.TH LIBCOLOUR_CIELAB 7 libcolour
.SH NAME
LIBCOLOUR_CIELAB - CIE L*a*b*
.SH DESCRIPTION
CIE L*a*b*
colours are presented with either of
.nf

	typedef struct libcolour_cielab_f libcolour_cielab_f_t;
	typedef struct libcolour_cielab_lf libcolour_cielab_lf_t;
	typedef struct libcolour_cielab_llf libcolour_cielab_llf_t;

.fi
These
.BR struct s
use
.BR float ,
.BR double ,
and
.BR long\ double ,
respecitively, for the values stored in them,
and are otherwise identical.
.B struct libcolour_cielab_f
is defined as
.nf

	struct libcolour_cielab_f {
		enum libcolour_model \fImodel\fP;
		float \fIL\fP;
		float \fIa\fP;
		float \fIb\fP;
	};

.fi
.I .model
shall be set to
.BR LIBCOLOUR_CIELAB .
In
.BR union\ libcolour_colour_f ,
.BR union\ libcolour_colour_lf ,
and
.BR union\ libcolour_colour_llf ,
.I .cielab
is used for CIE L*a*b* colours.
.P
.IR .L ,
.IR .a ,
and
.I .b
hold the L*, a*, and b* values, respectively.
.P
L* is 0 in black and 100 in white.
Negative a* values are green,
positive a* values are red.
Negative b* values are blue,
positive b* values are yellow.
.P
CIE L*a*b* is not additive, since conversion from
CIE 1931 XYZ is non-linear. It’s white point is the
CIE Standard Illuminant D50.
.P
CIE L*a*b* approximates human colour perception with a
lightness parameter (L*) and two chromaticity parameters
(a* and b*), it is therefore useful in image
manipulation applications.
.SH SEE ALSO
.BR libcolour (7)
.SH AUTHORS
Mattias Andrée
.RI < maandree@kth.se >