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COLOR(3TIFF) COLOR(3TIFF) NAME TIFFYCbCrToRGBInit, TIFFYCbCrtoRGB, TIFFCIELabToRGBInit, TIFF- CIELabToXYZ, TIFFXYZToRGB - color conversion routines. SYNOPSIS #include <tiffio.h> int TIFFYCbCrToRGBInit(TIFFYCbCrToRGB *ycbcr, float *luma, float *ref- BlackWhite");" 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); 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 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) { 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 pa- rameters: 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 im- age 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.simplesystems.org/libtiff/ libtiff December 21, 2003 COLOR(3TIFF)
NAME | SYNOPSIS | DESCRIPTION | SEE ALSO
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