314 lines
11 KiB
HTML
314 lines
11 KiB
HTML
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<head>
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<meta name="generator" content="groff -Thtml, see www.gnu.org">
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<meta name="Content-Style" content="text/css">
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<title>COLOR</title>
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</head>
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<body>
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<h1 align=center>COLOR</h1>
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<a href="#NAME">NAME</a><br>
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<a href="#SYNOPSIS">SYNOPSIS</a><br>
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<a href="#DESCRIPTION">DESCRIPTION</a><br>
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<a href="#SEE ALSO">SEE ALSO</a><br>
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<hr>
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<!-- Creator : groff version 1.17.2 -->
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<!-- CreationDate: Mon Dec 22 00:39:20 2003 -->
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<a name="NAME"></a>
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<h2>NAME</h2>
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<table width="100%" border=0 rules="none" frame="void"
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cols="2" cellspacing="0" cellpadding="0">
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<tr valign="top" align="left">
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<td width="10%"></td><td width="90%">
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TIFFYCbCrToRGBInit, TIFFYCbCrtoRGB, TIFFCIELabToRGBInit, TIFFCIELabToXYZ, TIFFXYZToRGB - color conversion routines.</td></table>
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<a name="SYNOPSIS"></a>
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<h2>SYNOPSIS</h2>
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<table width="100%" border=0 rules="none" frame="void"
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cols="2" cellspacing="0" cellpadding="0">
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<tr valign="top" align="left">
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<td width="10%"></td><td width="90%">
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<b>#include <tiffio.h></b></td></table>
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<table width="100%" border=0 rules="none" frame="void"
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cols="2" cellspacing="0" cellpadding="0">
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<tr valign="top" align="left">
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<td width="10%"></td><td width="90%">
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<b>int TIFFYCbCrToRGBInit(TIFFYCbCrToRGB
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*</b><i>ycbcr</i><b>, float *</b><i>luma</i><b>, float
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*</b><i>refBlackWhite"</i><b>);"<br>
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void TIFFYCbCrtoRGB(TIFFYCbCrToRGB *</b><i>ycbcr</i><b>,
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uint32</b> <i>Y</i><b>, int32</b> <i>Cb</i><b>, int32</b>
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<i>Cr</i><b>, uint32 *</b><i>R</i><b>, uint32
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*</b><i>G</i><b>, uint32 *</b><i>B</i>
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<b>);</b></td></table>
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<table width="100%" border=0 rules="none" frame="void"
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cols="2" cellspacing="0" cellpadding="0">
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<tr valign="top" align="left">
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<td width="10%"></td><td width="90%">
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<b>int TIFFCIELabToRGBInit(TIFFCIELabToRGB
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*</b><i>cielab</i><b>, TIFFDisplay *</b><i>display</i><b>,
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float *</b><i>refWhite</i><b>);<br>
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void TIFFCIELabToXYZ(TIFFCIELabToRGB *</b><i>cielab</i><b>,
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uint32</b> <i>L</i><b>, int32</b> <i>a</i><b>, int32</b>
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<i>b</i><b>, float *</b><i>X</i><b>, float *</b><i>Y</i><b>,
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float *</b><i>Z</i><b>);<br>
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void TIFFXYZToRGB(TIFFCIELabToRGB *</b><i>cielab</i><b>,
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float</b> <i>X</i><b>, float</b> <i>Y</i><b>, float</b>
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<i>Z"</i><b>,</b><i>uint32</i><b>*"</b><i>R</i><b>,
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uint32 *</b><i>G</i><b>, uint32
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*</b><i>B</i><b>);</b></td></table>
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<a name="DESCRIPTION"></a>
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<h2>DESCRIPTION</h2>
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<table width="100%" border=0 rules="none" frame="void"
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cols="2" cellspacing="0" cellpadding="0">
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<tr valign="top" align="left">
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<td width="10%"></td><td width="90%">
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TIFF supports several color spaces for images stored in that
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format. There is usually a problem of application to handle
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the data properly and convert between different colorspaces
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for displaying and printing purposes. To simplify this task
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libtiff implements several color conversion routines itself.
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In particular, these routines used in
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<b>TIFFRGBAImage(3T)</b> interface.</td></table>
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<table width="100%" border=0 rules="none" frame="void"
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cols="2" cellspacing="0" cellpadding="0">
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<tr valign="top" align="left">
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<td width="10%"></td><td width="90%">
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<b>TIFFYCbCrToRGBInit()</b> used to initialize <i>YCbCr</i>
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to <i>RGB</i> conversion state. Allocating and freeing of
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the <i>ycbcr</i> structure belongs to programmer.
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<i>TIFFYCbCrToRGB</i> defined in <b>tiffio.h</b>
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as</td></table>
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<table width="100%" border=0 rules="none" frame="void"
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cols="2" cellspacing="0" cellpadding="0">
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<tr valign="top" align="left">
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<td width="21%"></td><td width="79%">
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<pre>typedef struct { /* YCbCr->RGB support */
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TIFFRGBValue* clamptab; /* range clamping table */
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int* Cr_r_tab;
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int* Cb_b_tab;
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int32* Cr_g_tab;
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int32* Cb_g_tab;
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int32* Y_tab;
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} TIFFYCbCrToRGB;
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</pre></td></table>
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<table width="100%" border=0 rules="none" frame="void"
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cols="2" cellspacing="0" cellpadding="0">
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<tr valign="top" align="left">
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<td width="10%"></td><td width="90%">
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<i>luma</i> is a float array of three values representing
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proportions of the red, green and blue in luminance, Y (see
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section 21 of the TIFF 6.0 specification, where the YCbCr
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images discussed). <i>TIFFTAG_YCBCRCOEFFICIENTS</i> holds
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that values in TIFF file. <i>refBlackWhite</i> is a float
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array of 6 values which specifies a pair of headroom and
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footroom image data values (codes) for each image component
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(see section 20 of the TIFF 6.0 specification where the
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colorinmetry fields discussed).
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<i>TIFFTAG_REFERENCEBLACKWHITE</i> is responsible for
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storing these values in TIFF file. Following code snippet
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should helps to understand the the technique:</td></table>
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<table width="100%" border=0 rules="none" frame="void"
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cols="2" cellspacing="0" cellpadding="0">
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<tr valign="top" align="left">
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<td width="21%"></td><td width="79%">
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<pre>float *luma, *refBlackWhite;
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uint16 hs, vs;
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/* Initialize structures */
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ycbcr = (TIFFYCbCrToRGB*)
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_TIFFmalloc(TIFFroundup(sizeof(TIFFYCbCrToRGB), sizeof(long))
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+ 4*256*sizeof(TIFFRGBValue)
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+ 2*256*sizeof(int)
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+ 3*256*sizeof(int32));
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if (ycbcr == NULL) {
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TIFFError("YCbCr->RGB",
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"No space for YCbCr->RGB conversion state");
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exit(0);
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}
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TIFFGetFieldDefaulted(tif, TIFFTAG_YCBCRCOEFFICIENTS, &luma);
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TIFFGetFieldDefaulted(tif, TIFFTAG_REFERENCEBLACKWHITE, &refBlackWhite);
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if (TIFFYCbCrToRGBInit(ycbcr, luma, refBlackWhite) < 0)
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exit(0);
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/* Start conversion */
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uint32 r, g, b;
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uint32 Y;
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int32 Cb, Cr;
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for each pixel in image
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TIFFYCbCrtoRGB(img->ycbcr, Y, Cb, Cr, &r, &g, &b);
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/* Free state structure */
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_TIFFfree(ycbcr);
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</pre></td></table>
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<table width="100%" border=0 rules="none" frame="void"
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cols="2" cellspacing="0" cellpadding="0">
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<tr valign="top" align="left">
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<td width="10%"></td><td width="90%">
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<b>TIFFCIELabToRGBInit()</b> initializes the <i>CIE L*a*b*
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1976</i> to <i>RGB</i> conversion state.
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<b>TIFFCIELabToRGB</b> defined as</td></table>
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<table width="100%" border=0 rules="none" frame="void"
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cols="2" cellspacing="0" cellpadding="0">
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<tr valign="top" align="left">
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<td width="21%"></td><td width="79%">
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<pre>#define CIELABTORGB_TABLE_RANGE 1500
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typedef struct { /* CIE Lab 1976->RGB support */
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int range; /* Size of conversion table */
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float rstep, gstep, bstep;
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float X0, Y0, Z0; /* Reference white point */
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TIFFDisplay display;
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float Yr2r[CIELABTORGB_TABLE_RANGE + 1]; /* Conversion of Yr to r */
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float Yg2g[CIELABTORGB_TABLE_RANGE + 1]; /* Conversion of Yg to g */
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float Yb2b[CIELABTORGB_TABLE_RANGE + 1]; /* Conversion of Yb to b */
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} TIFFCIELabToRGB;
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</pre></td></table>
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<table width="100%" border=0 rules="none" frame="void"
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cols="2" cellspacing="0" cellpadding="0">
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<tr valign="top" align="left">
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<td width="10%"></td><td width="90%">
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<i>display</i> is a display device description, declared
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as</td></table>
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<table width="100%" border=0 rules="none" frame="void"
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cols="2" cellspacing="0" cellpadding="0">
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<tr valign="top" align="left">
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<td width="21%"></td><td width="79%">
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<pre>typedef struct {
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float d_mat[3][3]; /* XYZ -> luminance matrix */
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float d_YCR; /* Light o/p for reference white */
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float d_YCG;
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float d_YCB;
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int d_Vrwr; /* Pixel values for ref. white */
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int d_Vrwg;
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int d_Vrwb;
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float d_Y0R; /* Residual light for black pixel */
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float d_Y0G;
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float d_Y0B;
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float d_gammaR; /* Gamma values for the three guns */
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float d_gammaG;
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float d_gammaB;
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} TIFFDisplay;
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</pre></td></table>
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<table width="100%" border=0 rules="none" frame="void"
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cols="2" cellspacing="0" cellpadding="0">
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<tr valign="top" align="left">
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<td width="10%"></td><td width="90%">
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For example, the one can use sRGB device, which has the
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following parameters:</td></table>
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<table width="100%" border=0 rules="none" frame="void"
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cols="2" cellspacing="0" cellpadding="0">
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<tr valign="top" align="left">
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<td width="21%"></td><td width="79%">
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<pre>TIFFDisplay display_sRGB = {
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{ /* XYZ -> luminance matrix */
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{ 3.2410F, -1.5374F, -0.4986F },
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{ -0.9692F, 1.8760F, 0.0416F },
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{ 0.0556F, -0.2040F, 1.0570F }
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},
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100.0F, 100.0F, 100.0F, /* Light o/p for reference white */
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255, 255, 255, /* Pixel values for ref. white */
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1.0F, 1.0F, 1.0F, /* Residual light o/p for black pixel */
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2.4F, 2.4F, 2.4F, /* Gamma values for the three guns */
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};
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</pre></td></table>
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<table width="100%" border=0 rules="none" frame="void"
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cols="2" cellspacing="0" cellpadding="0">
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<tr valign="top" align="left">
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<td width="10%"></td><td width="90%">
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<i>refWhite</i> is a color temperature of the reference
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white. The <i>TIFFTAG_WHITEPOINT</i> contains the
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chromaticity of the white point of the image from where the
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reference white can be calculated using following
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formulae:</td></table>
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<table width="100%" border=0 rules="none" frame="void"
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cols="2" cellspacing="0" cellpadding="0">
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<tr valign="top" align="left">
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<td width="21%"></td><td width="79%">
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refWhite_Y = 100.0<br>
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refWhite_X = whitePoint_x / whitePoint_y * refWhite_Y<br>
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refWhite_Z = (1.0 - whitePoint_x - whitePoint_y) /
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whitePoint_y * refWhite_X</td></table>
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<table width="100%" border=0 rules="none" frame="void"
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cols="2" cellspacing="0" cellpadding="0">
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<tr valign="top" align="left">
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<td width="10%"></td><td width="90%">
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The conversion itself performed in two steps: at the first
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one we will convert <i>CIE L*a*b* 1976</i> to <i>CIE XYZ</i>
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using <b>TIFFCIELabToXYZ()</b> routine, and at the second
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step we will convert <i>CIE XYZ</i> to <i>RGB</i> using
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<b>TIFFXYZToRGB().</b> Look at the code sample
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below:</td></table>
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<table width="100%" border=0 rules="none" frame="void"
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cols="2" cellspacing="0" cellpadding="0">
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<tr valign="top" align="left">
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<td width="21%"></td><td width="79%">
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<pre>float *whitePoint;
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float refWhite[3];
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/* Initialize structures */
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img->cielab = (TIFFCIELabToRGB *)
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_TIFFmalloc(sizeof(TIFFCIELabToRGB));
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if (!cielab) {
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TIFFError("CIE L*a*b*->RGB",
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"No space for CIE L*a*b*->RGB conversion state.");
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exit(0);
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}
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TIFFGetFieldDefaulted(tif, TIFFTAG_WHITEPOINT, &whitePoint);
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refWhite[1] = 100.0F;
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refWhite[0] = whitePoint[0] / whitePoint[1] * refWhite[1];
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refWhite[2] = (1.0F - whitePoint[0] - whitePoint[1])
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/ whitePoint[1] * refWhite[1];
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if (TIFFCIELabToRGBInit(cielab, &display_sRGB, refWhite) < 0) {
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TIFFError("CIE L*a*b*->RGB",
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"Failed to initialize CIE L*a*b*->RGB conversion state.");
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_TIFFfree(cielab);
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exit(0);
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}
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/* Now we can start to convert */
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uint32 r, g, b;
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uint32 L;
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int32 a, b;
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float X, Y, Z;
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for each pixel in image
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TIFFCIELabToXYZ(cielab, L, a, b, &X, &Y, &Z);
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TIFFXYZToRGB(cielab, X, Y, Z, &r, &g, &b);
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/* Don't forget to free the state structure */
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_TIFFfree(cielab);
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</pre></td></table>
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<a name="SEE ALSO"></a>
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<h2>SEE ALSO</h2>
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<table width="100%" border=0 rules="none" frame="void"
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cols="2" cellspacing="0" cellpadding="0">
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<tr valign="top" align="left">
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<td width="10%"></td><td width="90%">
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<i>libtiff</i>(3T),<i>TIFFRGBAImage(3T)</i></td></table>
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