Device and a method for encoding an image and corresponding decoding method and decoding device

1. A method for encoding an image represented in a perceptual color space, wherein the image has at least a luminance component, the method comprising:

transforming by a wavelet transform the luminance component to form at least one low frequency subband of wavelet coefficients;

determining a low frequency component from the wavelet coefficient(s) of at least one spatial area of the low frequency subband;

quantizing and inverse quantizing said determined low frequency component to obtain a reconstructed low frequency component {circumflex over (L)} lf for the at least one spatial area;

subtracting the reconstructed low frequency component from the wavelet coefficients of the spatial area;

quantizing the wavelet coefficients in the spatial area responsive to a threshold, wherein said threshold is proportional to the reconstructed low frequency component {circumflex over (L)} lf ; and

encoding the quantized wavelet coefficients and the low frequency component.

2. The method of claim 1 , wherein the spatial area comprises a single wavelet coefficient and wherein reconstructing a low frequency component for the spatial area of the low frequency subband comprises quantizing and inverse quantizing the single wavelet coefficient.

3. The method of claim 1 , wherein the spatial area comprises at least two wavelet coefficients and wherein reconstructing a low frequency component for the spatial area of the low frequency subband comprises averaging the at least two wavelet coefficients into an averaged coefficient, quantizing and inverse quantizing the average coefficient.

4. The method of claim 1 , wherein quantizing the wavelet coefficients of the spatial area comprises:

normalizing the wavelet coefficients using the threshold of the spatial area; and

quantizing the normalized wavelet coefficients using a quantization step size independent of the spatial area.

5. The method according to claim 3 , wherein quantizing the wavelet coefficients in the spatial area comprises:

determining a quantization step size for the spatial area from the threshold of the spatial area; and

quantizing the wavelet coefficients using the determined quantization step size.

6. The method of claim 1 , wherein the image has at least one chrominance component and wherein the method further comprises:

transforming by the wavelet transform the at least one chrominance component into chrominance wavelet coefficients; and

normalizing the chrominance wavelet coefficients by the threshold of the spatial area to which the chrominance wavelet coefficients belong.

7. The method of claim 1 , wherein the at least one low frequency subband of wavelet coefficients being divided into spatial areas, the method applies on each spatial area.

8. A coding device for encoding an image represented in a perceptual color space, the image having at least a luminance component, wherein the coding device comprises at least a processor configured to:

transform by a wavelet transform the luminance component to form at least one low frequency subband of wavelet coefficients;

determine a low frequency component from the wavelet coefficient(s) of at least one spatial area of the low frequency subband;

quantize and inverse quantize said determined low frequency component to obtain a reconstructed low frequency component {circumflex over (L)} lf for the at least one spatial area;

subtract the reconstructed low frequency component from the wavelet coefficients of the spatial area;

quantize the wavelet coefficients in the spatial area responsive to a threshold, wherein said threshold is proportional to the reconstructed low frequency component; and

encode the quantized wavelet coefficients and the low frequency component.

9. The coding device of claim 8 , wherein the spatial area comprises a single wavelet coefficient and wherein to reconstruct a low frequency component for the spatial area of the low frequency subband comprises quantizing and inverse quantizing the single wavelet coefficient.

10. The coding device of claim 8 , wherein the spatial area comprises at least two wavelet coefficients and wherein to reconstruct a low frequency component for the spatial area of the low frequency subband comprises averaging the at least two wavelet coefficients into an averaged coefficient, quantizing and inverse quantizing the average coefficient.

11. The coding device of claim 8 , wherein to quantize the wavelet coefficients of the spatial area comprises:

normalizing the wavelet coefficients using the threshold of the spatial area; and

quantizing the normalized wavelet coefficients using a quantization step size independent of the spatial area.

12. The coding device of claim 10 , wherein to quantize the wavelet coefficients in the spatial area comprises:

determining a quantization step size for the spatial area from the threshold of the spatial area; and

quantizing the wavelet coefficients using the determined quantization step size.

13. The coding device of claim 8 , wherein the image has at least one chrominance component and wherein the at least one processor is further configured to:

transform by the wavelet transform the at least one chrominance component into chrominance wavelet coefficients; and

normalize the chrominance wavelet coefficients by the threshold of the spatial area to which the chrominance wavelet coefficients belong.

14. The coding device of claim 8 , wherein the at least one low frequency subband of wavelet coefficients being divided into spatial areas, the at least a processor processes each spatial area.

15. A method for decoding an image having at least a luminance component represented in a perceptual color space, the method comprising:

decoding wavelet coefficients of at least a low frequency subband and a low frequency component for at least one spatial area of the low frequency subband;

inverse quantizing the decoded wavelet coefficients of the spatial area responsive to a threshold, the threshold being proportional to the decoded low frequency component of the spatial area;

adding the decoded low frequency component to the wavelet coefficients of the spatial area; and

transforming by an inverse wavelet transform the wavelet coefficients to form a reconstructed luminance component.

16. The method according to claim 15 , wherein inverse quantizing the wavelet coefficients of the spatial area comprises:

inverse quantizing the wavelet coefficients; and

de-normalizing the wavelet coefficients using the threshold of the spatial area.

17. The method according to claim 15 , wherein inverse quantizing the wavelet coefficients of the spatial area comprises:

determining a quantization step size for the spatial area from the threshold of the spatial area; and

inverse quantizing the wavelet coefficients using the determined quantization step size.

18. The method according to claim 15 , wherein the image has at least one chrominance component and wherein the method further comprises:

decoding chrominance wavelet coefficients; and

de-normalizing the chrominance wavelet coefficients by the threshold of the spatial area to which the chrominance wavelet coefficients belong.

19. The method according to claim 15 , wherein the at least one low frequency subband of wavelet coefficients being divided into spatial areas, the method applies on each spatial area.

20. A decoding device for decoding an image having at least a luminance component represented in a perceptual color space, wherein the decoding device comprises at least a processor configured to:

decode wavelet coefficients of at least a low frequency subband and a low frequency component for at least one spatial area of the low frequency subband;

inverse quantize the decoded wavelet coefficients of the spatial area responsive to a threshold, the threshold being proportional to the decoded low frequency component of the spatial area;

add the decoded low frequency component to the wavelet coefficients of the spatial area; and

transform by an inverse wavelet transform the wavelet coefficients to form a reconstructed luminance component.

21. The decoding device according to claim 20 , wherein to inverse quantize the wavelet coefficients of the spatial area comprises:

inverse quantizing the wavelet coefficients; and

de-normalizing the wavelet coefficients using the threshold of the spatial area.

22. The decoding device according to claim 20 , wherein to inverse quantize the wavelet coefficients of the spatial area comprises:

determining a quantization step size for the spatial area from the threshold of the spatial area; and

inverse quantizing the wavelet coefficients using the determined quantization step size.

23. The decoding device according to claim 20 , wherein the image has at least one chrominance component and wherein the at least one processor is further configured to:

decode chrominance wavelet coefficients; and

de-normalize the chrominance wavelet coefficients by the threshold of the spatial area to which the chrominance wavelet coefficients belong.

24. The decoding device according to claim 20 , wherein the at least one low frequency subband of wavelet coefficients being divided into spatial areas, at least a processor processes each spatial area.