im_xyz2disp(3)

NAME

im_XYZ2disp, im_disp2XYZ, im_Lab2XYZ, im_XYZ2Lab,

im_XYZ2Yxy,
im_Yxy2XYZ, im_XYZ2sRGB, im_sRGB2XYZ, im_Lab2LCh,

im_LCh2Lab,
im_LCh2UCS, im_UCS2LCh - convert images between various

colour spaces

SYNOPSIS

#include <vips/vips.h>
int im_XYZ2disp(in, out, display)
IMAGE *in, *out;
struct im_col_display *display;
int im_disp2XYZ(in, out, display)
IMAGE *in, *out;
struct im_col_display *display;
int im_Lab2XYZ(in, out)
IMAGE *in, *out;
int im_XYZ2Lab(in, out)
IMAGE *in, *out;
int im_XYZ2Yxy(in, out)
IMAGE *in, *out;
int im_Yxy2XYZ(in, out)
IMAGE *in, *out;
int im_XYZ2sRGB(in, out)
IMAGE *in, *out;
int im_sRGB2XYZ(in, out)
IMAGE *in, *out;
int im_Lab2LCh(in, out)
IMAGE *in, *out;
int im_LCh2Lab(in, out)
IMAGE *in, *out;
int im_LCh2UCS(in, out)
IMAGE *in, *out;
int im_UCS2LCh(in, out)
IMAGE *in, *out;

DESCRIPTION

Functions to convert images between the different

colour spaces supported by VIPS: RGB, sRGB, XYZ, Yxy, Lab, LCh and UCS. RGB

and sRGB are
three band uchar files. XYZ, Lab, LCh and UCS are

three band float
files.

These are the basic conversion routines provided by VIPS.

Other conversions, such as im_Lab2disp(3), are built by composing

these functions
and are provided as a convenience to the programmer.

The VIPS colour spaces inter-convert as follows:

+------- sRGB

LabQ ----- Lab ----- XYZ ----- RGB

|
| | +------- Yxy

| +------- LCh ----- UCS

+-------- LabS

The colour spaces are:

LabQ --- This is the principal VIPS colorimetric storage

format. See
im_LabQ2Lab(3) for an explanation. You cannot perform

calculations on
LabQ images. They are for storage only.

LabS --- This format represents coordinates in CIE Lab

space as 16-bit
integers. It is the best format for computation, being

relatively compact, quick, and accurate. Colour values expressed in this

way are hard
to visualise. See the page for im_LabQ2LabS().

Lab --- Coordinates in CIE Lab space are represented as

float values in
the usual range. This is the easiest format for general

work: adding 50
to the L channel, for example, has the expected result.

XYZ --- CIE XYZ colour space.

Yxy --- CIE Yxy colour space.

RGB --- This format is compatible with the RGB colour

systems used in
other packages. If you want to export your image to a PC,

for example,
convert your colorimetric image to RGB, then turn it

to TIFF with
vips2TIFF(1). You need to supply a structure which char

acterises your
display. See the manual page for im_col_XYZ2rgb(3) for

hints on these
guys.

sRGB --- This is a standard RGB, as defined by:

http://www.color.org/contrib/sRGB.html

it's handy for carrying colour information through JPEG

compressors,
for example.

LCh --- Like Lab, but the rectangular ab coordinates are

replaced with
the polar Ch (Chroma and hue) coordinates. Hue angles are

expressed in
degrees.

UCS --- A colour space based on the CMC(1:1) colour dif

ference measurement. This is a highly uniform colour space, much better

than Lab for
expressing small differences. Conversions to and from UCS

are extremely
slow.

These conversions set the Type field in the image header

to LABQ, LABS,
LAB, XYZ, RGB, sRGB, LCH and UCS respectively. The Type

field is for
user information only --- no function reads the Type field

to determine
its behaviour. Visualisation programs, such as ip(1),

use the Type
field to help present the information to the user.

All VIPS colour spaces are based around D65.

VIPS has functions for finding colour difference

images. See
im_dE_fromLab(3).

RETURN VALUE

The functions return 0 on success and -1 on error.

SEE ALSO

im_col_XYZ2rgb(3), im_dE_fromLab(3), im_LabQ2Lab(3),

im_Lab2disp(3).

COPYRIGHT

National Gallery and Birkbeck College, 1990 - 1993

AUTHOR

K. Martinez - 2/12/1992
J. Cupitt - 21/7/93

2 Decemder 1992

IM_XYZ2disp(3)