.\" $Id: TIFFcolor.3tiff,v 1.3 2006/03/23 14:54:02 dron Exp $

.\"

.\" Copyright (c) 2003, Andrey Kiselev <dron@ak4719.spb.edu>

.\"

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.\" THE SOFTWARE IS PROVIDED "AS-IS" AND WITHOUT WARRANTY OF ANY KIND, 

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.\" WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.  

.\" 

.\" IN NO EVENT SHALL SAM LEFFLER OR SILICON GRAPHICS BE LIABLE FOR

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.\"

.if n .po 0

.TH COLOR 3TIFF "December 21, 2003" "libtiff"

.SH NAME

TIFFYCbCrToRGBInit, TIFFYCbCrtoRGB, TIFFCIELabToRGBInit, TIFFCIELabToXYZ,

TIFFXYZToRGB \- color conversion routines.

.SH SYNOPSIS

.B "#include <tiffio.h>"

.sp

.BI "int TIFFYCbCrToRGBInit(TIFFYCbCrToRGB *" ycbcr ", float *" luma ", float *"refBlackWhite" );"

.br

.BI "void TIFFYCbCrtoRGB(TIFFYCbCrToRGB *" ycbcr ", uint32 " Y ", int32 " Cb ", int32 " Cr ", uint32 *" R ", uint32 *" G ", uint32 *" B " );"

.sp

.BI "int TIFFCIELabToRGBInit(TIFFCIELabToRGB *" cielab ", TIFFDisplay *" display ", float *" refWhite ");"

.br

.BI "void TIFFCIELabToXYZ(TIFFCIELabToRGB *" cielab ", uint32 " L ", int32 " a ", int32 " b ", float *" X ", float *" Y ", float *" Z ");"

.br

.BI "void TIFFXYZToRGB(TIFFCIELabToRGB *" cielab ", float " X ", float " Y ", float " Z" , uint32 *" R ", uint32 *" G ", uint32 *" B ");"

.SH DESCRIPTION

TIFF supports several color spaces for images stored in that format. There is

usually a problem of application to handle the data properly and convert

between different colorspaces for displaying and printing purposes. To

simplify this task libtiff implements several color conversion routines

itself. In particular, these routines used in

.B TIFFRGBAImage(3TIFF)

interface.

.PP

.B TIFFYCbCrToRGBInit()

used to initialize

.I YCbCr

to

.I RGB

conversion state. Allocating and freeing of the

.I ycbcr

structure belongs to programmer.

.I TIFFYCbCrToRGB

defined in

.B tiffio.h

as

.PP

.RS

.nf

typedef struct {                /* YCbCr->RGB support */

        TIFFRGBValue* clamptab; /* range clamping table */

        int*	      Cr_r_tab;

        int*	      Cb_b_tab;

        int32*	      Cr_g_tab;

        int32*	      Cb_g_tab;

        int32*        Y_tab;

} TIFFYCbCrToRGB;

.fi

.RE

.PP

.I luma

is a float array of three values representing proportions of the red, green

and blue in luminance, Y (see section 21 of the TIFF 6.0 specification, where

the YCbCr images discussed).

.I TIFFTAG_YCBCRCOEFFICIENTS

holds that values in TIFF file.

.I refBlackWhite

is a float array of 6 values which specifies a pair of headroom and footroom

image data values (codes) for each image component (see section 20 of the

TIFF 6.0 specification where the colorinmetry fields discussed).

.I TIFFTAG_REFERENCEBLACKWHITE

is responsible for storing these values in TIFF file. Following code snippet

should helps to understand the the technique:

.PP

.RS

.nf

float *luma, *refBlackWhite;

uint16 hs, vs;

/* Initialize structures */

ycbcr = (TIFFYCbCrToRGB*)

	_TIFFmalloc(TIFFroundup(sizeof(TIFFYCbCrToRGB), sizeof(long))

        	+ 4*256*sizeof(TIFFRGBValue)

        	+ 2*256*sizeof(int)

        	+ 3*256*sizeof(int32));

if (ycbcr == NULL) {

        TIFFError("YCbCr->RGB",

		"No space for YCbCr->RGB conversion state");

        exit(0);

}

TIFFGetFieldDefaulted(tif, TIFFTAG_YCBCRCOEFFICIENTS, &luma);

TIFFGetFieldDefaulted(tif, TIFFTAG_REFERENCEBLACKWHITE, &refBlackWhite);

if (TIFFYCbCrToRGBInit(ycbcr, luma, refBlackWhite) < 0)

	exit(0);

/* Start conversion */

uint32 r, g, b;

uint32 Y;

int32 Cb, Cr;

for each pixel in image

	TIFFYCbCrtoRGB(img->ycbcr, Y, Cb, Cr, &r, &g, &b);

/* Free state structure */

_TIFFfree(ycbcr);

.fi

.RE

.PP

.PP

.B TIFFCIELabToRGBInit()

initializes the

.I CIE L*a*b* 1976

to

.I RGB

conversion state.

.B TIFFCIELabToRGB

defined as

.PP

.RS

.nf

#define CIELABTORGB_TABLE_RANGE 1500

typedef struct {		     /* CIE Lab 1976->RGB support */

	int	range;		     /* Size of conversion table */

	float	rstep, gstep, bstep;

	float	X0, Y0, Z0;	     /* Reference white point */

	TIFFDisplay display;

	float	Yr2r[CIELABTORGB_TABLE_RANGE + 1]; /* Conversion of Yr to r */

	float	Yg2g[CIELABTORGB_TABLE_RANGE + 1]; /* Conversion of Yg to g */

	float	Yb2b[CIELABTORGB_TABLE_RANGE + 1]; /* Conversion of Yb to b */

} TIFFCIELabToRGB;

.fi

.RE

.PP

.I display

is a display device description, declared as

.PP

.RS

.nf

typedef struct {

	float d_mat[3][3]; /* XYZ -> luminance matrix */

	float d_YCR;       /* Light o/p for reference white */

	float d_YCG;

	float d_YCB;

	uint32 d_Vrwr;     /* Pixel values for ref. white */

	uint32 d_Vrwg;

	uint32 d_Vrwb;

	float d_Y0R;       /* Residual light for black pixel */

	float d_Y0G;

	float d_Y0B;

	float d_gammaR;    /* Gamma values for the three guns */

	float d_gammaG;

	float d_gammaB;

} TIFFDisplay;

.fi

.RE

.PP

For example, the one can use sRGB device, which has the following parameters:

.PP

.RS

.nf

TIFFDisplay display_sRGB = {

	{       /* XYZ -> luminance matrix */

		{  3.2410F, -1.5374F, -0.4986F },

		{  -0.9692F, 1.8760F, 0.0416F },

		{  0.0556F, -0.2040F, 1.0570F }

	},	

	100.0F, 100.0F, 100.0F, /* Light o/p for reference white */

	255, 255, 255,      /* Pixel values for ref. white */

	1.0F, 1.0F, 1.0F,   /* Residual light o/p for black pixel */

	2.4F, 2.4F, 2.4F,   /* Gamma values for the three guns */

};

.fi

.RE

.PP

.I refWhite

is a color temperature of the reference white. The

.I TIFFTAG_WHITEPOINT

contains the chromaticity of the white point of the image from where the

reference white can be calculated using following formulae:

.PP

.RS

refWhite_Y = 100.0

.br

refWhite_X = whitePoint_x / whitePoint_y * refWhite_Y

.br

refWhite_Z = (1.0 - whitePoint_x - whitePoint_y) / whitePoint_y * refWhite_X

.br

.RE

.PP

The conversion itself performed in two steps: at the first one we will convert

.I CIE L*a*b* 1976

to

.I CIE XYZ

using

.B TIFFCIELabToXYZ()

routine, and at the second step we will convert

.I CIE XYZ

to

.I RGB

using

.B TIFFXYZToRGB().

Look at the code sample below:

.PP

.RS

.nf

float   *whitePoint;

float   refWhite[3];

/* Initialize structures */

img->cielab = (TIFFCIELabToRGB *)

	_TIFFmalloc(sizeof(TIFFCIELabToRGB));

if (!cielab) {

	TIFFError("CIE L*a*b*->RGB",

		"No space for CIE L*a*b*->RGB conversion state.");

	exit(0);

}

TIFFGetFieldDefaulted(tif, TIFFTAG_WHITEPOINT, &whitePoint);

refWhite[1] = 100.0F;

refWhite[0] = whitePoint[0] / whitePoint[1] * refWhite[1];

refWhite[2] = (1.0F - whitePoint[0] - whitePoint[1])

	      / whitePoint[1] * refWhite[1];

if (TIFFCIELabToRGBInit(cielab, &display_sRGB, refWhite) < 0) {

	TIFFError("CIE L*a*b*->RGB",

		"Failed to initialize CIE L*a*b*->RGB conversion state.");

	_TIFFfree(cielab);

	exit(0);

}

/* Now we can start to convert */

uint32 r, g, b;

uint32 L;

int32 a, b;

float X, Y, Z;

for each pixel in image

	TIFFCIELabToXYZ(cielab, L, a, b, &X, &Y, &Z);

	TIFFXYZToRGB(cielab, X, Y, Z, &r, &g, &b);

/* Don't forget to free the state structure */

_TIFFfree(cielab);

.fi

.RE

.PP

.SH "SEE ALSO"

.BR TIFFRGBAImage (3TIFF)

.BR libtiff (3TIFF),

.PP

Libtiff library home page:

.BR http://www.remotesensing.org/libtiff/