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<head><title>Colour</title>
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<h3 class="sectionHead"><span class="titlemark">5.1 </span> <a
id="x23-450005.1"></a>Colour</h3>
<!--l. 30--><p class="noindent" ><a name="nip_label_sec:menu-colour"></a> <!--l. 32--><p class="indent" > 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.
<!--l. 39--><p class="indent" > <span
class="pcrr7t-">nip2 </span>has 9 main types of colorimetric image, see
Table <a
href="#x23-450011">5.1<!--tex4ht:ref: tb:colour --></a>. All these types are D65 (that is, daylight)
absolute colorimetric. When it displays an image, <span
class="pcrr7t-">nip2</span>
uses the <span
class="pcrr7t-">Type </span>field in the image header as a hint on how to
transform the numbers in the image into RGB for the
display. The current <span
class="pcrr7t-">Type </span>is displayed at the end of the
caption line below an image thumbnail.
<!--l. 46--><p class="indent" > The <span
class="pcrr7t-">Mono</span>, <span
class="pcrr7t-">GREY16 </span>and <span
class="pcrr7t-">RGB16 </span>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.
<!--l. 52--><p class="indent" > <a
id="x23-450011"></a><hr class="float"><div class="float"
>
<div class="center"
>
<!--l. 52--><p class="noindent" >
<div class="tabular"> <table id="TBL-2" class="tabular"
cellspacing="0" cellpadding="0" rules="groups"
><colgroup id="TBL-2-1g"><col
id="TBL-2-1"></colgroup><colgroup id="TBL-2-2g"><col
id="TBL-2-2"></colgroup><colgroup id="TBL-2-3g"><col
id="TBL-2-3"></colgroup><tr
class="hline"><td><hr></td><td><hr></td><td><hr></td></tr><tr
style="vertical-align:baseline;" id="TBL-2-1-"><td style="white-space:nowrap; text-align:left;" id="TBL-2-1-1"
class="td11">Name </td><td style="white-space:nowrap; text-align:left;" id="TBL-2-1-2"
class="td11">Format </td><td style="white-space:nowrap; text-align:left;" id="TBL-2-1-3"
class="td11">Notes </td></tr><tr
class="hline"><td><hr></td><td><hr></td><td><hr></td></tr><tr
style="vertical-align:baseline;" id="TBL-2-2-"><td style="white-space:nowrap; text-align:left;" id="TBL-2-2-1"
class="td11"><span
class="pcrr7t-">Mono </span></td><td style="white-space:nowrap; text-align:left;" id="TBL-2-2-2"
class="td11">One band 8 bit </td><td style="white-space:nowrap; text-align:left;" id="TBL-2-2-3"
class="td11">Not calibrated</td>
</tr><tr
style="vertical-align:baseline;" id="TBL-2-3-"><td style="white-space:nowrap; text-align:left;" id="TBL-2-3-1"
class="td11"><span
class="pcrr7t-">sRGB </span></td><td style="white-space:nowrap; text-align:left;" id="TBL-2-3-2"
class="td11">Three band 8 bit </td><td style="white-space:nowrap; text-align:left;" id="TBL-2-3-3"
class="td11">Screen device space for the sRGB standard </td>
</tr><tr
style="vertical-align:baseline;" id="TBL-2-4-"><td style="white-space:nowrap; text-align:left;" id="TBL-2-4-1"
class="td11"><span
class="pcrr7t-">GREY16</span></td><td style="white-space:nowrap; text-align:left;" id="TBL-2-4-2"
class="td11">One band 16 bit </td><td style="white-space:nowrap; text-align:left;" id="TBL-2-4-3"
class="td11">Not calibrated </td></tr><tr
style="vertical-align:baseline;" id="TBL-2-5-"><td style="white-space:nowrap; text-align:left;" id="TBL-2-5-1"
class="td11"><span
class="pcrr7t-">RGB16 </span></td><td style="white-space:nowrap; text-align:left;" id="TBL-2-5-2"
class="td11">Three band 16 bit</td><td style="white-space:nowrap; text-align:left;" id="TBL-2-5-3"
class="td11">Not calibrated</td>
</tr><tr
style="vertical-align:baseline;" id="TBL-2-6-"><td style="white-space:nowrap; text-align:left;" id="TBL-2-6-1"
class="td11"><span
class="pcrr7t-">Lab </span></td><td style="white-space:nowrap; text-align:left;" id="TBL-2-6-2"
class="td11">Three band float </td><td style="white-space:nowrap; text-align:left;" id="TBL-2-6-3"
class="td11">The 1976 version of the CIE perceptual colourspace</td>
</tr><tr
style="vertical-align:baseline;" id="TBL-2-7-"><td style="white-space:nowrap; text-align:left;" id="TBL-2-7-1"
class="td11"><span
class="pcrr7t-">LabQ </span></td><td style="white-space:nowrap; text-align:left;" id="TBL-2-7-2"
class="td11">Four band 8 bit </td><td style="white-space:nowrap; text-align:left;" id="TBL-2-7-3"
class="td11">Like <span
class="pcrr7t-">Lab</span>, but represented as 10:11:11 bits </td>
</tr><tr
style="vertical-align:baseline;" id="TBL-2-8-"><td style="white-space:nowrap; text-align:left;" id="TBL-2-8-1"
class="td11"><span
class="pcrr7t-">LabS </span></td><td style="white-space:nowrap; text-align:left;" id="TBL-2-8-2"
class="td11">Three band 16 bit</td><td style="white-space:nowrap; text-align:left;" id="TBL-2-8-3"
class="td11">Like <span
class="pcrr7t-">Lab</span>, but represented as 15:16:16 bits </td>
</tr><tr
style="vertical-align:baseline;" id="TBL-2-9-"><td style="white-space:nowrap; text-align:left;" id="TBL-2-9-1"
class="td11"><span
class="pcrr7t-">LCh </span></td><td style="white-space:nowrap; text-align:left;" id="TBL-2-9-2"
class="td11">Three band float </td><td style="white-space:nowrap; text-align:left;" id="TBL-2-9-3"
class="td11"><span
class="pcrr7t-">Lab</span>, but with polar coordinates </td>
</tr><tr
style="vertical-align:baseline;" id="TBL-2-10-"><td style="white-space:nowrap; text-align:left;" id="TBL-2-10-1"
class="td11"><span
class="pcrr7t-">XYZ </span></td><td style="white-space:nowrap; text-align:left;" id="TBL-2-10-2"
class="td11">Three band float </td><td style="white-space:nowrap; text-align:left;" id="TBL-2-10-3"
class="td11">The base CIE colourspace </td>
</tr><tr
style="vertical-align:baseline;" id="TBL-2-11-"><td style="white-space:nowrap; text-align:left;" id="TBL-2-11-1"
class="td11"><span
class="pcrr7t-">Yxy </span></td><td style="white-space:nowrap; text-align:left;" id="TBL-2-11-2"
class="td11">Three band float </td><td style="white-space:nowrap; text-align:left;" id="TBL-2-11-3"
class="td11">Sometimes useful for colour meters </td>
</tr><tr
style="vertical-align:baseline;" id="TBL-2-12-"><td style="white-space:nowrap; text-align:left;" id="TBL-2-12-1"
class="td11"><span
class="pcrr7t-">UCS </span></td><td style="white-space:nowrap; text-align:left;" id="TBL-2-12-2"
class="td11">Three band float </td><td style="white-space:nowrap; text-align:left;" id="TBL-2-12-3"
class="td11">Highly uniform space from the CMC(l:c) standard </td>
</tr><tr
class="hline"><td><hr></td><td><hr></td><td><hr></td></tr><tr
style="vertical-align:baseline;" id="TBL-2-13-"><td style="white-space:nowrap; text-align:left;" id="TBL-2-13-1"
class="td11"> </td></tr></table></div></div>
<br /><div class="caption"
><span class="id">Table 5.1: </span><span
class="content"><span
class="pcrr7t-">nip2 </span>colourspaces</span></div><!--tex4ht:label?: x23-450011 -->
<!--l. 83--><p class="noindent" ><a name="nip_label_tb:colour"></a>
</div><hr class="endfloat" />
<dl class="description"><dt class="description">
<span
class="pcrb7t-">New</span> </dt><dd
class="description">Make a patch of colour, or pick a colour from a
slice through CIELAB colourspace.
</dd><dt class="description">
<span
class="pcrb7t-">Convert To Colour</span> </dt><dd
class="description">Convert anything into a
Colour object.
</dd><dt class="description">
<span
class="pcrb7t-">Colourspace</span> </dt><dd
class="description">Change the colourspace. The stored
numbers change, but the visual appearance should
stay the same.
</dd><dt class="description">
<span
class="pcrb7t-">Tag As</span> </dt><dd
class="description">Change the colourspace tag (the <span
class="pcrr7t-">Type </span>field
in the image header). The stored numbers stay the
same, but the visual appearance should change.
</dd><dt class="description">
<span
class="pcrb7t-">Colour Temperature</span> </dt><dd
class="description">Change
the colour temperature. <span
class="pcrr7t-">Move Whitepoint </span>just
adjusts the ratios of X and Z using the CIE standard
illuminants.
<!--l. 108--><p class="noindent" ><span
class="pcrr7t-">D65 to</span>
<span
class="pcrr7t-">D50 </span>and <span
class="pcrr7t-">D50 to D65 </span>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.
<!--l. 113--><p class="noindent" >The final two items go from XYZ to LAB and
back, but with D50 normalisation rather than the
default D65.
</dd><dt class="description">
<span
class="pcrb7t-">ICC</span> </dt><dd
class="description">Transform images (not patches of colour) device
space to profile connection space (LAB float) and
back.
<!--l. 120--><p class="noindent" >You need to be careful about colour temperature
issues: all printers work with D50, and <span
class="pcrr7t-">nip2 </span>is all
D65. Use the D65 to D50 interchange items in the
<span
class="pcrr7t-">Colour Temperature </span>menu to swap back and
forth.
<!--l. 125--><p class="noindent" >All printers also work with relative colorimetry,
and <span
class="pcrr7t-">nip2 </span>is generally absolute. Use <span
class="pcrr7t-">Absolute</span>
<span
class="pcrr7t-">to Relative </span>to scale an absolute colorimetric
image by a media white point. </dd><dt class="description">
<span
class="pcrb7t-">Radiance</span> </dt><dd
class="description"><span
class="pcrr7t-">nip2 </span>can read and write images written
by the Radiance family of programs (usually
with the suffix <span
class="pcrr7t-">.hdr</span>), commonly used in HDR
photrography.
<!--l. 134--><p class="noindent" >Images in this format used a packed floating point
layout for their pixels. Items in this menu pack and
unpack pixels for you.
</dd><dt class="description">
<span
class="pcrb7t-">Difference</span> </dt><dd
class="description">Calculate various colour difference
metrics. You can mix patches of colour and colour
images.
</dd><dt class="description">
<span
class="pcrb7t-">Adjust</span> </dt><dd
class="description">Change colour in
a colorimetric way. <span
class="pcrr7t-">Recombination </span>multiplies
each pixel in an image through a matrix. <span
class="pcrr7t-">Cast</span>
displaces the neutral axis in LAB space. <span
class="pcrr7t-">HSB </span>lets
you adjust an image in LCh colourspace.
</dd><dt class="description">
<span
class="pcrb7t-">Similar Colour</span> </dt><dd
class="description">find pixels in an image with a
similar colour to a patch of colour.
</dd><dt class="description">
<span
class="pcrb7t-">Measure Colour Chart</span> </dt><dd
class="description">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.
<!--l. 156--><p class="noindent" >Use <span
class="pcrr7t-">Make Synthetic Colour Chart </span>to
make a colour chart image from a matrix of
measurements.
</dd><dt class="description">
<span
class="pcrb7t-">Plot ab Scatter</span> </dt><dd
class="description">draws a 2
dimensional histogram of the distribution of pixel
colours in LAB colourspace.
</dd></dl>
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