COLOR

NAME

SYNOPSIS

int TIFFYCbCrToRGBInit(TIFFYCbCrToRGB *ycbcr, float *luma, float *refBlackWhite );
void TIFFYCbCrtoRGB(TIFFYCbCrToRGB *ycbcr, uint32 Y, int32 Cb, int32 Cr, uint32 *R, uint32 *G, uint32 *B );

int TIFFCIELabToRGBInit(TIFFCIELabToRGB *cielab, TIFFDisplay *display, float *refWhite);
void TIFFCIELabToXYZ(TIFFCIELabToRGB *cielab, uint32 L, int32 a, int32 b, float *X, float *Y, float *Z);
void TIFFXYZToRGB(TIFFCIELabToRGB *cielab, float X, float Y, float Z , uint32 *R, uint32 *G, uint32 *B);  

DESCRIPTION

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 */
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;
        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;
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

Libtiff library home page: http://www.remotesensing.org/libtiff/