1COLOR(3TIFF) COLOR(3TIFF)
2
6 TIFFYCbCrToRGBInit, TIFFYCbCrtoRGB, TIFFCIELabToRGBInit, TIFF‐
7 CIELabToXYZ, TIFFXYZToRGB - color conversion routines.
8
10 #include <tiffio.h>
11 int TIFFYCbCrToRGBInit(TIFFYCbCrToRGB *ycbcr, float *luma, float *ref‐
13 BlackWhite");"
14 void TIFFYCbCrtoRGB(TIFFYCbCrToRGB *ycbcr, uint32 Y, int32 Cb, int32
15 Cr, uint32 *R, uint32 *G, uint32 *B );
16 int TIFFCIELabToRGBInit(TIFFCIELabToRGB *cielab, const TIFFDisplay
18 *display, float *refWhite);
19 void TIFFCIELabToXYZ(TIFFCIELabToRGB *cielab, uint32 L, int32 a, int32
20 b, float *X, float *Y, float *Z);
21 void TIFFXYZToRGB(TIFFCIELabToRGB *cielab, float X, float Y, float
22 Z",uint32*"R, uint32 *G, uint32 *B);
23
25 TIFF supports several color spaces for images stored in that format.
26 There is usually a problem of application to handle the data properly
27 and convert between different colorspaces for displaying and printing
28 purposes. To simplify this task libtiff implements several color con‐
29 version routines itself. In particular, these routines used in TIFFRG‐
30 BAImage(3TIFF) interface.
31 TIFFYCbCrToRGBInit() used to initialize YCbCr to RGB conversion state.
33 Allocating and freeing of the ycbcr structure belongs to programmer.
34 TIFFYCbCrToRGB defined in tiffio.h as
35 typedef struct { /* YCbCr->RGB support */
37 TIFFRGBValue* clamptab; /* range clamping table */
38 int* Cr_r_tab;
39 int* Cb_b_tab;
40 int32* Cr_g_tab;
41 int32* Cb_g_tab;
42 int32* Y_tab;
43 } TIFFYCbCrToRGB;
44 luma is a float array of three values representing proportions of the
46 red, green and blue in luminance, Y (see section 21 of the TIFF 6.0
47 specification, where the YCbCr images discussed). TIFFTAG_YCBCRCOEFFI‐
48 CIENTS holds that values in TIFF file. refBlackWhite is a float array
49 of 6 values which specifies a pair of headroom and footroom image data
50 values (codes) for each image component (see section 20 of the TIFF 6.0
51 specification where the colorinmetry fields discussed). TIFFTAG_REFER‐
52 ENCEBLACKWHITE is responsible for storing these values in TIFF file.
53 Following code snippet should helps to understand the the technique:
54 float *luma, *refBlackWhite;
56 uint16 hs, vs;
57 /* Initialize structures */
59 ycbcr = (TIFFYCbCrToRGB*)
60 _TIFFmalloc(TIFFroundup(sizeof(TIFFYCbCrToRGB), sizeof(long))
61 + 4*256*sizeof(TIFFRGBValue)
62 + 2*256*sizeof(int)
63 + 3*256*sizeof(int32));
64 if (ycbcr == NULL) {
65 TIFFError("YCbCr->RGB",
66 "No space for YCbCr->RGB conversion state");
67 exit(0);
68 }
69 TIFFGetFieldDefaulted(tif, TIFFTAG_YCBCRCOEFFICIENTS, &luma);
71 TIFFGetFieldDefaulted(tif, TIFFTAG_REFERENCEBLACKWHITE, &refBlackWhite);
72 if (TIFFYCbCrToRGBInit(ycbcr, luma, refBlackWhite) < 0)
73 exit(0);
74 /* Start conversion */
76 uint32 r, g, b;
77 uint32 Y;
78 int32 Cb, Cr;
79 for each pixel in image
81 TIFFYCbCrtoRGB(img->ycbcr, Y, Cb, Cr, &r, &g, &b);
82 /* Free state structure */
84 _TIFFfree(ycbcr);
85 TIFFCIELabToRGBInit() initializes the CIE L*a*b* 1976 to RGB conversion
87 state. TIFFCIELabToRGB defined as
88 #define CIELABTORGB_TABLE_RANGE 1500
90 typedef struct { /* CIE Lab 1976->RGB support */
92 int range; /* Size of conversion table */
93 float rstep, gstep, bstep;
94 float X0, Y0, Z0; /* Reference white point */
95 TIFFDisplay display;
96 float Yr2r[CIELABTORGB_TABLE_RANGE + 1]; /* Conversion of Yr to r */
97 float Yg2g[CIELABTORGB_TABLE_RANGE + 1]; /* Conversion of Yg to g */
98 float Yb2b[CIELABTORGB_TABLE_RANGE + 1]; /* Conversion of Yb to b */
99 } TIFFCIELabToRGB;
100 display is a display device description, declared as
102 typedef struct {
104 float d_mat[3][3]; /* XYZ -> luminance matrix */
105 float d_YCR; /* Light o/p for reference white */
106 float d_YCG;
107 float d_YCB;
108 uint32 d_Vrwr; /* Pixel values for ref. white */
109 uint32 d_Vrwg;
110 uint32 d_Vrwb;
111 float d_Y0R; /* Residual light for black pixel */
112 float d_Y0G;
113 float d_Y0B;
114 float d_gammaR; /* Gamma values for the three guns */
115 float d_gammaG;
116 float d_gammaB;
117 } TIFFDisplay;
118 For example, the one can use sRGB device, which has the following
120 parameters:
121 TIFFDisplay display_sRGB = {
123 { /* XYZ -> luminance matrix */
124 { 3.2410F, -1.5374F, -0.4986F },
125 { -0.9692F, 1.8760F, 0.0416F },
126 { 0.0556F, -0.2040F, 1.0570F }
127 },
128 100.0F, 100.0F, 100.0F, /* Light o/p for reference white */
129 255, 255, 255, /* Pixel values for ref. white */
130 1.0F, 1.0F, 1.0F, /* Residual light o/p for black pixel */
131 2.4F, 2.4F, 2.4F, /* Gamma values for the three guns */
132 };
133 refWhite is a color temperature of the reference white. The TIFF‐
135 TAG_WHITEPOINT contains the chromaticity of the white point of the
136 image from where the reference white can be calculated using following
137 formulae:
138 refWhite_Y = 100.0
140 refWhite_X = whitePoint_x / whitePoint_y * refWhite_Y
141 refWhite_Z = (1.0 - whitePoint_x - whitePoint_y) / whitePoint_y
142 * refWhite_X
143 The conversion itself performed in two steps: at the first one we will
145 convert CIE L*a*b* 1976 to CIE XYZ using TIFFCIELabToXYZ() routine, and
146 at the second step we will convert CIE XYZ to RGB using TIFFXYZToRGB().
147 Look at the code sample below:
148 float *whitePoint;
150 float refWhite[3];
151 /* Initialize structures */
153 img->cielab = (TIFFCIELabToRGB *)
154 _TIFFmalloc(sizeof(TIFFCIELabToRGB));
155 if (!cielab) {
156 TIFFError("CIE L*a*b*->RGB",
157 "No space for CIE L*a*b*->RGB conversion state.");
158 exit(0);
159 }
160 TIFFGetFieldDefaulted(tif, TIFFTAG_WHITEPOINT, &whitePoint);
162 refWhite[1] = 100.0F;
163 refWhite[0] = whitePoint[0] / whitePoint[1] * refWhite[1];
164 refWhite[2] = (1.0F - whitePoint[0] - whitePoint[1])
165 / whitePoint[1] * refWhite[1];
166 if (TIFFCIELabToRGBInit(cielab, &display_sRGB, refWhite) < 0) {
167 TIFFError("CIE L*a*b*->RGB",
168 "Failed to initialize CIE L*a*b*->RGB conversion state.");
169 _TIFFfree(cielab);
170 exit(0);
171 }
172 /* Now we can start to convert */
174 uint32 r, g, b;
175 uint32 L;
176 int32 a, b;
177 float X, Y, Z;
178 for each pixel in image
180 TIFFCIELabToXYZ(cielab, L, a, b, &X, &Y, &Z);
181 TIFFXYZToRGB(cielab, X, Y, Z, &r, &g, &b);
182 /* Don't forget to free the state structure */
184 _TIFFfree(cielab);
185
187 TIFFRGBAImage(3TIFF) libtiff(3TIFF),
188 Libtiff library home page: http://www.remotesensing.org/libtiff/
190libtiff December 21, 2003 COLOR(3TIFF)