TIFFcolor man page on FreeBSD

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       CIELabToXYZ, TIFFXYZToRGB - color conversion routines.

       #include <tiffio.h>

       int TIFFYCbCrToRGBInit(TIFFYCbCrToRGB *ycbcr, float *luma, float	 *ref‐
       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);

       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 TIFFRG‐
       BAImage(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_YCBCRCOEFFI‐
       CIENTS  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_REFER‐
       ENCEBLACKWHITE  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) {
			"No space for YCbCr->RGB conversion state");

	      TIFFGetFieldDefaulted(tif, TIFFTAG_YCBCRCOEFFICIENTS, &luma);
	      TIFFGetFieldDefaulted(tif, TIFFTAG_REFERENCEBLACKWHITE, &refBlackWhite);
	      if (TIFFYCbCrToRGBInit(ycbcr, luma, refBlackWhite) < 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 */

       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

	      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	 TIFF‐
       TAG_WHITEPOINT  contains	 the  chromaticity  of	the white point of the
       image from where the reference white can be calculated using  following

	      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 *)
	      if (!cielab) {
		   TIFFError("CIE L*a*b*->RGB",
			"No space for CIE L*a*b*->RGB conversion state.");

	      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.");

	      /* 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 */

       TIFFRGBAImage(3TIFF) libtiff(3TIFF),

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

libtiff			       December 21, 2003		  COLOR(3TIFF)

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