144 lines
3.9 KiB
Groff
144 lines
3.9 KiB
Groff
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.TH IM_COL_XYZ2RGB 3 "2 December 1992"
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.SH NAME
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im_col_Lab2LCh, im_col_LCh2ab, im_col_Lab2XYZ, im_col_XYZ2Lab,
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im_col_pythagoras, im_col_display, im_col_XYZ2rgb, im_col_rgb2XYZ,
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im_col_L2Lucs, im_col_Lucs2L, im_col_C2Cucs, im_col_Cucs2C, im_col_Ch2hucs,
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im_col_Chucs2h, im_col_make_tables_UCS, im_col_dECMC
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\- colour space conversion
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.SH SYNOPSIS
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#include <vips/vips.h>
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int im_col_ab2Ch( a, b, C, h )
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.br
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float a, b, *C, *h;
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int im_col_Ch2ab( C, h, a, b )
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.br
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float C, h, *a, *b;
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int im_col_Lab2XYZ( L, a, b, X, Y, Z )
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.br
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float L, a, b, *X, *Y, *Z;
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int im_col_XYZ2Lab( X, Y, Z, L, a, b )
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.br
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float X, Y, Z, *L, *a, *b;
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float im_col_pythagoras( L1, a1, b1, L2, a2, b2 )
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.br
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float L1, a1, b1, L2, a2, b2;
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extern struct im_col_display *im_col_displays[];
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struct im_col_tab_disp *im_col_make_tables_RGB( im, display )
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.br
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IMAGE *im;
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.br
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struct im_col_display *display;
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int im_col_XYZ2rgb( display, table, X, Y, Z, r, g, b, oflow )
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.br
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struct im_col_display *display;
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.br
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struct im_col_tab_disp *table;
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.br
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float X, Y, Z;
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.br
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int *r, *g, *b;
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.br
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int *oflow;
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int im_col_rgb2XYZ( display, table, r, g, b, X, Y, Z )
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.br
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struct im_col_display *display;
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.br
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struct im_col_tab_disp *table;
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.br
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int r, g, b;
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.br
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float *X, *Y, *Z;
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float im_col_L2Lucs( L )
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.br
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float L;
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float im_col_Lucs2L( Lucs )
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.br
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float Lucs;
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float im_col_C2Cucs( C )
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.br
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float C;
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float im_col_Cucs2C( Cucs )
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.br
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float Cucs;
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float im_col_Ch2hucs( C, h )
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.br
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float h, C;
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float im_col_Chucs2h( C, hucs )
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.br
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float hucs, C;
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void im_col_make_tables_UCS( void )
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float im_col_dECMC( L1, a1, b1, L2, a2, b2 )
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.br
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float L1, a1, b1, L2, a2, b2;
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.SH DESCRIPTION
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Colour space conversion.
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These functions convert colour values between four different formats: XYZ
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(float), Lab (float), UCS (float), and RGB (unsigned char)
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displayable. Additionally, functions are provided to move from (a,b)-style
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rectangular colour coordinates to (C,h)-style coordinates. h is always
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in degrees.
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UCS is a colour space derived from the CMC(1:1) equations. There is no easy
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analytical conversion from UCS to Lab, so look-up tables are used. These have
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to be built with a call to im_col_make_tables_UCS(). Once built, these
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tables are shared by all UCS functions. You may call im_col_make_tables_UCS()
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many times - tables are only built on the first call.
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im_col_pythagoras() returns the pythagoran distance between two points in a
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colour space. It can be used for finding CIELAB delta E's. im_col_dECMC()
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returns the colour difference between two LAB points in CMC(1:1).
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An im_col_display structure characterises a CRT screen (see <vips/colour.h>).
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You can make up your own (if you can find a TV analyser), or use one of the
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structures provied by VIPS in the NULL-terminated array im_col_displays[]. See
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the source for disp2XYZ(1) for ideas on extracting a display struct from this
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list.
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im_make_tables_RGB(3) has a display type as argument, and returns a pointer
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to the structure im_col_tab_RGB. This latter contains the matrices to go from
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XYZ to luminances (and back), and the tables to go from the luminances (in r,
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g, b) to the effective signal values to be applied to the monitor input (and
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back). The function returns NULL on error. The IMAGE argument is passed on to
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im_malloc() to make the space required for the tables. Pass either NULL (if
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you need to free the memory yourself) or an IMAGE descriptor (if you want the
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memory to be freed automatically when that descriptor is closed).
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im_col_XYZ2rgb() takes a display, a look-up table and an XYZ coordinate are
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returns three values in the range 0-255. The extra value oflow is set to 0 if
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the specified XYZ position aflls within the display gamut, and to 1 if the
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point lies outside the gamut. im_col_rgb2XYZ() is the reverse transformation.
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.SH RETURN VALUE
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The functions (usually) return 0 on success and -1 on error.
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.SH SEE\ ALSO
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.nf
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im_XYZ2disp(3), im_dE_fromdisp(3).
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.SH COPYRIGHT
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National Gallery, 1990-1993.
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.SH AUTHOR
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D. Saunders \- 1988
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.br
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J.Ph. Laurent \- 2/12/1992
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.br
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J.Cupitt \- 21/7/93
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