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, const 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);
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:
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);
Libtiff library home page: http://www.simplesystems.org/libtiff/