gradient functions that represent the amount of contrast amplification as a function of gray value at the corresponding scale, and wherein the gradient functions at the smaller scales larger than a predefined large scale are derived from the specified contrast amplification.