Provided by: libtiff4-dev_3.9.5-2ubuntu1_amd64 bug

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  conversion  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/