COLOR(3TIFF) Image Format Library C++ Reference Manual COLOR(3TIFF)NAME
TIFFYCbCrToRGBInit, TIFFYCbCrtoRGB, TIFFCIELabToRGBInit, TIFFCIELabToXYZ,
TIFFXYZToRGB - color conversion routines.
SYNOPSIS
#include <tiffio.h>
int TIFFYCbCrToRGBInit(TIFFYCbCrToRGB *ycbcr, float *luma, float
*refBlackWhite
void TIFFYCbCrtoRGB(TIFFYCbCrToRGB *ycbcr, uint32 Y, int32 Cb
int TIFFCIELabToRGBInit(TIFFCIELabToRGB *cielab, TIFFDisplay *display,
float *refWhite
void TIFFCIELabToXYZ(TIFFCIELabToRGB *cielab, uint32 L, int32 a
void TIFFXYZToRGB(TIFFCIELabToRGB *cielab, float X, float Y
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
TIFFRGBAImage(3TIFF) interface.
TIFFYCbCrToRGBInit() used to initialize YCbCr to RGB conversion state.
Allocating and freeing of the ycbcr structure belongs to programmer.
TIFFYCbCrToRGB defined in tiffio.h as
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;
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).
TIFFTAG_YCBCRCOEFFICIENTS holds that values in TIFF file. 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). TIFFTAG_REFERENCEBLACKWHITE is responsible for storing these
values in TIFF file. Following code snippet should helps to understand
the the technique:
float *luma, *refBlackWhite;
uint16 hs, vs;
/* Initialize structures */
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COLOR(3TIFF) Image Format Library C++ Reference Manual COLOR(3TIFF)
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);
TIFFCIELabToRGBInit() initializes the CIE L*a*b* 1976 to RGB conversion
state. TIFFCIELabToRGB defined as
#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;
display is a display device description, declared as
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;
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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;
For example, the one can use sRGB device, which has the following
parameters:
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 */
};
refWhite is a color temperature of the reference white. The
TIFFTAG_WHITEPOINT contains the chromaticity of the white point of the
image from where the reference white can be calculated using following
formulae:
refWhite_Y = 100.0
refWhite_X = whitePoint_x / whitePoint_y * refWhite_Y
refWhite_Z = (1.0 - whitePoint_x - whitePoint_y) / whitePoint_y *
refWhite_X
The conversion itself performed in two steps: at the first one we will
convert CIE L*a*b* 1976 to CIE XYZ using TIFFCIELabToXYZ() routine, and
at the second step we will convert CIE XYZ to RGB using TIFFXYZToRGB().
Look at the code sample below:
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;
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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);
SEE ALSOTIFFRGBAImage(3TIFF)libtiff(3TIFF),
Libtiff library home page: http://www.remotesensing.org/libtiff/
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