Take a sequence of colorSpec objects and compute their product. The product is associative. Only certain types of sequences are allowed, see Details.
Unnamed arguments are colorSpec objects,
and possibly a single character string, see Details.
Named arguments (if any) are passed to
To explain the allowable product sequences it is helpful to introduce some simple notation for the objects:
|notation|| colorSpec ||description of the object|
|L|| ||a light source|
|M|| ||a material|
|R_L|| ||a light responder (aka detector)|
|R_M|| ||a material responder|
It is also helpful to define a sequence of positive integers
to be conformable iff it has at most one value greater than 1.
a sequence of all 1s is conformable. A sequence of all q's is conformable.
c(1,1,4,1,1,4,1) are conformable,
c(1,1,4,1,3,4,1) is not.
There are 6 types of sequences for which the product is defined:
1. M_1 * M_2 * ... * M_m ↦ M'
The product of m materials is another material. Think of a stack of m transmitting filters effectively forming a new filter. If we think of each object as a matrix (with the spectra in the columns), then the product is element-by-element using R's
* - the Hadamard product.
The numbers of spectra in the terms must be conformable.
If some objects have 1 spectrum and all the others have q,
then the column-vector spectrums are repeated q times to form a
matrix with q columns.
If the numbers of spectra are not conformable,
it is an ERROR and the function returns
As an example, suppose M_1 has 1 spectrum and M_2 has q spectra, and m=2. Then the product is a material with q spectra. Think of an IR-blocking filter followed by the RGB filters in a 3-CCD camera.
2. L * M_1 * M_2 * ... * M_m ↦ L'
The product of a light source followed by m materials is a light source. Think of a light source followed by a stack of m transmitting filters, effectively forming a new light source. The numbers of spectra in the terms must be conformable as in sequence 1, and the matrices are multiplied element by element.
As an example, suppose L has 1 spectrum and M_1 has q spectra, and m=1. Then the product is a light source with q spectra. Think of a light source followed by a filter wheel with q filters.
3. M_1 * M_2 * ... * M_m * R_L ↦ R_L'
The product of m materials followed by a light responder, is a light responder. Think of a stack of m transmitting filters in front of a camera, effectively forming a new camera. The numbers of spectra in the terms must be conformable as in sequence 1, and the matrices are multiplied element by element.
As an example, suppose R_L has 1 spectrum and M_1 has q spectra, and m=1. Then the product is a light responder with q spectra. Think of a 3-CCD camera in which all 3 CCDs have exactly the same responsivity and so can be modeled with a single object R_L.
4. L * M_1 * M_2 * ... * M_m * R_L ↦ matrix
The product of a light source, followed by m materials, followed by a light responder, is a matrix! The numbers of spectra in the terms must splittable into a conformable left part (L' from sequence 2.) and a conformable right part (R_L' from sequence 3.). There is a row for each spectrum in L', and a column for each spectrum in R_L'. Suppose the element-by-element product of the left part is n×p and the element-by-element product of the right part is and n×q, where n is the number of wavelengths. Then the output matrix is the usual matrix product
of the transpose of the left part times and right part,
which is p×q.
As an example, think of a light source followed by a reflective color target with 24 patches followed by an RGB camera. The sequence of spectra is
which is splittable into
The product matrix is 24×3.
See the gallery vignette for a worked-out example.
Note that is OK for there to be no materials in this product; it is OK if m=0. See the blueflame vignette for a worked-out example.
5. L * M_1 * ... * • * ... * M_m * R_L ↦ R_M'
This is the strangest product. The bullet symbol • means that a variable material is inserted at that slot in the sequence (or light path). For each material spectrum inserted there is a response from R_L. Therefore the product of this sequence is a material responder R_M. Think of a light source L going through a transparent object • on a flatbed scanner and into a camera R_L. For more about the mathematics of this product, see the colorSpec-guide.pdf in the doc directory. These material responder spectra are the same as the effective spectral responsivities in Digital Color Management. The numbers of spectra in the terms must be conformable as in sequence 1.
In the function
product() the location of the • is marked
by any character string whatsoever - it's up to the user who might choose
something that describes the typical material.
For example one might choose:
scanner = product( A.1nm, 'photo', Flea2.RGB, wave='auto')
to model a scanner that is most commonly used to scan photographs. Other possible strings could be
See the gallery vignette for a worked-out example.
6. M_1 * M_2 * ... * M_m * R_M ↦ matrix
The product of m materials followed by a material responder, is a matrix ! The sequence of numbers of spectra must be splittable into left and right parts as in sequence 4, and the product matrix is formed the same way. One reason for computing this matrix in 2 steps is that one can
calibrate the material responder separately in a customizable way.
See the gallery vignette for a worked-out example with a flatbed scanner.
Note that sequences 4. and 6. are the only ones that
use the usual matrix product
wavelength can also be
In this case the intersection of all the wavelength ranges of the objects is computed.
If the intersection is empty, it is an ERROR and the function returns
The wavelength step
is taken to be the smallest over all the object wavelength sequences.
If the minimum
step.wl is less than 1 nanometer,
it is rounded off to the nearest power of 2 (e.g 1, 0.5, 0.25, ...).
Although the function signature shows the colorSpec objects, followed by
wavelength, followed by more arguments,
actually they can come in any order.
The unnamed arguments are taken to be colorSpec objects
and the named arguments are taken to be arguments for
product() returns a colorSpec object or a matrix, see Details.
In case of a colorSpec object, the
extradata is lost.
However, all terms in the product are saved in
One can use
str to inspect this attribute.
All terms are converted to
radiometric on-the-fly and the returned
colorSpec object is also radiometric.
In case of ERROR it returns
Edward J. Giorgianni and Thomas E. Madden. Digital Color Management: Encoding Solutions. 2nd Edition John Wiley. 2009. Figure 10.11a. page 141.
Wikipedia. Hadamard product (matrices). http://en.wikipedia.org/wiki/Hadamard_product_%28matrices%29
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# sequence 1. path = system.file( "extdata/objects/Midwest-SP700-2014.txt", package='colorSpec' ) blocker.IR = readSpectra( path ) product( blocker.IR, Hoya, wave='auto' ) # sequence 2. product( subset(solar.irradiance,1), atmosphere2003, blocker.IR, Hoya, wave='auto' ) # sequence 3. plumbicon = readSpectra( system.file( "extdata/cameras/plumbicon30mm.txt", package='colorSpec' ) ) product( blocker.IR, subset(Hoya,1:3), plumbicon, wave='auto' ) # sequence 4. product( D65.1nm, Flea2.RGB ) # a 1x3 matrix, no materials product( D65.1nm, neutralMaterial(0.01), Flea2.RGB, wave='auto' ) # a 1x3 matrix, 1 material path = system.file( "extdata/sources/Lumencor-SpectraX.txt", package='colorSpec' ) lumencor = readSpectra( path, wave=340:660 ) product( lumencor, Flea2.RGB, wave='auto' ) # a 7x3 matrix, no materials # sequence 5. # make an RGB scanner bluebalancer = subset(Hoya,'LB') # combine tungsten light source A.1nm with blue light-balance filter # use the string 'artwork' to mark the variable material slot scanner = product( A.1nm, bluebalancer, 'artwork', Flea2.RGB, wave='auto' ) # sequence 6. scanner = calibrate( scanner ) target = readSpectra( system.file( "extdata/targets/N130501.txt", package='colorSpec') ) product( target, scanner, wave='auto' ) # a 288x3 matrix
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