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<html>
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<head>
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<meta name="generator" content="groff -Thtml, see www.gnu.org">
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<meta name="Content-Style" content="text/css">
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<title>COLOR</title>
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</head>
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<body>
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COLOR
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NAME
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SYNOPSIS
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DESCRIPTION
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SEE ALSO
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NAME
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       cols="2" cellspacing="0" cellpadding="0">
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TIFFYCbCrToRGBInit, TIFFYCbCrtoRGB, TIFFCIELabToRGBInit,
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TIFFCIELabToXYZ, TIFFXYZToRGB − color conversion
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routines.
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SYNOPSIS
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       cols="2" cellspacing="0" cellpadding="0">
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#include <tiffio.h>
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int TIFFYCbCrToRGBInit(TIFFYCbCrToRGB
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*ycbcr, float *luma, float
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*refBlackWhite");"
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void TIFFYCbCrtoRGB(TIFFYCbCrToRGB *ycbcr,
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uint32 Y, int32 Cb, int32
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Cr, uint32 *R, uint32
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*G, uint32 *B );
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int TIFFCIELabToRGBInit(TIFFCIELabToRGB
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*cielab, TIFFDisplay *display,
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float *refWhite);
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void TIFFCIELabToXYZ(TIFFCIELabToRGB *cielab,
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uint32 L, int32 a, int32
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b, float *X, float *Y,
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float *Z);
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void TIFFXYZToRGB(TIFFCIELabToRGB *cielab,
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float X, float Y, float
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Z",uint32*"R,
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uint32 *G, uint32 *B);
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DESCRIPTION
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       cols="2" cellspacing="0" cellpadding="0">
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TIFF supports several color spaces for images stored in
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that format. There is usually a problem of application to
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handle the data properly and convert between different
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colorspaces for displaying and printing purposes. To
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simplify this task libtiff implements several color
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conversion routines itself. In particular, these routines
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used in TIFFRGBAImage(3TIFF) interface.
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TIFFYCbCrToRGBInit() used to initialize
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YCbCr to RGB conversion state. Allocating and
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freeing of the ycbcr structure belongs to programmer.
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TIFFYCbCrToRGB defined in tiffio.h as
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typedef struct {                /* YCbCr->RGB support */
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        TIFFRGBValue* clamptab; /* range clamping table */
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int*
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Cr_r_tab;
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int*
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Cb_b_tab;
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int32*
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Cr_g_tab;
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int32*
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Cb_g_tab;
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int32* Y_tab;
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} TIFFYCbCrToRGB;
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luma is a float array of three values representing
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proportions of the red, green and blue in luminance, Y (see
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section 21 of the TIFF 6.0 specification, where the YCbCr
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images discussed). TIFFTAG_YCBCRCOEFFICIENTS holds
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that values in TIFF file. refBlackWhite is a float
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array of 6 values which specifies a pair of headroom and
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footroom image data values (codes) for each image component
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(see section 20 of the TIFF 6.0 specification where the
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colorinmetry fields discussed).
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TIFFTAG_REFERENCEBLACKWHITE is responsible for
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storing these values in TIFF file. Following code snippet
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should helps to understand the the technique:
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float *luma, *refBlackWhite;
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uint16 hs, vs;
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/* Initialize structures */
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ycbcr = (TIFFYCbCrToRGB*)
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       cols="2" cellspacing="0" cellpadding="0">
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_TIFFmalloc(TIFFroundup(sizeof(TIFFYCbCrToRGB),
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sizeof(long))
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+ 4*256*sizeof(TIFFRGBValue)
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+ 2*256*sizeof(int)
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+ 3*256*sizeof(int32));
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       cols="2" cellspacing="0" cellpadding="0">
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if (ycbcr == NULL) {
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TIFFError("YCbCr->RGB",
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       cols="4" cellspacing="0" cellpadding="0">
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"No space for YCbCr->RGB conversion
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state");
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exit(0);
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}
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TIFFGetFieldDefaulted(tif, TIFFTAG_YCBCRCOEFFICIENTS,
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&luma);
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TIFFGetFieldDefaulted(tif, TIFFTAG_REFERENCEBLACKWHITE,
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&refBlackWhite);
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if (TIFFYCbCrToRGBInit(ycbcr, luma, refBlackWhite) <
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0)
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exit(0);
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       cols="2" cellspacing="0" cellpadding="0">
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/* Start conversion */
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uint32 r, g, b;
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uint32 Y;
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int32 Cb, Cr;
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for each pixel in image
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TIFFYCbCrtoRGB(img->ycbcr, Y, Cb, Cr, &r, &g,
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&b);
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/* Free state structure */
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_TIFFfree(ycbcr);
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TIFFCIELabToRGBInit() initializes the CIE
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L*a*b* 1976 to RGB conversion state.
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TIFFCIELabToRGB defined as
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#define CIELABTORGB_TABLE_RANGE 1500
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typedef struct {
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/* CIE Lab 1976->RGB support */
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int
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range;
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/* Size of conversion table */
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float
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rstep, gstep, bstep;
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float
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X0, Y0, Z0;
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/* Reference white point */
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TIFFDisplay display;
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float
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Yr2r[CIELABTORGB_TABLE_RANGE + 1]; /* Conversion of Yr
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to r */
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float
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Yg2g[CIELABTORGB_TABLE_RANGE + 1]; /* Conversion of Yg
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to g */
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float
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Yb2b[CIELABTORGB_TABLE_RANGE + 1]; /* Conversion of Yb
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to b */
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       cols="2" cellspacing="0" cellpadding="0">
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} TIFFCIELabToRGB;
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       cols="2" cellspacing="0" cellpadding="0">
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display is a display device description, declared
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as
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typedef struct {
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float d_mat[3][3]; /* XYZ -> luminance matrix */
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float d_YCR; /* Light o/p for reference white */
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float d_YCG;
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float d_YCB;
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uint32 d_Vrwr; /* Pixel values for ref. white */
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uint32 d_Vrwg;
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uint32 d_Vrwb;
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float d_Y0R; /* Residual light for black pixel */
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float d_Y0G;
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float d_Y0B;
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float d_gammaR; /* Gamma values for the three guns
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*/
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float d_gammaG;
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float d_gammaB;
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       cols="2" cellspacing="0" cellpadding="0">
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} TIFFDisplay;
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       cols="2" cellspacing="0" cellpadding="0">
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For example, the one can use sRGB device, which has the
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following parameters:
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       cols="2" cellspacing="0" cellpadding="0">
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TIFFDisplay display_sRGB = {
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       cols="5" cellspacing="0" cellpadding="0">
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{ /* XYZ -> luminance matrix */
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{ 3.2410F, -1.5374F, -0.4986F },
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{ -0.9692F, 1.8760F, 0.0416F },
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{ 0.0556F, -0.2040F, 1.0570F }
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},
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100.0F, 100.0F, 100.0F, /* Light o/p for reference white
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*/
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255, 255, 255, /* Pixel values for ref. white */
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1.0F, 1.0F, 1.0F, /* Residual light o/p for black pixel
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*/
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2.4F, 2.4F, 2.4F, /* Gamma values for the three guns
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*/
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       cols="2" cellspacing="0" cellpadding="0">
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};
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       cols="2" cellspacing="0" cellpadding="0">
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refWhite is a color temperature of the reference
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white. The TIFFTAG_WHITEPOINT contains the
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chromaticity of the white point of the image from where the
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reference white can be calculated using following
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formulae:
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       cols="2" cellspacing="0" cellpadding="0">
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refWhite_Y = 100.0
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refWhite_X = whitePoint_x / whitePoint_y * refWhite_Y
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refWhite_Z = (1.0 - whitePoint_x - whitePoint_y) /
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whitePoint_y * refWhite_X
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       cols="2" cellspacing="0" cellpadding="0">
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The conversion itself performed in two steps: at the
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first one we will convert CIE L*a*b* 1976 to CIE
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XYZ using TIFFCIELabToXYZ() routine, and at the
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second step we will convert CIE XYZ to RGB
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using TIFFXYZToRGB(). Look at the code sample
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below:
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       cols="2" cellspacing="0" cellpadding="0">
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float   *whitePoint;
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float   refWhite[3];
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/* Initialize structures */
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img->cielab = (TIFFCIELabToRGB *)
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       cols="2" cellspacing="0" cellpadding="0">
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_TIFFmalloc(sizeof(TIFFCIELabToRGB));
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       cols="2" cellspacing="0" cellpadding="0">
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if (!cielab) {
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       cols="5" cellspacing="0" cellpadding="0">
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TIFFError("CIE L*a*b*->RGB",
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"No space for CIE L*a*b*->RGB conversion
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state.");
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exit(0);
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       cols="2" cellspacing="0" cellpadding="0">
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}
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TIFFGetFieldDefaulted(tif, TIFFTAG_WHITEPOINT,
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&whitePoint);
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refWhite[1] = 100.0F;
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refWhite[0] = whitePoint[0] / whitePoint[1] *
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refWhite[1];
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refWhite[2] = (1.0F - whitePoint[0] -
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whitePoint[1])
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       cols="2" cellspacing="0" cellpadding="0">
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/ whitePoint[1] * refWhite[1];
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       cols="2" cellspacing="0" cellpadding="0">
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if (TIFFCIELabToRGBInit(cielab, &display_sRGB,
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refWhite) < 0) {
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       cols="5" cellspacing="0" cellpadding="0">
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TIFFError("CIE L*a*b*->RGB",
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"Failed to initialize CIE L*a*b*->RGB conversion
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state.");
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_TIFFfree(cielab);
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exit(0);
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       cols="2" cellspacing="0" cellpadding="0">
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}
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/* Now we can start to convert */
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uint32 r, g, b;
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uint32 L;
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int32 a, b;
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float X, Y, Z;
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for each pixel in image
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       cols="2" cellspacing="0" cellpadding="0">
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TIFFCIELabToXYZ(cielab, L, a, b, &X, &Y,
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&Z);
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TIFFXYZToRGB(cielab, X, Y, Z, &r, &g,
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&b);
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       cols="2" cellspacing="0" cellpadding="0">
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/* Don’t forget to free the state structure */
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_TIFFfree(cielab);
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SEE ALSO
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       cols="2" cellspacing="0" cellpadding="0">
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TIFFRGBAImage(3TIFF) libtiff(3TIFF),
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Libtiff library home page:
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http://www.simplesystems.org/libtiff/
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</body>
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