Misc. Reference Manual Pages COLOR(3TIFF)
NAME
TIFFYCbCrToRGBInit, TIFFYCbCrtoRGB, TIFFCIELabToRGBInit,TIFFCIELabToXYZ, TIFFXYZToRGB - color conversion routines.
SYNOPSIS
#include
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 YDESCRIPTION
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 conver-
sion state. Allocating and freeing of the ycbcr structure belongs to programmer. TIFFYCbCrToRGB defined in tiffio.h astypedef 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 propor-
tions of the red, green and blue in luminance, Y (see sec-
tion 21 of the TIFF 6.0 specification, where the YCbCrimages 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 fieldsdiscussed). TIFFTAG_REFERENCEBLACKWHITE is responsible for
storing these values in TIFF file. Following code snippet should helps to understand the the technique: libtiff Last change: December 21, 2003 1 Misc. Reference Manual Pages COLOR(3TIFF) 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 imageTIFFYCbCrtoRGB(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;
libtiff Last change: December 21, 2003 2 Misc. Reference Manual Pages COLOR(3TIFF)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;For example, the one can use sRGB device, which has the fol-
lowing 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. TheTIFFTAG_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.");
libtiff Last change: December 21, 2003 3 Misc. Reference Manual Pages COLOR(3TIFF) 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);
SEE ALSO
TIFFRGBAImage(3TIFF) libtiff(3TIFF), Libtiff library home page: http://www.remotesensing.org/libtiff/ libtiff Last change: December 21, 2003 4