Method and apparatus for shoot suppression in image detail enhancement

• US 20030113032A1
• Filed: 01/16/2003
• Published: 06/19/2003
• Est. Priority Date: 01/16/2003
• Status: Active Grant

• Current Assignee: Samsung Electronics Co. Ltd.
• Original Assignee: Samsung Electronics Co. Ltd.
• Inventors: Kim, Yeong-Taeg, Wang, Xianglin
• Granted Patent: US 7,269,296 B2
• Time in Patent Office: Days
• US Class Current: 382/275
• CPC Class Codes: G06T 5/20 using local operators
• CPC Class Codes: G06T 5/75 Unsharp masking
• CPC Class Codes: H04N 1/4092 Edge or detail enhancement

Abstract

A general shoot suppression method and system for image detail enhancement that provides good suppression in shoot areas while preserving the enhancement (with less or no suppression) in non-shoot areas. Asymmetry checker checks the luminance variation of pixels around the current pixel within a filtering range. Based on the checking result, different patterns of luminance variation in the neighborhood of the current pixel are categorized. Higher suppression is applied to those patterns that are more likely to cause overshoot/undershoot. Optionally, suppression is further associated with the intensity of detail signal at each pixel position. An intensity checker checks the magnitude of the detail signal. If the detail signal is weak at a current pixel position, it is unlikely that obvious shoot artifacts appear at that position, and little or no suppression is applied. Such a method and system provide shoot suppression only in shoot areas while maintaining good enhancement in non-shoot areas.

Claims

1. A method for shoot suppression in image detail enhancement, comprising the steps of:

(a) selecting image details for enhancement, wherein the selected image details are represented by a detail signal;

(b) determining a shoot suppression factor s for the detail signal at each pixel position based on the image characteristics of the pixels neighboring a current image pixel in order to substantially suppress shoot artifacts in the enhanced image details; and

(c) applying the shoot suppression factor s to the detail signal.

2. The method of claim 1, wherein step (b) further includes the steps of:

determining a symmetry factor α

based on luminance values of pixels neighboring a current image pixel, wherein the symmetry factor represents amount of symmetry in the luminance curve of the neighboring pixels with reference to the current pixel position, and

determining the shoot suppression factor s based on the symmetry factor α

, wherein;

for an essentially symmetric luminance curve, lower shoot suppression is selected, and

for an essentially asymmetric luminance curve, higher shoot suppression is selected.

3. The method of claim 2, wherein:

the symmetry factor α

at a pixel position is determined as;

$\mathrm{\alpha <br><br>}=\{\begin{array}{cc}0,& \mathrm{if}<br><br>\text{\hspace{1em}<br><br>}<br><br>M_r*M_l=0\\ \min <br><br>\left(A_l/M_r,A_r/M_l\right),& \mathrm{otherwise},\end{array}$

such that M_l is the maximum absolute luminance difference between the current pixel and any pixel on its left side, M_r is the maximum absolute luminance difference between the current pixel and any pixel on its right side, A, is the average of the absolute luminance difference values on the left side of the current pixel, A_r is the average of the absolute luminance difference values on the right side of the current pixel.

4. The method of claim 3, wherein:

the detail signal is a function of the difference between an image signal f representing the image, and a filtered signal f₁ representing the signal f after application of a low pass filter, such that signal f₁ at each pixel position is determined based on pixels of signal f within the filtering range, represented by a one dimensional sequence of pixels centered with current pixels, wherein samples of signal f are represented by X[i], i=−

L, . . . , 1, 0, 1, . . . , L, such that X[0] is the current pixel to be enhanced, L is a constant where the value (2*L+1) is the length of the filter, and X[i], i=−

L, . . . , −

1, 0, 1, . . . , L are essentially all the pixels involved in filtering when X[0] is being enhanced; and

M₁=max(d<sub>0,−

L</sub>,d<sub>0,−

L+1</sub>, . . . ,d<sub>0,−

1</sub>), M_r=max(d_L,0,d<sub>L−

1,0</sub>, . . . ,d_1,0), $\begin{array}{c}{A}_l=\frac{1}{L}<br><br>\underset{i=0}{\overset{L-1}{\sum <br><br>}}<br><br>{<br><br>}_{0,-L+i},\\ A_r=\frac{1}{L}<br><br>\underset{i=0}{\overset{L-1}{\sum <br><br>}}<br><br>{<br><br>}_{L-i,0},\end{array}$

wherein d_m,n denotes the absolute luminance difference between pixel X[m] at position m and pixel X[n] at position n, such that;

d_m,n=|X[m]−

X[n]|.

5. The method of claim 2, wherein the shoot suppression factor s at a pixel position is represented by:

s=min(0,K₁α

)

wherein K₁ is a constant, K₁≧

1.

6. A method for shoot suppression in image detail enhancement, wherein selected image details are represented by a detail signal, comprising the steps of:

(a) determining a symmetry factor α

based on luminance values of pixels neighboring a current image pixel, wherein the symmetry factor represents amount of symmetry in the luminance curve of the neighboring pixels with reference to the current pixel position;

(b) determining a detail signal intensity factor β

based on the intensity of the detail signal at the current pixel position;

(c) determining a shoot suppression factor s based on the symmetry factor α and

the intensity factor β

, wherein;

if said luminance curve is essentially symmetric, then selecting lower shoot suppression,

if said luminance curve is essentially asymmetric, then selecting higher shoot suppression,

if the detail signal is weak in intensity, then selecting lower or no shoot suppression to allow a better enhancement at that pixel position; and

(d) applying the shoot suppression factor s to the detail signal, in order to substantially suppress shoot artifacts in the enhanced image details.

7. The method of claim 6, wherein:

if β

is close to 0 then the detail signal at current pixel position is considered weak, and if β

is close to 1 then the detail signal at current pixel position is considered strong,

such that;

β

=min(1,max(0,(|dd|−

T₁)/(T₂−

T₁))),

wherein dd is the detail signal at the current pixel position, and threshold values T₁ and T₂ (0≦

T₁≦

T₂) are selected such that essentially no obvious shoot artifacts are introduced during enhancement when detail signal is considered weak.

8. The method of claim 6, wherein the shoot suppression factor s is represented as:

s=1−

(1−

min(1,K₁α

))*β

,

wherein K₁ is a constant, K₁≧

1.

9. The method of claim 6, wherein:

the symmetry factor α

at a pixel position is determined as;

$\mathrm{\alpha <br><br>}=\{\begin{array}{cc}0,& \mathrm{if}<br><br>\text{\hspace{1em}<br><br>}<br><br>M_r*M_l=0\\ \min <br><br>\left(A_l/M_r,A_r/M_l\right),& \mathrm{otherwise},\end{array}$

such that M_l is the maximum absolute luminance difference between the current pixel and any pixel on its left side, M_r is the maximum absolute luminance difference between the current pixel and any pixel on its right side, A_l is the average of the absolute luminance difference values on the left side of the current pixel, A_r is the average of the absolute luminance difference values on the right side of the current pixel.

10. The method of claim 9, wherein:

the detail signal is a function of the difference between an image signal f representing the image, and a filtered signal f₁ representing the signal f after application of a low pass filter (LPF), such that signal f₁ at each pixel position is determined based on pixels of signal f within the filtering range, represented by a one dimensional sequence of pixels centered with current pixels, wherein samples of signal f are represented by X[i], i=−

L, . . . , −

1, 0, 1, . . . , L, such that X[0] is the current pixel to be enhanced, L is a constant where the value (2*L+1) is the length of the filter, and X[i], i=−

L, . . . , −

1, 0, 1, . . . , L are essentially all the pixels involved in filtering when X[0] is being enhanced; and

M_l=max(d<sub>0,−

L</sub>,d<sub>0,−

L+1</sub>, . . . ,d<sub>0,−

1</sub>) M_r=max(d_L,0,d<sub>L−

1,0</sub>, . . . ,d_1,0) $\begin{array}{c}{A}_l=\frac{1}{L}<br><br>\underset{i=0}{\overset{L-1}{\sum <br><br>}}<br><br>{<br><br>}_{0,-L+i},\\ A_r=\frac{1}{L}<br><br>\underset{i=0}{\overset{L-1}{\sum <br><br>}}<br><br>{<br><br>}_{L-i,0},\end{array}$

wherein d_m,n denotes the absolute difference between pixel X[m] at position m and pixel X[n] at position n, such that;

d_m,n=|X[m]−

X[n]|.

11. A method for enhancing image details in an image signal f, comprising the steps of:

(a) generating a detail signal f₂ as a function of the image signal f, the detail signal f₂ representing the image details;

(b) determining a shoot suppression factor s for the detail signal f₂ at each pixel position based on the image characteristics of the pixels neighboring a current pixel, in order to substantially suppress shoot artifacts in the enhanced image details; and

(c) applying the shoot suppression factor s to the detail signal f₂.

12. The method of claim 11, wherein step (b) further includes the steps of:

determining a symmetry factor α

based on luminance values of pixels neighboring a current image pixel, wherein the symmetry factor represents amount of symmetry in the luminance curve of the neighboring pixels with reference to the current pixel position, and

determining the shoot suppression factor s based on the symmetry factor α

, wherein;

for an essentially symmetric luminance curve, lower shoot suppression is selected, and

for an essentially asymmetric luminance curve, higher shoot suppression is selected.

13. The method of claim 12, wherein:

the symmetry factor α

at a pixel position is determined as;

$\mathrm{\alpha <br><br>}=\{\begin{array}{cc}0,& \mathrm{if}<br><br>\text{\hspace{1em}<br><br>}<br><br>M_r*M_l=0\\ \min <br><br>\left(A_l/M_r,A_r/M_l\right),& \mathrm{otherwise},\end{array}$

such that M_l is the maximum absolute luminance difference between the current pixel and any pixel on its left side, M_r is the maximum absolute luminance difference between the current pixel and any pixel on its right side, A_l is the average of the absolute luminance difference values on the left side of the current pixel, A_r is the average of the absolute luminance difference values on the right side of the current pixel.

14. The method of claim 13, wherein:

the detail signal is a function of the difference between an image signal f representing the image, and a filtered signal f₁ representing the signal f after application of a low pass filter, such that signal f₁ at each pixel position is determined based on pixels of signal f within the filtering range, represented by a one dimensional sequence of pixels centered with current pixels, wherein samples of signal f are represented by X[i], i=−

L, . . . , −

1, 0, 1, . . . , L, such that X[0] is the current pixel to be enhanced, L is a constant where the value (2*L+1) is the length of the filter, and X[i], i=−

L, . . . , −

1, 0, 1, . . . , L are essentially all the pixels involved in filtering when X[0] is being enhanced; and

M_l=max(d<sub>0,−

L</sub>,d<sub>0,−

L+1</sub>, . . . ,d<sub>0,−

1</sub>), M_r=max(d_L,0,d<sub>L−

1,0</sub>, . . . ,d_1,0) $\begin{array}{c}{A}_l=\frac{1}{L}<br><br>\underset{i=0}{\overset{L-1}{\sum <br><br>}}<br><br>{<br><br>}_{0,-L+i},\\ A_r=\frac{1}{L}<br><br>\underset{i=0}{\overset{L-1}{\sum <br><br>}}<br><br>{<br><br>}_{L-i,0},\end{array}$

wherein d_m,n denotes the absolute luminance difference between pixel X[m] at position m and pixel X[n] at position n, such that;

d_m,n=|X[m]−

X[n]|.

15. The method of claim 12, wherein the shoot suppression factor s at a pixel position is represented by:

s=min(1,K₁α

)

wherein K₁ is a constant, K₁≧

1.

16. The method of claim 11, further including the steps of determining a detail signal intensity factor β

based on the intensity of the detail signal at the current pixel position;

wherein in step (b), determining the shoot suppression factor s is based on the symmetry factor α and

the intensity factor β

, wherein;

if said luminance curve is essentially symmetric, then selecting lower shoot suppression,

if said luminance curve is essentially asymmetric, then selecting higher shoot suppression,

if the detail signal is weak in intensity, then selecting lower or no shoot suppression to allow a better enhancement at that pixel position.

17. The method of claim 16, wherein:

if β

is close to 0 then the detail signal at current pixel position is considered weak, and if β

is close to 1 then the detail signal at current pixel position is considered strong,

such that;

β

=min(1,max(0,(|dd|−

T₁)/(T₂−

T₁))),

wherein dd is the detail signal at the current pixel position, and threshold values T₁ and T₂ (0≦

T₁<

T₂) are selected such that essentially no obvious shoot artifacts are introduced during enhancement when detail signal is considered weak.

18. The method of claim 16, wherein the shoot suppression factor s is represented as:

s=1−

(1−

min(1,K₁α

))*β

,

wherein K₁ is a constant, K₁≧

1.

19. An apparatus for shoot suppression in image detail enhancement, wherein selected image details are represented by a detail signal, comprising:

a shoot suppressor that determining a shoot suppression factor s for the detail signal based on the image characteristics of the pixels neighboring a current pixel in order to substantially suppress shoot artifacts in the enhanced image details; and

a combiner that applies the shoot suppression factor s to the detail signal.

20. The apparatus of claim 19, wherein the shoot suppressor comprises:

a symmetry checker for determining a symmetry factor α

based on luminance values of pixels neighboring a current image pixel, wherein the symmetry factor represents amount of symmetry in the luminance curve of the neighboring pixels with reference to the current pixel position,

such that the shoot suppressor determines the shoot suppression factor s based on the symmetry factor α

, wherein;

for an essentially symmetric luminance curve, lower shoot suppression is selected, and

for an essentially asymmetric luminance curve, higher shoot suppression is selected.

21. The apparatus of claim 20, wherein:

the symmetry checker determines the symmetry factor α

at a pixel position according to the relation;

$\mathrm{\alpha <br><br>}=\{\begin{array}{cc}0,& \mathrm{if}<br><br>\text{\hspace{1em}<br><br>}<br><br>M_r*M_l=0\\ \min <br><br>\left(A_l/M_r,A_r/M_l\right),& \mathrm{otherwise},\end{array}$

wherein M_l is the maximum absolute luminance difference between the current pixel and any pixel on its left side, M_r is the maximum absolute luminance difference between the current pixel and any pixel on its right side, A_l is the average of the absolute luminance difference values on the left side of the current pixel, A_r is the average of the absolute luminance difference values on the right side of the current pixel.

22. The apparatus of claim 21, wherein:

the detail signal is a function of the difference between an image signal f representing the image, and a filtered signal f₁ representing the signal f after application of a low pass filter, such that signal f₁ at each pixel position is determined based on pixels of signal f within the filtering range, represented by a one dimensional sequence of pixels centered with current pixels, wherein samples of signal f are represented by X[i], i=−

L, . . . , 1, 0, 1, . . . , L, such that X[0] is the current pixel to be enhanced, L is a constant where the value (2*L+1) is the length of the filter, and X[i], i=−

L, . . . , −

1, 0, 1, . . . , L are essentially all the pixels involved in filtering when X[0] is being enhanced; and

M₁=max(d<sub>0,−

L</sub>,d<sub>0,−

L+1</sub>,d<sub>0,−

1</sub>), M_r=max(d_L,0,d<sub>L−

1,0</sub>, . . . d_1,0),

$\begin{array}{c}{A}_l=\frac{1}{L}<br><br>\underset{i=0}{\overset{L-1}{\sum <br><br>}}<br><br>{<br><br>}_{0,-L+i},\\ A_r=\frac{1}{L}<br><br>\underset{i=0}{\overset{L-1}{\sum <br><br>}}<br><br>{<br><br>}_{L-i,0},\end{array}$

wherein d_m,n denotes the absolute luminance difference between pixel X[m] at position m and pixel X[n] at position n, such that;

d_m,n=|X[m]−

X[n]|.

23. The apparatus of claim 20, wherein the shoot suppressor determined the shoot suppression factor s at a pixel position according to the relation:

s=min(1,K₁α

),

wherein K₁ is a constant, K₁≧

1.

24. An image enhancement apparatus for enhancing image details in an image signal f, comprising:

(a) a detail signal generator for generating a detail signal f₂ as a function of the image signal f, the detail signal f₂ representing the image details;

(b) a shoot suppressor that determines a shoot suppression factor s for the detail signal f₂ based on the image characteristics of the pixels neighboring a current pixel, in order to substantially suppress shoot artifacts in the enhanced image details; and

(c) a combiner that applies the shoot suppression factor s to the detail signal f₂.

25. The apparatus of claim 24, wherein:

the shoot suppressor includes a symmetry checker that determines a symmetry factor α

based on luminance values of pixels neighboring a current image pixel, wherein the symmetry factor represents amount of symmetry in the luminance curve of the neighboring pixels with reference to the current pixel position, and

the shoot suppressor determines the suppression factor s based on the symmetry factor α

, wherein;

for an essentially symmetric luminance curve, lower shoot suppression is selected, and

for an essentially asymmetric luminance curve, higher shoot suppression is selected.

26. The apparatus of claim 25, wherein:

the symmetry checker determined the symmetry factor α

at a pixel position according to the relation;

$\mathrm{\alpha <br><br>}=\{\begin{array}{cc}0,& \mathrm{if}<br><br>\text{\hspace{1em}<br><br>}<br><br>M_r*M_l=0\\ \min <br><br>\left(A_l/M_r,A_r/M_l\right)& \mathrm{otherwise},\end{array}$

wherein M_l is the maximum absolute luminance difference between the current pixel and any pixel on its left side, M_r is the maximum absolute luminance difference between the current pixel and any pixel on its right side, A_l is the average of the absolute luminance difference values on the left side of the current pixel, A_r is the average of the absolute luminance difference values on the right side of the current pixel.

27. The apparatus of claim 26, wherein:

the detail signal is a function of the difference between an image signal f representing the image, and a filtered signal f₁ representing the signal f after application of a low pass filter, such that signal f₁ at each pixel position is determined based on pixels of signal f within the filtering range, represented by a one dimensional sequence of pixels centered with current pixels, wherein samples of signal f are represented by X[i], i=−

L, . . . , -1, 0, 1, . . . , L, such that X[0] is the current pixel to be enhanced, L is a constant where the value (2*L+1) is the length of the filter, and X[i], i=−

L, . . . , -1, 0, 1, . . . , L are essentially all the pixels involved in filtering when X[0] is being enhanced; and

M_l=max(d<sub>0,−

L</sub>,d<sub>0,−

L+1</sub>, . . . ,d<sub>0,−

1</sub>) M_r=max(d_L,0,d<sub>L−

1,0</sub>, . . . ,d_1,0)

$\begin{array}{c}{A}_l=\frac{1}{L}<br><br>\underset{i=0}{\overset{L-1}{\sum <br><br>}}<br><br>{<br><br>}_{0,-L+i},\\ A_r=\frac{1}{L}<br><br>\underset{i=0}{\overset{L-1}{\sum <br><br>}}<br><br>{<br><br>}_{L-i,0},\end{array}$

wherein d_m,n denotes the absolute luminance difference between pixel X[m] at position m and pixel X[n] at position n, such that;

d_m,n=|X[m]−

X[n]|.

28. The apparatus of claim 25, wherein the shoot suppressor determined the shoot suppression factor s at a pixel position according to the relation:

s=min(1,K₁α

)

wherein K₁ is a constant, K₁≧

1.

29. The apparatus of claim 24, wherein:

the shoot suppressor further includes an intensity checker that determines a detail signal intensity factor β

based on the intensity of the detail signal at the current pixel position; and

the shoot suppressor determines the shoot suppression factor s based on the symmetry factor α and

the intensity factor β

, wherein;

if said luminance curve is essentially symmetric, then selecting lower shoot suppression,

if said luminance curve is essentially asymmetric, then selecting higher shoot suppression,

if the detail signal is weak in intensity, then selecting lower or no shoot suppression to allow a better enhancement at that pixel position.

30. The apparatus of claim 29, wherein:

if β

is close to 0 then the detail signal at current pixel position is considered weak, and if β

is close to 1 then the detail signal at current pixel position is considered strong,

such that;

β

=min(1,max(0,(|dd|−

T₁)/(T₂−

T₁))),

wherein dd is the detail signal at the current pixel position, and threshold values T₁ and T₂ (0≦

T₁<

T₂) are selected such that essentially no obvious shoot artifacts are introduced during enhancement when detail signal is considered weak.

31. The apparatus of claim 29, wherein the shoot suppressor determines the suppression factor s according to the relation:

s=1−

(1−

min(1,K₁α

))*β

,

wherein K₁ is a constant, K₁≧

1.

32. An image enhancement apparatus for enhancing image details in an image signal f, comprising:

a shoot suppressor that determines a suppression factor s by checking the luminance pattern of image pixels neighboring a current pixel, in order to substantially suppress shoot artifacts in the enhanced image details.

33. The apparatus of claim 32, further comprising:

a detail signal generator for generating the detail signal f₂ as a function of the image signal f, the detail signal representing the image details; and

a combiner that applies the shoot suppression factor s to the detail signal f2.

34. The apparatus of claim 32, wherein the shoot suppressor further determines the suppression factor s by checking the luminance pattern of image pixels neighboring a current pixel, and intensity of detail signal, in order to substantially suppress shoot artifacts in the enhanced image details.

35. The shoot suppression system for an image detail enhancer, comprising:

a shoot suppressor including a symmetry checker that determines the maximum absolute difference values and average absolute difference values between a current image pixel and neighboring pixels, and based on said values determines a symmetry factor α

that is an indication of the symmetry of the luminance curve of said neighboring pixels with reference to the current pixel;

wherein the shoot suppressor determines a suppression factor based on the symmetry factor, in order to substantially suppress shoot artifacts in the enhanced image details.

36. The system of claim 35, wherein the shoot suppressor further includes:

an intensity checker that determines the intensity factor β

of a detail signal at a current pixel position, wherein the detail signal represents the image details;

such that, the shoot suppressor determines the suppression factor s based on the symmetry factor α and

the intensity factor β

, wherein when the detail signal is weak in intensity, shoot suppression for that pixel position is reduced or bypassed to allow a better enhancement at such a pixel position.