jp2_colour::init

Must be one of the following (JP2/JPX enumerated colour space codes are also given to resolve any possible lack of clarity for those who have copies of the standards documents):

• JP2_bilevel1_SPACE (EnumCS=0) — used to indicate a bi-level image in which 0 means white and 1 means black.
• JP2_YCbCr1_SPACE (EnumCS=1) — commonly used for data that originated from a video signal. (*) The primaries are based on ITU-R Rec. BT709-4, having chromaticities (Rx,Ry)=(0.640,0.330), (Gx,Gy)=(0.300,0.600) and (Bx,By)=(0.150,0.060), with a whitepoint of (Wx,Wy)=(0.3127,0.3290) under CIE illuminant D65, and a gamma function with parameters gamma=1/0.45 and beta=0.099. (*) The opponent transform which maps gamma-corrected RGB to Y1, Cb1 and Cr1 is identical to the ICT (irreversible colour transform) defined by JPEG2000 Part 1, except that the Y1, Cb1 and Cr1 channels have been scaled so as to occupy less than the full nominal dynamic range. In particular, writing Y, Cb and Cr for the values which would be produced by the ICT, with Y in the range 0 to 1, and Cb and Cr in the range -0.5 to +0.5, we have Y1 = (219*Y'+16) / 256; Cb1 = Cb' * 224/256; and Cr1 = Cr' * 224/256. As an example, to express Y1, Cb1 and Cr1 as 8-bit unsigned numbers, the normalized quantities are multiplied by 256, and the chrominance quantities are then offset by 128.
• JP2_YCbCr2_SPACE (EnumCS=3) — commmonly used for data that originated as RGB, but has been converted to luminance and chrominance samples. (*) The primaries are based on ITU-R Rec. BT601-5, having chromaticities (Rx,Ry)=(0.630,0.340), (Gx,Gy)=(0.310,0.595) and (Bx,By)=(0.155,0.070), with a whitepoint of (Wx,Wy)=(0.3127,0.3290) under CIE illuminant D65, and with a gamma function governed by parameters gamma=1/0.45 and beta=0.099. Note that this is the same gamma as the colour space above, but with different RGB primaries. (*) The opponent colour transform used to map RGB to Y2, Cb2 and Cr2 values is identical to the ICT (irreversible colour transform) defined by JPEG2000 Part 1, without any additional scaling.
• JP2_YCbCr3_SPACE (EnumCS=4) — commonly used for data that originated from a video signal. (*) The primaries are based on ITU-R Rec. BT601-5, having exactly the same chromaticities, whitepoint and gamma as JP2_YCbCr2_SPACE. (*) The opponent colour transform used to map RGB to Y3, Cb3 and Cr3 values is identical to the ICT (irreversible colour transform) defined by JPEG2000 Part 1, except that it includes the additional scaling factors described above for the JP2_YCbCr1_SPACE.
• JP2_PhotoYCC_SPACE (EnumCS=9) — this is the colour encoding method used in the Photo CD system. (*) The primaries are based on ITU-R Rec. BT.709, having exactly the same chromaticities, whitepoint and gamma as JP2_YCbCr1_SPACE. (*) The opponent colour transform used to map RGB to Y, C1 and C2 values is identical to the ICT (irreversible colour transform) defined by JPEG2000 Part 1, but the coefficients are scaled and offset in an unusual manner. In particular, treating the Y component as an unsigned quantity in the range 0 to 1, and the Cb and Cr components as signed quantities in the range -0.5 to +0.5, the Y component is scaled by 0.7133, while the Cb and Cr components are both scaled and offset. The Cb component is scaled by 0.7711 and then offset by the addition of +0.1094, while the Cr component is scaled by 0.7428 and offset by the addition of +0.0352.
• JP2_CMY_SPACE (EnumCS=11) — A Cyan-Magenta-Yellow colour space suitable for printing. Not formally defined — device dependent. Values of 0 mean zero ink coverage, while the maximum nominal value corresponds to an ink coverage of 100% for the relevant colourant.
• JP2_CMYK_SPACE (EnumCS=12) — Same as JP2_CMY_SPACE but with an extra channel for black ink.
• JP2_YCCK_SPACE (EnumCS=13) — This is an opponent representation for the JP2_CMYK_SPACE. The first 3 components (C, M and Y) are first complemented to form pseudo-RGB coordinates. Specifically, if C, M and Y are regarded as numbers in the range 0 to 1, complementing them forms R=1-C, G=1-M and B=1-Y. The ICT (irreversible colour transform) defined by JPEG2000 Part 1 code-streams is then applied to the complemented colour channels to form luminance and chrominance channels. The black (K) channel is unaffected by the above transformations.
• JP2_CIELab_SPACE (EnumCS=14) — This is the CIE Lab colour space. If you choose to use this form of the init function, you will get the default Lab parameters, as described in the documentation provided with the second form of the init function.
• JP2_bilevel2_SPACE (EnumCS=15) — Same as JP2_bilevel1_SPACE, exept that a value of 1 means white and 0 means black.
• JP2_sRGB_SPACE (EnumCS=16) — This is the standard RGB space used by JP2 files. Try to use this colour space if you have a choice, since that will encourage JP2-compatibility. (*) The primaries are those of ITU-R Rec. BT709, having the same chromaticities and whitepoint as JP2_YCbCr1_SPACE, except that the gamma function uses parameters gamma=2.4 and beta=0.055, which are different from those defined by Rec. BT709. These are defined in IEC 61966-2-1 (sRGB).
• JP2_sLUM_SPACE (EnumCS=17) — This is the standard luminance space used by JP2 files. Try to use this space if you have a choice of luminance specifications, since that will encourage JP2-compatibility. (*) Uses a gamma function with gamma=2.4 and beta=0.055, which is the same as that defined by IEC 61966-2-1 for the sRGB space.
• JP2_sYCC_SPACE (EnumCS=18) — This is the standard opponent colour space used by JP2 files. Try to use this space if you have a choice of opponent colour specifications, since that will encourage JP2-compatibility. (*) The primaries are those of ITU-R Rec. BT709, except that the gamma function uses parameters gamma=2.4 and beta=0.055, exactly as for JP2_sRGB_SPACE. (*) The opponent colour transform used to map RGB to Y, C1 and C2 values is identical to the ICT (irreversible colour transform) defined by JPEG2000 Part 1, without any additional scaling.
• JP2_CIEJab_SPACE (EnumCS=19) — This colour space is based on CIECAM97s (CIE Publication 131). If you choose to use this form of the init function, you will get the default Jab parameters, as described in the documentation provided with the second form of the init function.
• JP2_esRGB_SPACE (EnumCS=20) — This is an extended form of the sRGB colour space, designed to allow colours with a larger gamut than that of sRGB to be represented. The specification may be found in PIMA 7667; however, the extension is simple enough to understand: (*) The underlying primaries and gamma function are the same as those of JP2_sRGB_SPACE (i.e., Rec. 709 primaries with a different gamma of 2.4 and beta=0.055). However, negative and abnormally large values for the linear R, G and B primary intensities are considered legal. The gamma function processes negative channel values by processing their magnitude in the usual fashion and passing the sign across the gamma function — that is, the gamma function is defined to be and odd (symmetric) function of its input. (*) Regarding the gamma corrected channel intensities as unsigned quantities with a nominal range of 0 to 1 (remember that negative and abnormally large values are allowed, though), these values are scaled by 0.5 and then offset by adding exactly 0.375.
• JP2_ROMMRGB_SPACE (EnumCS=21) — As defined in PIMA 7666.
• JP2_YPbPr60_SPACE (EnumCS=22) — Opponent colour space used by HDTV systems. (*) The primaries have chromaticities (0.67,0.33), (0.21,0.71) and (0.15,0.06), with a D65 whitepoint (0.3127,0.3290), and gamma=1/0.45, with beta=0.099. (*) The opponent colour transform used to map RGB to Y, Pb and Pr values yields: Y'=0.2122*R+0.7013*G+0.0865*B; Pb'=-0.1162*R-0.3838*G+0.5*B; Pr'=0.5*R-0.4451*G-0.0549*B, where Y, Pb and Pr are obtained by scaling and offsetting Y', Pb' and Pr'. Exactly as for the JP2_YCbCr1_SPACE, if R, G and B lie in the nominal range 0 to 1, so that Y' has the same range and Pb' and Pr' lie in the range -0.5 to +0.5, the actual opponent channels are scaled such that Y = (219*Y'+16) / 256, Pb = Pb' * 224/256 and Pr = Pr' * 224/256. This ensures that when these quantities are represented as unsigned 8-bit numbers, they will lie in the range 16 to 235 (for Y) and 16 to 240 (for Pb and Pr).
• JP2_YPbPr50_SPACE (EnumCS=23) — The standard indicates that this space has a slightly different opponent colour transform than JP2_YPbPr60_SPACE; however, I have been unable to confirm this. All information I have been able to lay my hands on concerning YPbPr provides the definition given above; it may be that the text in the standard was borrowed from the SPIFF documentation, and that this had become outdated.
• JP2_esYCC_SPACE (EnumCS=24) — This is an extended form of sYCC, whose definition may be found in Annex B of PIMA 7667. Although the basic principles are similar to those associated with esRGB, this colour space is NOT obtained simply by applying the irreversible colour transform to the esRGB coordinates. Instead, the following procedure is used to obtain esYCC coordinates. (*) The colour space is based on the same primaries and gamma function as sRGB and esRGB (i.e., Rec. 709 primaries, with the different gamma of 2.4, with beta=0.055). As with esRGB, the linear primary RGB coordinates are allowed to take on negative and abnormally large values. (*) The opponent transform is obtained by first applying the ICT (irreversible colour transform) defined by JPEG2000 Part 1 to the above-mentioned primaries, and then scaling the Cb and Cr coordinates by 0.5. Once this is done, negative or abnormally large values for the luminance coordinate are clipped so that the extended gamut information is retained only in the chrominance channels.