This menu groups operations on colorimetric images and
patches of colour. A colour patch is three float numbers plus
a tag saying how those number should be interpreted as
colour (for example, as a colour in CIE LAB colourspace).
You can drag and drop between colour patches, and
into and from the inkwell in an image paint window.
Double-left-click on a colour patch to open a colour select
dialog.
nip2 has 9 main types of colorimetric image, see
Table 5.1. All these types are D65 (that is, daylight)
absolute colorimetric. When it displays an image, nip2 uses
the Type field in the image header as a hint on how to
transform the numbers in the image into RGB for the
display. The current Type is displayed at the end of the
caption line below an image thumbnail.
The Mono, GREY16 and RGB16 types are not really
calibrated themselves: they are usually whatever you get by
loading an image from a file. You’ll usually need an extra
step, such as applying an embedded ICC profile, before you
get accurate colour.
Name
Format
Notes
Mono
One band 8 bit
Not calibrated
sRGB
Three band 8 bit
Screen device space for the sRGB standard
GREY16
One band 16 bit
Not calibrated
RGB16
Three band 16 bit
Not calibrated
Lab
Three band float
The 1976 version of the CIE perceptual colourspace
LabQ
Four band 8 bit
Like Lab, but represented as 10:11:11 bits
LabS
Three band 16 bit
Like Lab, but represented as 15:16:16 bits
LCh
Three band float
Lab, but with polar coordinates
XYZ
Three band float
The base CIE colourspace
Yxy
Three band float
Sometimes useful for colour meters
UCS
Three band float
Highly uniform space from the CMC(l:c) standard
Table 5.1: nip2 colourspaces
New
Make a patch of colour, or pick a colour from a
slice through CIELAB colourspace.
Convert To Colour
Convert anything into a Colour
object.
Colourspace
Change the colourspace. The stored
numbers change, but the visual appearance should
stay the same.
Tag As
Change the colourspace tag (the Type field in the
image header). The stored numbers stay the same,
but the visual appearance should change.
Colour Temperature
Change the colour temperature.
Move Whitepoint just adjusts the ratios of X and Z
using the CIE standard illuminants.
D65 to D50 and D50 to D65 transform using either a
3x3 matrix which is numerically minimal in XYZ
space with respect to the colours on a Macbeth
Color Checker, or via Bradford cone space. The
Bradford transform omits the power term.
The final two items go from XYZ to LAB and
back, but with D50 normalisation rather than the
default D65.
ICC
Transform images (not patches of colour) device
space to profile connection space (LAB float) and
back.
You need to be careful about colour temperature
issues: all printers work with D50, and nip2 is all
D65. Use the D65 to D50 interchange items in the
Colour Temperature menu to swap back and forth.
All printers also work with relative colorimetry,
and nip2 is generally absolute. Use Absolute toRelative to scale an absolute colorimetric image by
a media white point.
Radiance
nip2 can read and write images written by
the Radiance family of programs (usually with the
suffix .hdr), commonly used in HDR photrography.
Images in this format used a packed floating point
layout for their pixels. Items in this menu pack and
unpack pixels for you.
Difference
Calculate various colour difference metrics.
You can mix patches of colour and colour images.
Adjust
Change colour in a colorimetric way. Recombination
multiplies each pixel in an image through a matrix.
Cast displaces the neutral axis in LAB space. HSB
lets you adjust an image in LCh colourspace.
Similar Colour
find pixels in an image with a similar
colour to a patch of colour.
Measure Colour Chart
This takes a trimmed image of
a colour chart (a rectangular grid of coloured
squares), measures the average pixel value in the
centre 50% of each square, and returns a matrix of
the measured values.
Use Make Synthetic Colour Chart to make a colour
chart image from a matrix of measurements.
Plot ab Scatter
draws a 2 dimensional histogram of the
distribution of pixel colours in LAB colourspace.