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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