Basic Concepts#
This page puts an emphasis on basic concepts of Colour, those are important to understand.
Object Name Categorisation#
The API tries to bundle the objects by categories by naming them with common prefixes which makes introspection and autocompletion easier.
For example, in IPython or Jupyter Notebook, most of the definitions pertaining to the spectral distribution handling can be found as follows:
In [1]: import colour
In [2]: colour.sd_
sd_blackbody() sd_gaussian() sd_rayleigh_scattering() sd_zeros
sd_CIE_illuminant_D_series() sd_mesopic_luminous_efficiency_function() sd_single_led()
sd_CIE_standard_illuminant_A() sd_multi_leds() sd_to_aces_relative_exposure_values()
sd_constant() sd_ones() sd_to_XYZ
Likewise, for the spectral distribution handling related attributes:
In [2]: colour.SD
SD_GAUSSIAN_METHODS SD_TO_XYZ_METHODS SDS_ILLUMINANTS SDS_LIGHT_SOURCES
SD_MULTI_LEDS_METHODS SDS_COLOURCHECKERS SDS_LEFS
SD_SINGLE_LED_METHODS SDS_FILTERS SDS_LENSES
Similarly, all the RGB colourspaces can be individually accessed from the
colour.models
namespace:
In [2]: colour.models.RGB_COLOURSPACE
RGB_COLOURSPACE_ACES2065_1 RGB_COLOURSPACE_ACESPROXY RGB_COLOURSPACE_APPLE_RGB RGB_COLOURSPACE_BT470_525
RGB_COLOURSPACE_ACESCC RGB_COLOURSPACE_ADOBE_RGB1998 RGB_COLOURSPACE_BEST_RGB RGB_COLOURSPACE_BT470_625
RGB_COLOURSPACE_ACESCCT RGB_COLOURSPACE_ADOBE_WIDE_GAMUT_RGB RGB_COLOURSPACE_BETA_RGB RGB_COLOURSPACE_BT709 >
RGB_COLOURSPACE_ACESCG RGB_COLOURSPACE_ALEXA_WIDE_GAMUT RGB_COLOURSPACE_BT2020 RGB_COLOURSPACE_CIE_RGB
Abbreviations#
The following abbreviations are in use in Colour:
CAM : Colour Appearance Model
CCS : Chromaticity Coordinates
CCTF : Colour Component Transfer Function
CCT : Correlated Colour Temperature
CMY : Cyan, Magenta, Yellow
CMYK : Cyan, Magenta, Yellow, Black
CVD : Colour Vision Deficiency
CV : Code Value
EOTF : ElectroOptical Transfer Function
IDT : Input Device Transform
MSDS : MultiSpectral Distributions
OETF : OpticalElectrical Transfer Function
OOTF : OpticalOptical Transfer Function
SD : Spectral Distribution
TVS : Tristimulus Values
NDimensional Array Support#
Most of Colour definitions are fully vectorised and support ndimensional array by leveraging Numpy.
While it is recommended to use ndarray as input for the API objects, it is possible to use tuples or lists:
import colour
xyY = (0.4316, 0.3777, 0.1008)
colour.xyY_to_XYZ(xyY)
array([ 0.11518475, 0.1008 , 0.05089373])
xyY = [0.4316, 0.3777, 0.1008]
colour.xyY_to_XYZ(xyY)
array([ 0.11518475, 0.1008 , 0.05089373])
xyY = [
(0.4316, 0.3777, 0.1008),
(0.4316, 0.3777, 0.1008),
(0.4316, 0.3777, 0.1008),
]
colour.xyY_to_XYZ(xyY)
array([[ 0.11518475, 0.1008 , 0.05089373],
[ 0.11518475, 0.1008 , 0.05089373],
[ 0.11518475, 0.1008 , 0.05089373]])
As shown in the above example, there is widespread support for ndimensional arrays:
import numpy as np
xyY = np.array([0.4316, 0.3777, 0.1008])
xyY = np.tile(xyY, (6, 1))
colour.xyY_to_XYZ(xyY)
array([[ 0.11518475, 0.1008 , 0.05089373],
[ 0.11518475, 0.1008 , 0.05089373],
[ 0.11518475, 0.1008 , 0.05089373],
[ 0.11518475, 0.1008 , 0.05089373],
[ 0.11518475, 0.1008 , 0.05089373],
[ 0.11518475, 0.1008 , 0.05089373]])
colour.xyY_to_XYZ(xyY.reshape([2, 3, 3]))
array([[[ 0.11518475, 0.1008 , 0.05089373],
[ 0.11518475, 0.1008 , 0.05089373],
[ 0.11518475, 0.1008 , 0.05089373]],
[[ 0.11518475, 0.1008 , 0.05089373],
[ 0.11518475, 0.1008 , 0.05089373],
[ 0.11518475, 0.1008 , 0.05089373]]])
Which enables image processing:
RGB = colour.read_image('_static/Logo_Small_001.png')
RGB = RGB[..., 0:3] # Discarding alpha channel.
XYZ = colour.sRGB_to_XYZ(RGB)
colour.plotting.plot_image(XYZ, text_kwargs={'text': 'sRGB to XYZ'})
Spectral Representation and Continuous Signal#
Floating Point Wavelengths#
Colour current representation of spectral data is atypical and has been influenced by the failures and shortcomings of the previous implementation that required less than ideal code to support floating point wavelengths. Wavelengths should not have to be defined as integer values and it is effectively common to get data from instruments whose domain is returned as floating point values.
For example, the data from an Ocean Insight (Optics) STSVIS spectrometer is typically saved with 3 digits decimal precision:
Data from Subt2_143615210.txt Node
Date: Sat Nov 17 14:36:15 NZDT 2018
User: kelsolaar
Spectrometer: S12286
Trigger mode: 0
Resolution mode: 1024 pixels
Integration Time (sec): 5.000000E0
Scans to average: 3
Nonlinearity correction enabled: true
Boxcar width: 3
Baseline correction enabled: true
XAxis mode: Wavelengths
Number of Pixels in Spectrum: 1024
>>>>>Begin Spectral Data<<<<<
338.028 279.71
338.482 285.43
338.936 291.33
...
821.513 3112.65
822.008 3133.74
822.503 3107.11
A solution to the problem is to quantize the data at integer values but it is often nondesirable. The spectra representation implementation prior to Colour 0.3.11 was relying on a custom mutable mapping which was allowing to retrieve decimal keys within a given precision:
data_1 = {0.1999999998: 'Nemo', 0.2000000000: 'John'}
apm_1 = ArbitraryPrecisionMapping(data_1, key_decimals=10)
tuple(apm_1.keys())
(0.1999999998, 0.2)
apm_2 = ArbitraryPrecisionMapping(data_1, key_decimals=7)
tuple(apm_2.keys())
(0.2,)
While functional, the approach was brittle and not elegant which triggered a significant amount of rework.
Continuous Signal#
All the spectral distributions in Colour are instances of the
colour.SpectralDistribution
class (or its subclasses), a subclass of
the colour.continuous.Signal
class which is itself an implementation
of the colour.continuous.AbstractContinuousFunction
ABCMeta
class:
Likewise, the multispectral distributions are instances
colour.MultiSpectralDistributions
class (or its subclasses), a
subclass of the colour.continuous.MultiSignals
class which is a
container for multiple colour.continuous.Signal
subclass instances
and also implements the colour.continuous.AbstractContinuousFunction
ABCMeta class.
The colour.continuous.Signal
class implements the
Signal.function()
method so that evaluating the function for any
independent domain \(x \in\mathbb{R}\) variable returns a corresponding
range \(y \in\mathbb{R}\) variable.
It adopts an interpolating function encapsulated inside an extrapolating
function. The resulting function independent domain, stored as discrete values
in the colour.continuous.Signal.domain
attribute corresponds with the
function dependent and already known range stored in the
colour.continuous.Signal.range
attribute.
Consequently, it is possible to get the value of a spectral distribution at any given wavelength:
data = {
500: 0.0651,
520: 0.0705,
540: 0.0772,
560: 0.0870,
580: 0.1128,
600: 0.1360
}
sd = colour.SpectralDistribution(data)
sd[555.5]
0.083453673782958995
Getting, Setting, Indexing and Slicing#
Attention
Indexing a spectral distribution (or multispectral distribution) with a numeric (or a numeric sequence) returns the corresponding value(s). Indexing a spectral distribution (or multispectral distribution) with a slice returns the values for the corresponding wavelength indexes.
While it is tempting to think that the colour.SpectralDistribution
and colour.MultiSpectralDistributions
classes behave like Numpy’s
ndarray,
they do not entirely and some peculiarities exist that make them different.
An important difference lies in the behaviour with respect to getting and setting the values of the data.
Getting the value(s) for a single (or multiple wavelengths) is done by indexing
the colour.SpectralDistribution
(or
colour.MultiSpectralDistributions
) class with the a single numeric
or array of numeric wavelengths, e.g. sd[555.5]
or
sd[555.25, 555.25, 555.75]
.
However, if getting the values using a slice
class instance, e.g.
sd[0:3]
, the underlying discrete values for the indexes represented by the
slice
class instance are returned instead.
As shown in the previous section, getting the value of a wavelength is done as follows:
data = {
500: 0.0651,
520: 0.0705,
540: 0.0772,
560: 0.0870,
580: 0.1128,
600: 0.1360
}
sd = colour.SpectralDistribution(data)
sd[555]
0.083135180664062502,
Multiple wavelength values can be retrieved as follows:
sd[(555.0, 556.25, 557.5, 558.75, 560.0)]
array([ 0.08313518, 0.08395997, 0.08488108, 0.085897 , 0.087 ])
However, slices will return the values for the corresponding wavelength indexes:
sd[0:3]
array([ 0.0651, 0.0705, 0.0772])
sd[:]
array([ 0.0651, 0.0705, 0.0772, 0.087 , 0.1128, 0.136 ])
Note
Indexing a multispectral distribution is achieved similarly, it can however be sliced along multiple axes because the data is2dimensional, e.g. msds[0:3, 0:2].
A copy of the underlying colour.SpectralDistribution
and
colour.MultiSpectralDistributions
classes discretized data can be
accessed via the wavelengths
and values
properties. However, it cannot
be changed directly via the properties or slicing:
Attention
The data returned by the wavelengths
and values
properties is a
copy of the underlying colour.SpectralDistribution
and
colour.MultiSpectralDistributions
classes discretized data: It
can only be changed indirectly.
data = {
500: 0.0651,
520: 0.0705,
540: 0.0772,
560: 0.0870,
580: 0.1128,
600: 0.1360
}
sd = colour.SpectralDistribution(data)
# Note: The wavelength 500nm is at index 0.
sd.values[0] = 0
sd[500]
0.065100000000000019
Instead, the values can be set indirectly:
values = sd.values
values[0] = 0
sd.values = values
sd.values
array([ 0. , 0.0705, 0.0772, 0.087 , 0.1128, 0.136 ])
DomainRange Scales#
Note
This section contains important information.
Colour adopts 4 main input domains and output ranges:
Scalars usually in domainrange [0, 1] (or [0, 10] for Munsell Value).
Percentages usually in domainrange [0, 100].
Degrees usually in domainrange [0, 360].
Integers usually in domainrange [0, 2**n 1] where n is the bit depth.
It is error prone but it is also a direct consequence of the inconsistency of the colour science field itself. We have discussed at length about this and we were leaning toward normalisation of the whole API to domainrange [0, 1], we never committed for reasons highlighted by the following points:
Colour Scientist performing computations related to Munsell Renotation System would be very surprised if the output Munsell Value was in range [0, 1] or [0, 100].
A Visual Effect Industry artist would be astonished to find out that conversion from CIE XYZ to sRGB was yielding values in range [0, 100].
However benefits of having a consistent and predictable domainrange scale are numerous thus with Colour 0.3.12 we have introduced a mechanism to allow users to work within one of the two available domainrange scales.
Scale  Reference#
‘Reference’ is the default domainrange scale of Colour, objects adopt the implemented reference, i.e. paper, publication, etc.., domainrange scale.
The ‘Reference’ domainrange scale is inconsistent, e.g. colour appearance
models, spectral conversions are typically in domainrange [0, 100] while RGB
models will operate in domainrange [0, 1]. Some objects, e.g.
colour.colorimetry.lightness_Fairchild2011()
definition have mismatched
domainrange: input domain [0, 1] and output range [0, 100].
Scale  1#
‘1’ is a domainrange scale converting all the relevant objects from Colour public API to domainrange [0, 1]:
Scalars in domainrange [0, 10], e.g Munsell Value are scaled by 10.
Percentages in domainrange [0, 100] are scaled by 100.
Degrees in domainrange [0, 360] are scaled by 360.
Integers in domainrange [0, 2**n 1] where n is the bit depth are scaled by 2**n 1.
Dimensionless values are unaffected and are indicated with DN.
Unaffected values are unaffected and are indicated with UN.
Warning
The conversion to ‘1’ domainrange scale is a soft normalisation and
similarly to the ‘Reference’ domainrange scale it is normal to
encounter values exceeding 1, e.g. High Dynamic Range Imagery (HDRI) or
negative values, e.g. outofgamut RGB colourspace values. Some definitions
such as colour.models.eotf_ST2084()
which decodes absolute luminance
values are not affected by any domainrange scales and are indicated with
UN.
Understanding the DomainRange Scale of an Object#
Using colour.adaptation.chromatic_adaptation_CIE1994()
definition
docstring as an example, the Notes section features two tables.
The first table is for the domain, and lists the input arguments affected by the two domainrange scales and which normalisation they should adopt depending the domainrange scale in use:
Domain 
Scale  Reference 
Scale  1 


[0, 100] 
[0, 1] 

[0, 100] 
[0, 1] 
The second table is for the range and lists the return value of the definition:
Range 
Scale  Reference 
Scale  1 


[0, 100] 
[0, 1] 
Working with the DomainRange Scales#
The current domainrange scale is returned with the
colour.get_domain_range_scale()
definition:
import colour
colour.get_domain_range_scale()
u'reference'
Changing from the ‘Reference’ default domainrange scale to ‘1’ is done
with the colour.set_domain_range_scale()
definition:
XYZ_1 = [28.00, 21.26, 5.27]
xy_o1 = [0.4476, 0.4074]
xy_o2 = [0.3127, 0.3290]
Y_o = 20
E_o1 = 1000
E_o2 = 1000
colour.adaptation.chromatic_adaptation_CIE1994(XYZ_1, xy_o1, xy_o2, Y_o, E_o1, E_o2)
array([ 24.03379521, 21.15621214, 17.64301199])
colour.set_domain_range_scale('1')
XYZ_1 = [0.2800, 0.2126, 0.0527]
Y_o = 0.2
colour.adaptation.chromatic_adaptation_CIE1994(XYZ_1, xy_o1, xy_o2, Y_o, E_o1, E_o2)
array([ 0.24033795, 0.21156212, 0.17643012])
The output tristimulus values with the ‘1’ domainrange scale are equal to those from ‘Reference’ default domainrange scale divided by 100.
Passing incorrectly scaled values to the
colour.adaptation.chromatic_adaptation_CIE1994()
definition
would result in unexpected values and a warning in that case:
colour.set_domain_range_scale('Reference')
colour.adaptation.chromatic_adaptation_CIE1994(XYZ_1, xy_o1, xy_o2, Y_o, E_o1, E_o2)
File "<ipythoninput...>", line 4, in <module>
E_o2)
File "/colourscience/colour/colour/adaptation/cie1994.py", line 134, in chromatic_adaptation_CIE1994
warning(('"Y_o" luminance factor must be in [18, 100] domain, '
/colourscience/colour/colour/utilities/verbose.py:207: ColourWarning: "Y_o" luminance factor must be in [18, 100] domain, unpredictable results may occur!
warn(*args, **kwargs)
array([ 0.17171825, 0.13731098, 0.09972054])
Setting the ‘1’ domainrange scale has the following effect on the
colour.adaptation.chromatic_adaptation_CIE1994()
definition:
As it expects values in domain [0, 100], scaling occurs and the
relevant input values, i.e. the values listed in the domain table, XYZ_1
and Y_o
are converted from domain [0, 1] to domain [0, 100] by
colour.utilities.to_domain_100()
definition and conversely
return value XYZ_2
is converted from range [0, 100] to range [0, 1] by
colour.utilities.from_range_100()
definition.
A convenient alternative to the colour.set_domain_range_scale()
definition is the colour.domain_range_scale
context manager and
decorator. It temporarily overrides Colour domainrange scale with given
scale value:
with colour.domain_range_scale('1'):
colour.adaptation.chromatic_adaptation_CIE1994(XYZ_1, xy_o1, xy_o2, Y_o, E_o1, E_o2)
[ 0.24033795 0.21156212 0.17643012]
Multiprocessing on Windows with DomainRange Scales#
Windows does not have a fork system call, a consequence is that child processes do not necessarily inherit from changes made to global variables.
It has crucial consequences as Colour stores the current domainrange scale into a global variable.
The solution is to define an initialisation definition that defines the scale upon child processes spawning.
The colour.utilities.multiprocessing_pool
context manager conveniently
performs the required initialisation so that the domainrange scale is
propagated appropriately to child processes.
Safe Power and Division#
Colour default handling of fractional power and zerodivision occurring during practical applications is managed via varous definitions and context managers.
Safe Power#
NaNs generation occurs when a negative number \(a\) is raised to the
fractional power \(p\). This can be avoided using the
colour.algebra.spow()
definition that raises to the power as follows:
\(sign(a) * a^p\).
To the extent possible, the colour.algebra.spow()
definition has been
used throughout the codebase. The default behaviour is controlled with the
following definitions:
colour.algebra.set_spow_enabled()
colour.algebra.spow_enable()
(Context Manager & Decorator)
Safe Division#
NaNs and +/ infs generation occurs when a number \(a\) is divided 0. This
can be avoided using the colour.algebra.sdiv()
definition. It has been
used wherever deemed relevant in the codebase. The default behaviour is
controlled with the following definitions:
colour.algebra.sdiv_mode()
(Context Manager & Decorator)
The following modes are available:
Numpy
: The current Numpy zerodivision handling occurs.Ignore
: Zerodivision occurs silently.Warning
: Zerodivision occurs with a warning.Ignore Zero Conversion
: Zerodivision occurs silently and NaNs or +/ infs values are converted to zeros. Seenumpy.nan_to_num()
definition for more details.Warning Zero Conversion
: Zerodivision occurs with a warning and NaNs or +/ infs values are converted to zeros. Seenumpy.nan_to_num()
definition for more details.Ignore Limit Conversion
: Zerodivision occurs silently and NaNs or +/ infs values are converted to zeros or the largest +/ finite floating point values representable by the division resultnumpy.dtype
. Seenumpy.nan_to_num()
definition for more details.Warning Limit Conversion
: Zerodivision occurs with a warning and NaNs or +/ infs values are converted to zeros or the largest +/ finite floating point values representable by the division resultnumpy.dtype
.
colour.algebra.get_sdiv_mode()
'Ignore Zero Conversion'
colour.algebra.set_sdiv_mode("Numpy")
colour.UCS_to_uv([0, 0, 0])
/Users/kelsolaar/Documents/Development/colourscience/colour/colour/algebra/common.py:317: RuntimeWarning: invalid value encountered in true_divide
c = a / b
array([ nan, nan])
colour.algebra.set_sdiv_mode("Ignore Zero Conversion")
colour.UCS_to_uv([0, 0, 0])
array([ 0., 0.])