libtiff/man/TIFFcolor.3tiff

.\" $Id: TIFFcolor.3tiff,v 1.3 2006-03-23 14:54:02 dron Exp $
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.\" Copyright (c) 2003, Andrey Kiselev <dron@ak4719.spb.edu>
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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/