COLOR(3TIFF)															      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, 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

       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 con-
       version 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 */
	      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

       TIFFRGBAImage(3TIFF) libtiff(3TIFF),

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

libtiff 							 December 21, 2003						      COLOR(3TIFF)