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COLOR(3TIFF)                                                      COLOR(3TIFF)

       TIFFYCbCrToRGBInit,    TIFFYCbCrtoRGB,    TIFFCIELabToRGBInit,    TIFF-
       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:

libtiff                        December 21, 2003                  COLOR(3TIFF)

Czas wygenerowania: 0.00014 sek.

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