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5.1 Colour
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.
Table 5.1:
nip2 colourspaces
Name |
Format |
Notes |
Mono |
One band 8 bit |
Not really calibrated at all |
sRGB |
Three band 8 bit |
Screen device space for the sRGB standard |
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 |
|
- New
- Make a patch of colour, or pick a colour from a slice through CIELAB
colourspace.
- Convert To
- Convert to a new 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 to Relative to scale an absolute
colorimetric image by a media white point.
- 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.
- Plot ab Scatter
- draws a 2 dimensional histogram of the distribution of pixel colours
in LAB colourspace.
- Image to Colour
- Turn images into patches of colour (by averaging their pixels), and
turn patches of colour back into images.
- 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.
Next: 5.2 Filter
Up: 5. Image processing menus
Previous: 5. Image processing menus
Contents
John Cupitt
2004-12-20