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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.

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 to Relative 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.

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