AU757081B2 - Small gamut L*a*b colour space - Google Patents

Description

S&FRef: 504003

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

Name and Address of Applicant: Actual Inventor(s): Address for Service: Invention Title: Canon Kabushiki Kaisha 30-2, Shimomaruko 3-chome, Ohta-ku Tokyo 146 Japan Peter Malcolm Roberts Spruson Ferguson St Martins Tower 31 Market Street Sydney NSW 2000 Small Gamut L*a*b* Colour Space ASSOCIATED PROVISIONAL APPLICATION DETAILS [33] Country [31] Applic. No(s) AU PQ0549 [32] Application Date 25 May 1999 The following statement is a full description of this invention, including the best method of performing it known to me/us:- 5815c 4 -1- SMALL GAMUT L*a*b* COLOUR SPACE Technical Field of the Invention The present invention relates generally to image processing and, in particular, to digital rendering and image storage with a reduced gamut colour space The present invention relates to a method and apparatus for rendering an image using a reduced gamut. The invention also relates to a computer program product including a computer readable medium having recorded thereon a computer program for digital rendering and image storage using a reduced gamut.

Background Art Research into the use of colour image processing is motivated by two principle factors. First, in automated image analysis, colour is a powerful descriptor that often simplifies object identification and extraction from a scene. Second, in image analysis performed by human beings, the motivation for better colour resolution is that the human eye can discern thousands of colour shades and intensities.

A colour space model is a specification of a 3-D coordinate system and a visible subspace within that system within which all colours in a particular colour gamut lie.

Most colour space models in use today are orientated either towards hardware (such as for colour monitors or printers) or toward applications where colour manipulation is a goal (such as in the creation of colour graphics for animation). The hardware-oriented model most commonly used in practice, for rendering images on colour monitors and on a broad class of colour video cameras, is the RGB (red, green, blue) model. In the RGB model, each colour is constructed from the three primary components of red, green and blue. The colour subspace of interest is the unit cube, as illustrated in Fig. 1. Points along the main diagonal have grey values, from black at the origin to white at point The colour gamut for the RGB colour model 201 is much smaller than the entire visual gamut 203, as 459940 CFP1708AU PEN55 I:\ELEC\CISRA\OPEN\Open55\504003.doc -2illustrated in Fig. 2. Therefore, when rendering in RGB, the range of colours able to be represented is limited. For example, saturated cyan cannot be represented on an RGB monitor and, under such circumstances, becomes a non-reproducible colour.

An alternative colour space that is a good space for rendering artificially generated images is the L*a*b* colour space. The L*a*b* colour space has an extremely large gamut which covers the entire visual gamut. A colour gamut in L*a*b* is illustrated in Fig. 3. Hue and chroma are derived from the coordinates and which can have both positive and negative values ranging from -128 to 127. The third characteristic, brightness, is represented by means of a brightness scale designated L with scale values ranging from 0 (black) to 100 (white), but is representable from 0 to 255.

These ranges are more than adequate to represent the full visible gamut. The L*a*b* colour space is approximately perceptually linear and is also output device independent.

Thus, colour blends and compositing produced using the L*a*b* colour space are more natural looking than that produced using the RGB colour space.

15 However, when smooth colour blends are represented in 8 bit banding artifacts can occur on the resulting image. The problem can be even more accentuated if :.:•-blended image data presented in the RGB colour space is converted into the L*a*b* colour space, operated on and then converted back to RGB. These artefacts are due to the :i:i limited precision of the L*a*b* and RGB colour spaces when stored in 8 bits.

One solution to the problem described above is to increase the number of bits within which the L*a*b* colour space is represented to provide increased resolution.

However, some systems and applications do not allow the number of bits to be increased.

It is an object of the present invention to substantially overcome, or at least ameliorate, one or more disadvantages of existing arrangements by provision of a method 459940 CFP1708AU OPEN55 I:\ELEC\CISRA\OPEN\Open55\504003.doc of modifying the L*a*b* colour space to preserve its important properties while increasing the effective resolution.

Disclosure of the Invention According to one aspect of the present invention there is provided a method of providing a representation of image data, the method comprising the following steps: accessing a plurality of data values of said image data; calculating colour coordinate values for each of said data values using a first L*a*b* colour space, wherein said first L*a*b* colour space is constructed by rotating a second L*a*b* colour space, about a brightness axis from a first inclination to a second inclination, and by scaling a first pair of boundaries of said second L*a*b* colour c space from a first range to a second range; and providing a representation of said image data using said calculated colour S. coordinate values.

According to another aspect of the present invention there is provided an S 15 apparatus for providing a representation of image data, the apparatus comprising: means for accessing a plurality of data values of said image data; ~means for calculating colour coordinate values for each of said data values using a first L*a*b* colour space, wherein said first L*a*b* colour space is constructed by rotating a second L*a*b* colour space, about a brightness axis from a first inclination to a second inclination, and by scaling a first pair of boundaries of said second L*a*b* colour space from a first range to a second range; and means for providing a representation of said image data using said calculated colour coordinate values.

According to still another aspect of the present invention there is provided a method of providing a representation of image data, the method comprising the following eps: accessing a plurality of data values of said image data; 504003.doc -4calculating colour coordinate values for each of said data values using a first colour space, wherein said first colour space is constructed by rotating a second colour space from a first inclination to a second inclination and scaling the boundaries of said second colour space from a first set of ranges to a second set of ranges; and providing a representation of said image data using said calculated colour coordinate values.

According to still another aspect of the present invention there is provided an apparatus for providing a representation of image data, the apparatus comprising: means for accessing a plurality of data values of said image data; 10 means for calculating colour coordinate values for each of said data values using •ooo a first colour space, wherein said first colour space is constructed by rotating a second colour space from a first inclination to a second inclination and scaling the boundaries of said second colour space from a first set of ranges to a second set of ranges; and °e :•***means for providing a representation of said image data using said calculated S. 15 colour coordinate values.

According to still another aspect of the present invention there is provided a computer readable medium for storing a program for apparatus which processes data, said processing comprising a process for providing a representation of image data, said program comprising: code for accessing a plurality of data values of said image data; code for calculating colour coordinate values for each of said data values using a first L*a*b* colour space, wherein said first L*a*b* colour space is constructed by rotating a second L*a*b* colour space, about a brightness axis from a first inclination to a second inclination, and by scaling a first pair of boundaries of said second 25 *ab* colour space from a first range to a second range; and 504003.doc code for providing a representation of said image data using said calculated colour coordinate values.

SIT

OFV-

50.03.do -6- According to still another aspect of the present invention there is provided a computer readable medium for storing a program for apparatus which processes data, said processing comprising a method of providing a representation of image data, the program comprising: code for accessing a plurality of data values of said image data; and code for calculating colour coordinate values for each of said data values using a first colour space, wherein said first colour space is constructed by rotating a second colour space from a first inclination to a second inclination and scaling the boundaries of said second colour space from a first set of ranges to a second set of ranges; and 10 code for providing a representation of said image data using said calculated colour coordinate values.

S

S

ooo 504003.doc -7- Brief Description of the Drawings A number of preferred embodiments of the present invention will now be described with reference to the drawings, in which: Fig. 1 shows the RGB colour space; Fig. 2 shows the colour gamut for a typical colour monitor within the entire visual gamut; Fig. 3 show the L*a*b* colour gamut; Fig. 4 is a flow diagram of a method of representing image data in accordance with a preferred embodiment of the present invention; 10 Fig. 5 shows thew RGB colour gamut within the L*a*b* colour gamut; Fig. 6 is a schematic block diagram of a general purpose computer upon which o* the preferred embodiment of the present invention can be practiced; and Fig. 7 is a diagram of a method of transforming a pixel value, mapped to the RGB colour space, to a pixel value mapped to the L*a*b* colour space, in accordance with the preferred embodiment.

0 *o 504003.doc -8- Detailed Description including Best Mode Where reference is made in any one or more of the accompanying drawings to steps and/or features, which have the same reference numerals, those steps and/or features have for the purposes of this description the same function(s) or operation(s), unless the contrary intention appears.

Fig. 4 is a flow diagram of a method of representing image data in accordance with a preferred embodiment of the present invention. The preferred method reduces quantisation artefacts in image data when used for rendering or image storage by mapping 0 0 e

S

o• 504003.doc -9individual pixel colour coordinates to a reduced L*a*b* colour gamut. The method commences at step 402, where any necessary processes and parameters are initialised, such as counters. At the next step 404, input pixel values are retrieved for processing.

Each input pixel value is preferably mapped to the RGB colour space with a set of

R

coordinates represented by the vector G Each channel of the image is preferably

B

represented by 3 x 8-bit bytes with a resulting (256) 3 different intensity levels. The process continues at decision block 406, where a check is carried out to find out if the final pixel has been processed. If the final pixel has been processed then the process will conclude. Otherwise, at the next step 408, a current pixel value is mapped from the RGB 10 colour space to the L*a*b* colour space. The pixel will then have a set of coordinates represented by the vector a* where hue and saturation are derived from the cob* ordinates a* and and L* is the brightness value. The process of transforming the input pixel values from the RGB colour space to the L*a*b* colour space, in accordance with the preferred embodiment, is described in more detail below with reference to Fig. 7. The process continues at the next step 410, where the following transformation is applied to the a* vector to produce a colour coordinate matrix according to the preferred [b* embodiment as follows: L* 1 0 0 L* a' =0 COS(-7*) -Sin(-7) a* (1) b' 0 Sin COS b* At the next step 412, a' is encoded into the range 0 255 using the encoding: 459940 CFP1708AU OPEN55 I:\ELEC\CISRA\OPEN\Open55\504003.doc a' (2) (2) Sa Similarly, b' is encoded into the range 0 255 using the encoding: b' (-93) b (3) Sb where Sa (98 (-90))/255 and Sb (93 The process continues at step 414, where the resulting pixel is output with its colour coordinates represented by the vector

L*

a' being mapped to the reduced L*a*b* colour gamut according to the preferred b' embodiment. The process will continue until the final pixel value has been processed.

Fig. 7 is a flowchart showing the method of transforming an input pixel value, *o*o mapped to the RGB colour space, to a pixel value mapped to the L*a*b* colour space, in 10 accordance with the preferred embodiment. The process begins at step 701, where a pixel

R

value represented by the vector G is converted to linear RGB which is represented by S. R' the vector G' where R'=R 2 2

G'=G

22 and B'=B 2 2 At the next step 703, the following

R'

transformation is applied to the vector G' to produce a pixel value mapped to the XYZ

B'

X

colour space with a set of coordinates represented by the vector Y as follows:

Z

X X, X, Xb R' Y, Yg Yb (4) z Z' Zb B' 504003.doc 11 where Xr, Xg and Xb represent the weights applied to the colours red green and blue to find X. Similarly, Yr and Yg, Yb represent the weights applied to the colours red, green and blue to find Y, and Zr, Zg and Zb represent the weights applied to the colours red, green and blue to find Z. The process continues at the next step 705, where X

X'

the pixel value represented by the vector Y is converted to the vector Y' where:

Z'

X X if- 0.00856 7.787 Xo X, 1/3 116 X 16 else 100 Xo) 100 Sif 0.00856 7.787 o Yo 1/3 (6) ele 116 Y 16 '100 YO) 100 and z z Sif-- 0.00856 7.787 Z

Z

Zo o 10 1/3(7) els 116 Z 16 100 Zo 100 o and Xo, Yo and Zo represents the white point, X

X

r Yg Xb R' Yo Y Yg Yb G' (8)

Z

o

Z

r Zg Zb B' 459940 CFP1708AU OPEN55 I:\ELEC\CISRA\OPEN\pef55\504003.doc 12- The process concludes at the next step 707, where the following transformation is applied

X'

to the Y' vector to produce a pixel value mapped to the L*a*b* colour space, in

Z'

accordance with the preferred embodiment, as follows: a* 500 -500 0 Y' (9) b* 0 200 200 Z' Preferably, the linear RGB values and the XYZ values, determined at steps 701 and 707, respectively, are represented by numbers with more than 8 bits precision.

The aforementioned preferred methods comprise a particular control flow. There are many other variants of the preferred methods, which use different control flows !*without departing from the spirit or scope of the invention. Furthermore, one or more of 10 the steps of the preferred methods shown in Figs. 4 and 7 may be performed in parallel rather then sequentially. For example, the 3 x 3 matrix of step 410, can be multiplied with ',the 3 x 3 matrix of step 707, to produce a mathematically equivalent matrix which can be used to perform a transformation straight from the XYZ colour space to the reduced L*a*b* colour space in accordance with the preferred embodiment.

The following paragraphs provide a more detailed explanation of the rotation and "i scaling of the L*a*b* colour gamut to produce a reduced L*a*b* colour gamut according 4*S*SS to the preferred embodiment.

A colour gamut in L*a*b* 501 is illustrated in Fig. 5. As described above, hue and saturation are derived from the coordinates a* and which can have both positive and negative values ranging from -128 to 127. The third characteristic, brightness, is represented by means of a brightness scale designated as axis L coming out of the page with scale values ranging from 0 (black) to 100 (white), but is representable from 0 to 459940 CFP1708AU OPEN55 I:\ELEC\CISRA\OPEN\Open55\504003.doc -13- 255. The visible gamut in L*a*b* colour space 501 is cone-shaped, as shown in Fig. 3.

The tip of the cone is positioned at the vertex 505 of the a*b* axis coinciding with zero luminance, with the cone protruding out of the page.

A colour gamut for an RGB colour space, represented by triangles 503 and 504, is also shown in Fig. 5. The triangle 503 represents the RGB limits of the RGB gamut.

and the triangle 504 represents the CMY limits of the RGB gamut. The point 0, 0) of the cubed shaped RGB colour gamut 503, 504 is at the vertex 505 of the a*b* axis, coinciding with zero luminance, and its diagonally opposite point 1, 1) protrudes out of the page being on the luminance axis L* at the point L*=100. As illustrated in Fig. the RGB colour gamut 503, 504 is contained completely within the L*a*b* colour gamut 501.

The reduced L*a*b* colour gamut 507 in accordance with the preferred embodiment was produced by firstly, rotating the a*b* axis through an angle 510 to produce the axis In the preferred embodiment, the angle 510 is seven degrees.

Secondly, a box was drawn around the RGB colour gamut 503,504 such that the box totally enclosed the colour gamut 503,504 and the boundaries of the box were parallel to 4 the axis. Thirdly, the boundaries of the box were scaled to the ranges -90 a' 98 and -93 b' 93. This process produced the minimum sized gamut 507 which would totally enclose the RGB gamut.

Mapping pixel values to the reduced L*a*b* gamut was found to produce increased colour resolution by reducing quantisation artefacts in image data when used for rendering or image storage.

While the method of producing the reduced L*a*b* colour gamut in accordance with the preferred embodiment was described in terms of scaling parallel lines, it will be apparent to a person skilled in the art that the method can be practised using other 459940 CFP1708AU OPEN55 I:\ELEC\CISRA\OPEN\Open55\504003.doc -14derivation processes for finding minimum values without departing from the scope and spirit of the invention.

The method of Fig. 4 is preferably practised using a conventional generalpurpose computer system 600, such as that shown in Fig. 6 wherein the processes of Figs. 1 to 6 may be implemented as software, such as an application program executing within the computer system 600. In particular, the steps of method of representing image data are effected by instructions in the software that are carried out by the computer. The software can be divided into two separate parts; one part for carrying out the preferred methods; and another part to manage the user interface between the latter and the user.

The software may be stored in a computer readable medium, including the storage devices described below, for example. The software is loaded into the computer from the lgo computer readable medium, and then executed by the computer. A computer readable o~o medium having such software or computer program recorded on it is a computer program product. The use of the computer program product in the computer preferably effects an s 15 advantageous apparatus for representing image data in accordance with the embodiments gg•• of the invention.

0 :'The computer system 600 comprises a computer module 601, input devices such Coo• sesas a keyboard 602 and mouse 603, output devices including a printer 615 and a display device 614. A Modulator-Demodulator (Modem) transceiver device 616 is used by the 20 computer module 601 for communicating to and from a communications network 620, for example connectable via a telephone line 621 or other functional medium. The modem 616 can be used to obtain access to the Internet, and other network systems, such as a Local Area Network (LAN) or a Wide Area Network (WAN).

The computer module 601 typically includes at least one processor unit 605, a memory unit 606, for example formed from semiconductor random access memory 459940 CFP1708AU OPEN55 I:\ELEC\CIRA\OPEN\Open5550400 3 .doc (RAM) and read only memory (ROM), input/output interfaces including a video interface607, and an I/O interface613 for the keyboard602 and mouse 603 and optionally a joystick (not illustrated), and an interface 608 for the modem 616. A storage device 609 is provided and typically includes a hard disk drive 610 and a floppy disk drive 611. A magnetic tape drive (not illustrated) may also be used. A CD-ROM drive 612 is typically provided as a non-volatile source of data. The components 605 to 613 of the computer module 601, typically communicate via an interconnected bus 604 and in a manner which results in a conventional mode of operation of the computer system 600 known to those in the relevant art. Examples of computers on which the embodiments can be practised include IBM-PC's and compatibles, Sun Sparcstations or alike computer systems evolved therefrom.

Typically, the application program of the preferred embodiment is resident on the hard disk drive 610 and read and controlled in its execution by the processor 605.

Intermediate storage of the program and any data fetched from the network 620 may be accomplished using the semiconductor memory 606, possibly in concert with the hard disk drive 610. In some instances, the application program may be supplied to the user encoded on a CD-ROM or floppy disk and read via the corresponding drive 612 or 611, or alternatively may be read by the user from the network 620 via the modem device 616.

Still further, the software can also be loaded into the computer system 600 from other computer readable medium including magnetic tape, a ROM or integrated circuit, a magneto-optical disk, a radio or infra-red transmission channel between the computer module 601 and another device, a computer readable card such as a PCMCIA card, and the Internet and Intranets including email transmissions and information recorded on websites and the like. The foregoing is merely exemplary of relevant computer readable 459940 CFP1708AU OPEN55 I:\ELEC\CISRA\OPEN\Open55504003.doc -16mediums. Other computer readable mediums may be practiced without departing from the scope and spirit of the invention.

The method of representing image data may alternatively be implemented in dedicated hardware such as one or more integrated circuits performing the functions or sub functions of Fig. 4. Such dedicated hardware may include graphic processors, digital signal processors, or one or more microprocessors and associated memories.

The foregoing describes only one embodiment of the present invention, and modifications and/or changes can be made thereto without departing from the scope and spirit of the invention, the embodiments being illustrative and not restrictive. For example, the preferred embodiment has been described in relation to the RGB-SMPTE colour space. However, the preferred embodiment could also be described using the sRGB colour space whereby the boundaries of the reduced L*a*b* colour gamut would be scaled to a different range.

Further, a person skilled in the art would appreciate that there are other methods S 15 (eg. look-up tables) which can be utilised in order to determine L*a*b* values from RGB values, and that the preferred methods described above can be modified in order to determine L*a"b' values directly from RGB values. For example, steps 408 and 410 can be combined by multiplying the 3x3 matrix of equation with the 3x3 matrix of equation to determine a single 3x3 matrix which can be utilised to convert directly 20 from X'Y'Z' values to L*a'b' values without determining L*a*b* values. In a further ••go example, the encoding operation of step 412 can be represented as a single matrix which ooooo can be multiplied by the single 3x3 matrix discussed directly above, in order to combine steps 408 to 412. Combining steps 408 to 412 can provide improved computational speed and efficiency.

459940 CFP1708AU OPEN55 I:\ELEC\CISRA\OPEN\Open55\504003.doc -17- In the context of this specification, the word "comprising" means "including principally but not necessarily solely" or "having" or "including" and not "consisting only of'. Variations of the word comprising, such as "comprise" and "comprises" have corresponding meanings.

459940 CFP1708AU OPEN55 4 :\ELEC\CISRA\OPEN\Open5 \504003.doc

Claims (4)

1. A method of providing a representation of image data, the method comprising the following steps: accessing a plurality of data values of said image data; calculating colour coordinate values for each of said data values using a first L*a*b* colour space, wherein said first L*a*b* colour space is constructed by rotating a second L*a*b* colour space, about a brightness axis from a first inclination to a second inclination, and by scaling a first pair of boundaries of said second L*a*b* colour space from a first range to a second range; and 10 providing a representation of said image data using said calculated colour coordinate values.

2. A method according to claim 1, comprising the further step of scaling a second pair of boundaries of said second L*a*b* colour space from a third range to a fourth range. S. S S S S

3. A method according to any one of the preceding claims, wherein said second L*a*b* colour space is reduced in size.

4. A method according to any one of the preceding claims, wherein said second inclination is approximately 70 greater than said first inclination. A method according to any one of claims 1 to 4, wherein said second inclination is 70 greater than said first inclination.

504003.doc -19- 6. A method according to any one of claims 1 to 5, wherein said first range is -128 to 127. 7. A method according to any one of claims 1 to 6, wherein said second range is -93 to 93. 8. A method according to any one of claims 2 to 7, wherein said third range is -128 to 127. 10 9. A method according to any one of claims 2 to 8, wherein said fourth range is 90 to 98. A method according to claim 1, wherein said colour coordinate values are calculated by applying the following transformation to said plurality of data values: oo L* 1 0 0 I L* 15 a' =0 COS -Sin a* b' _0 Sin COS b* L L wherein a' represents said colour coordinate values and a represents a plurality of b' b* original colour coordinate values associated with said plurality of data values. 11. A method according to claim 10, wherein a' is encoded into a fifth range a' using the encoding and wherein Sa (98 (-90))/255. Sa 504003.doc 12. A method according to claim 10, wherein b' is encoded into a fifth range using (-93) the encoding and wherein Sb (93 (-93))/255. Sb 13. A method according to claims 11 or 12, wherein said fifth range is 0 to 255. 14. An apparatus for providing a representation of image data, the apparatus comprising: means for accessing a plurality of data values of said image data; means for calculating colour coordinate values for each of said data values using 10 a first L*a*b* colour space, wherein said first L*a*b* colour space is constructed by rotating a second L*a*b* colour space, about a brightness axis from a first inclination to a second inclination, and by scaling a first pair of boundaries of said second L*a*b* colour space from a first range to a second range; and 1 o means for providing a representation of said image data using said calculated 15 colour coordinate values. 0o 0 An apparatus according to claim 14, further comprising means for scaling a second pair of boundaries of said second L*a*b* colour space from a third range to a fourth range. 16. An apparatus according to any one of claims 14 to 15, wherein said second L*a*b* colour space is reduced in size. SAn apparatus according to any one of claims 14 to 16, wherein said second Knation is approximately 7° greater than said first inclination. 504003.doc 18. An apparatus according to any one of claims 14 to 17, wherein said second inclination is 70 greater than said first inclination. 19. An apparatus according to any one of claims 14 to 18, wherein said first range is -128 to 127. An apparatus according to any one of claims 14 to 19, wherein said second range is -93 to 93. .21. An apparatus according to any one of claims 15 to 20, wherein said third range is -128 to 127. 22. An apparatus according to any one of claims 15 to 21, wherein said fourth range 15 is -90 to 98. 0 9 23. An apparatus according to claim 14, further comprising means for determining said colour coordinate values by applying the following transformation to said plurality of data values: L* 1 0 0 L* a' 0 COS a* b' Sin COS b L* L* wherein a' represents said colour coordinate values and a represents a plurality of b' b* original colour coordinate values associated with said plurality of data values. 504003.doc -22- 24. An apparatus according to claim 23, wherein a' is encoded into a fifth range a' using the encoding a and wherein Sa (98 (-90))/255. Sa An apparatus according to claim 23, wherein b' is encoded into a fifth range b' -93 using the encoding and wherein Sb (93 (-93))/255. Sb 26. An apparatus according to any one of claims 24 or 25, wherein said fifth range is 0 to 255. 27. A method of providing a representation of image data, the method comprising the following steps: accessing a plurality of data values of said image data; calculating colour coordinate values for each of said data values using a first colour space, wherein said first colour space is constructed by rotating a second colour space from a first inclination to a second inclination and scaling the boundaries of said S: second colour space from a first set of ranges to a second set of ranges; and providing a representation of said image data using said calculated colour coordinate values. 28. A method according to claim 27, comprising the further step of scaling a first pair of boundaries of said second colour space from a first range to a second range. 504003.doc -23- 29. A method according to claim 27 or 28, comprising the further step of scaling a second pair of boundaries of said second colour space from a third range to a fourth range. 30. A method according to any one of claims 27 to 29, wherein said second colour space is reduced in size. 31. A method according to any one of claims 27 to 30, wherein said second colour space is the L*a*b* colour space. 32. A method according to any one of claims 27 to 31, wherein said second inclination is 7' greater than said first inclination. r 33. A method according to any one of claims 28 to 32, wherein said first range is 128 to 127. 34. A method according to any one of claims 28 to 33, wherein said second range is -93 to 93. 35. A method according to any one of claims 29 to 34, wherein said third range is -128 to 127. 36. A method according to any one of claims 29 to 35, wherein said fourth range is to 98. 504003.doc -24- 37. A method according to claim 27, wherein said colour coordinate values are calculated by applying the following transformation to said plurality of data values: L* 1 0 0 [L*1 a' 0 COS(-7) Sin(-7°) a* b' 0 Sin COS b wherein a' represents said colour coordinate values and a represents a plurality of b' b* original colour coordinate values associated with said plurality of data values. 38. A method according to claim 37, wherein a' is encoded into a fifth range a' (-90) using the encoding and wherein Sa (98 (-90))/255. ~Sa S 10 39. A method according to claim 37, wherein b' is encoded into a fifth range using b' (-93) the encoding and wherein Sb (93 (-93))/255. Sb A method according to claims 38 or 39, wherein said fifth range is 0 to 255. 41. An apparatus for providing a representation of image data, the apparatus comprising: means for accessing a plurality of data values of said image data; means for calculating colour coordinate values for each of said data values using a first colour space, wherein said first colour space is constructed by rotating a second colour space from a first inclination to a second inclination and scaling the boundaries of said second colour space from a first set of ranges to a second set of ranges; and 504003.doc means for providing a representation of said image data using said calculated colour coordinate values. 42. A computer readable medium for storing a program for apparatus which processes data, said processing comprising a process for providing a representation of image data, said program comprising: code for accessing a plurality of data values of said image data; code for calculating colour coordinate values for each of said data values using a first L*a*b* colour space, wherein said first L*a*b* colour space is constructed by 10 rotating a second L*a*b* colour space, about a brightness axis from a first inclination to a second inclination, and by scaling a first pair of boundaries of said second L*a*b* colour space from a first range to a second range; and code for providing a representation of said image data using said calculated colour coordinate values. S43. A computer readable medium for storing a program for apparatus which processes data, said processing comprising a method of providing a representation of image data, the program comprising: code for accessing a plurality of data values of said image data; code for calculating colour coordinate values for each of said data values using a first colour space, wherein said first colour space is constructed by rotating a second colour space from a first inclination to a second inclination and scaling the boundaries of said second colour space from a first set of ranges to a second set of ranges; and code for providing a representation of said image data using said calculated 25 our coordinate values. 504003.doc 44. A method of providing a representation of image data, said method being substantially as herein described with reference to Figs. 4 to 7. 45. An apparatus for providing a representation of image data, said apparatus being substantially as herein described with reference to Figs. 4 to 7. 46. A computer readable medium for storing a program for apparatus which processes data, said processing comprising a process for providing a representation of 10 image data, said program being substantially as herein described with reference to Figs. 4 to 7. DATED this third Day of December, 2002 Canon Kabushiki Kaisha 15 Patent Attorneys for the Applicant SPRUSON FERGUSON *°eo DAE•go•hr ayo Cao•gbeek Kih 504003.doc