table of contents
conflicting packages
COLOR(3TIFF) | COLOR(3TIFF) |
NAME¶
TIFFYCbCrToRGBInit, TIFFYCbCrtoRGB, TIFFCIELabToRGBInit, TIFFCIELabToXYZ, TIFFXYZToRGB - color conversion routines.SYNOPSIS¶
#include <tiffio.h>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 astypedef 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;
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);
#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;
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;
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_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/December 21, 2003 | libtiff |