Data compression for colour images using wavelet transform

• US 20050169548A1
• Filed: 04/09/2003
• Published: 08/04/2005
• Est. Priority Date: 04/19/2002
• Status: Active Grant

• Current Assignee: Qinetiq Limited (QinetiQ Group PLC)
• Original Assignee: Qinetiq Limited (QinetiQ Group PLC)
• Inventors: Ducksburg, Paul Gerard, Varga, Margaret Jai
• Granted Patent: US 7,512,277 B2
• Time in Patent Office: Days
• US Class Current: 382/240
• CPC Class Codes: H04N 19/115 Selection of the code volume for a coding unit prior to coding
• CPC Class Codes: H04N 19/146 Data rate or code amount at the encoder output
• CPC Class Codes: H04N 19/162 User input
• CPC Class Codes: H04N 19/167 Position within a video image, e.g. region of interest [ROI]
• CPC Class Codes: H04N 19/17 the unit being an image region, e.g. an object
• CPC Class Codes: H04N 19/186 the unit being a colour or a chrominance component
• CPC Class Codes: H04N 19/61 in combination with predictive coding
• CPC Class Codes: H04N 19/63 using sub-band based transform, e.g. wavelets

Abstract

The invention describes a wavelet compression method for colour converted image planes (Y, Cb and Cr), wherein a relatively low (e.g. zero) degree of compression is performed in areas of an original colour image having relatively higher importance, a relatively higher degree of compression in those areas indicated to be of lower importance. It performs a hierarchical encoding of a reduced wavelet image by discarding wavelet coefficients which satisfy the two criteria of firstly corresponding to image areas of relatively lower importance and secondly being below a certain wavelet coefficient threshold. The wavelet coefficient threshold is determined from a calculation histogram to remove a percentage of the image and can be specified as an input parameter by a user.

Claims

1. A method of data compression for colour images wherein it incorporates the following steps:

a) establishing a value for a number of scales into which a wavelet transformation is to be made;

b) distinguishing areas in an original colour image of relatively higher importance from those of relatively lower importance;

c) transforming the colour image into a second image in a different colour system having relatively more image information in a first component and relatively less in other components;

d) sub-sampling the other components to reduce their respective numbers of pixels;

e) transforming the first component and the sub-sampled components into wavelet coefficients with the said number of scales;

f) transforming the importance-distinguished areas to correspond to location and number of scales of the wavelet transformation; and

g) establishing a wavelet coefficient threshold and forming a reduced wavelet image by discarding wavelet coefficients which both correspond to image areas of relatively lower importance and are below the said threshold.

2. A method according to claim 1 including the step of producing a reconstituted colour image, this step comprising forming an encoded image by hierarchically encoding the reduced wavelet image, transmitting the encoded image to another location, and implementing respective inverses of the steps of encoding, wavelet transformation, sub-sampling and colour image transformation.

3. A method according to claim 2 wherein the step of forming an encoded image comprises forming a progressive bitstream in which more important image features are encoded earlier, and which includes information on number image rows and columns, number of scales and filter type.

4. A method according to claim 1 step of distinguishing areas in an original colour image of relatively higher importance from those of relatively lower importance comprises associating differing binary digits therewith.

5. A method according to claim 1 wherein:

a) the step of distinguishing areas of relatively higher importance from those of relatively lower importance comprises specifying a plurality of different levels of relatively lower importance, and

b) the step of establishing a wavelet coefficient threshold and forming a reduced wavelet image includes discarding progressively more wavelet coefficients as area importance level diminishes.

6. A method according to claim 1 wherein the step of transforming the colour image into a second image comprises transformation into luminance, blue chrominance and red chrominance.

7. A method according to claim 1 wherein the step of sub-sampling reduces pixel number to one quarter that preceding.

8. A method according to claim 1 wherein the step of wavelet transformation employs a Daubechies-4 filter.

9. A method according to claim 1 wherein the number of scales is in the range three to six.

10. A method according to claim 1 wherein the step of establishing a wavelet coefficient threshold comprises forming a cumulative histogram of numbers of pixels not exceeding respective wavelet coefficient values.

11. A computer program for use in data compression of colour images and having instructions for controlling computer apparatus to implement the following steps:

a) receiving a value for a number of scales into which a wavelet transformation is to be made;

b) receiving an indication of areas in an original colour image having relatively higher importance and those of relatively lower importance;

c) transforming the colour image into a second image in a different colour system having relatively more image information in a first component and relatively less in other components;

d) sub-sampling the other components to reduce their respective numbers of pixels;

e) transforming the first component and the sub-sampled components into wavelet coefficients with the said number of scales;

f) transforming the importance-distinguished areas to correspond to location and number of scales of the wavelet transformation; and

g) establishing a wavelet coefficient threshold and forming a reduced wavelet image by discarding wavelet coefficients which both correspond to image areas of relatively lower importance and are below the said threshold.

12. A computer program according to claim 11 having instructions for controlling computer apparatus to implement production of a reconstituted colour image by forming an encoded image by hierarchically encoding the reduced wavelet image, transmitting the encoded image to another location, and implementing respective inverses of the steps of encoding, wavelet transformation, sub-sampling and colour image transformation.

13. A computer program according to claim 11 having instructions for controlling computer apparatus to implement formation of an encoded image by forming a progressive bitstream in which more important image features are encoded earlier, and which includes information on number image rows and columns, number of scales and filter type.

14. A computer program according to claim 11 having instructions for controlling computer apparatus to distinguish areas in an original colour image of relatively higher importance from those of relatively lower importance by associating differing binary digits therewith.

15. A computer program according to claim 11 having instructions for controlling computer apparatus to implement transformation of the colour image into a second image by implementing a transformation into luminance, blue chrominance and red chrominance.

16. A computer program according to claim 11 having instructions for controlling computer apparatus to:

a) distinguish areas of relatively higher importance from those of relatively lower importance by specifying a plurality of different levels of relatively lower importance, and

b) establish a wavelet coefficient threshold and forming a reduced wavelet image by discarding progressively more wavelet coefficients as area importance level diminishes.

17. A computer program according to claim 11 having instructions for controlling computer apparatus to sub-sample by reducing pixel number to one quarter that preceding.

18. A computer program according to claim 11 having instructions for controlling computer apparatus to implement wavelet transformation with a Daubechies-4 filter.

19. A computer program according to claim 11 wherein the number of scales is in the range three to six.

20. A computer program according to claim 11 having instructions for controlling computer apparatus to establish a wavelet coefficient threshold by forming a cumulative histogram of numbers of pixels not exceeding respective wavelet coefficient values.

21. Computer apparatus for use in data compression of colour images and programmed to implement the following steps:

a) receiving a value for a number of scales into which a wavelet transformation is to be made;

b) receiving an indication of areas in an original colour image having relatively higher importance and those of relatively lower importance;

c) transforming the colour image into a second image in a different colour system having relatively more image information in a first component and relatively less in other components;

d) sub-sampling the other components to reduce their respective numbers of pixels;

e) transforming the first component and the sub-sampled components into wavelet coefficients with the said number of scales;

f) transforming the importance-distinguished areas to correspond to location and number of scales of the wavelet transformation; and

g) establishing a wavelet coefficient threshold and forming a reduced wavelet image by discarding wavelet coefficients which both correspond to image areas of relatively lower importance and are below the said threshold.

22. Computer apparatus according to claim 21 programmed to implement production of a reconstituted colour image by forming an encoded image by hierarchically encoding the reduced wavelet image, transmitting the encoded image to another location, and implementing respective inverses of the steps of encoding, wavelet transformation, sub-sampling and colour image transformation.

23. Computer apparatus according to claim 21 programmed to form an encoded image by forming a progressive bitstream in which more important image features are encoded earlier, and which includes information on number image rows and columns, number of scales and filter type.

24. Computer apparatus according to claim 21 programmed to distinguish areas in an original colour image of relatively higher importance from those of relatively lower importance by associating differing binary digits therewith.

25. Computer apparatus according to claim 21 programmed to transform the colour image into a second image by implementing a transformation into luminance, blue chrominance and red chrominance.

26. Computer apparatus according to claim 21 programmed to:

a) distinguish areas of relatively higher importance from those of relatively lower importance by specifying a plurality of different levels of relatively lower importance, and

b) establish a wavelet coefficient threshold and forming a reduced wavelet image by discarding progressively more wavelet coefficients as area importance level diminishes.

27. Computer apparatus according to claim 21 programmed to sub-sample by reducing pixel number to one quarter that preceding.

28. Computer apparatus according to claim 21 programmed to implement wavelet transformation with a Daubechies-4 filter.

29. Computer apparatus according to claim 21 wherein the number of scales is in the range three to six.

30. Computer apparatus according to claim 21 programmed to establish a wavelet coefficient threshold by forming a cumulative histogram of numbers of pixels not exceeding respective wavelet coefficient values.