---
title: "Ray Tracing the Zonohedron Boundary and the 2-Transition Surface"
author: "Glenn Davis"
date: "`r Sys.Date()`"
output: 
  rmarkdown::html_vignette:
    toc: true
    toc_depth: 2
    number_sections: false
bibliography: bibliography.bib

# csl: iso690-numeric-brackets-cs.csl
csl: personal.csl
# csl: institute-of-mathematical-statistics.csl
# csl: transactions-on-mathematical-software.csl
vignette: >
  %\VignetteIndexEntry{Ray Tracing the Zonohedron Boundary and the 2-Transition Surface}
  %\VignetteEngine{knitr::rmarkdown}
---


```{css, echo=FALSE}
body {
  max-width: 870px;     /* make wider, default is 700px */
}
```


```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
old_opt = options( width=144 )
```

<br><br>

# Introduction

The focus of this vignette are the two
functions `raytrace()` and `raytrace2trans()`.
The former is for the boundary of the zonohedron 
and the latter is for the associated 2-transition surface.
We revisit the example at the end of section 6
in Scott Burns' paper @Burns2021,
which is also illustrated in the 1nm plot from Figure 8.
His example is from colorimetry, where the boundary of the zonohedron
is the set of optimal colors
and the 2-transition surface is the set of Schr&ouml;dinger colors
(both for Illuminant E).
The correspondence for the optimal colors was discovered by
Paul Centore, see @Centore2013.

Other featured functions are `invertboundary()`, `inside()` and `inside2trans()`.

```{r, echo=TRUE,  message=FALSE}
library(zonohedra)
```

<br><br>

# A Ray Tracing Example

In Burns' example, the base of the ray is the center of the zonohedron $Z$:
```{r, echo=TRUE,  message=FALSE}
matgen = colorimetry.genlist[[2]]   # the CIE 1931 CMFs at 1nm step
matgen = 100 * matgen / sum( matgen[2, ] )   # it is traditional to scale so the center has Y=50, recall we use Illuminant E
zono =  zonohedron( matgen )
base = getcenter(zono) ; base
```
The vector `base` corresponds to Burns' vector $XYZ_{\text{50%}}$.

The direction of the ray is given by spherical angles,
which define a unit vector `u`:
```{r, echo=TRUE,  message=FALSE}
theta = 1.478858 ; phi = 0.371322
u = c( sin(phi)*cos(theta), sin(phi)*sin(theta), cos(phi) ) ; u
```

Calculate the intersection of the ray with the boundary of $Z$.
```{r, echo=TRUE,  message=TRUE}
df_opt = raytrace( zono, base, u ) ; df_opt
xyz_opt = df_opt$point[1, ] ; xyz_opt
```
This matches Burns' value of $XYZ_{\text{LPsoln}}$.
From Figure 8 of @Burns2021 we see that this point
(and every point in the same parallelogram)
comes from a reflectance spectrum with 4 transitions.
This can be verified by inverting:
```{r, echo=TRUE,  message=TRUE}
invertboundary( zono, xyz_opt )$transitions
```


Now calculate the intersection of the ray with the 2-transition surface
associated with $Z$.
```{r, echo=TRUE,  message=TRUE}
df_2trans = raytrace2trans( zono, base, u ) ; df_2trans
xyz_2trans = df_2trans$point[1, ] ; xyz_2trans
```
This matches Burns' value of $XYZ_{\text{two-trans}}$ to 4 decimal places.
The transition wavelengths 629 and 575nm,
and the parallelogram coordinates 0.2246808 and 0.4459951
(these are the corresponding reflectances),
are clearly visible in Figure 8.

Now consider the distance between these 2 points
$XYZ_{\text{LPsoln}}$ and $XYZ_{\text{two-trans}}$.
The parameter `tmax` in both data frames is the parameter on the ray
where it intersects the boundary or the surface.
Since `u` is a unit vector, the difference between the parameters is this distance.
```{r, echo=TRUE,  message=FALSE}
df_opt$tmax - df_2trans$tmax
```
This matches Burns' value of $1.29 \times 10^{-3}$,
which is very tiny especially compared to the two $XYZ$s.

What is the maximum that this distance can be over the entire $\partial Z$ ?
To get a rough estimate, a search was made over the rays
passing though the centers of all the 21900 deficient parallelograms,
and with the same basepoint as before.
The largest distance over these rays was $2.47 \times 10^{-3}$.
This distance is for the parallelogram with generators corresponding
to 592 and 608 nm; the generating 'spectrum' has 8 transitions.
The actual maximum distance between the boundary of the color solid
and the 2-transition surface is not much larger than this sampling.
This confirms Burns' statement from @Burns2021 that the distance between
these surfaces has
"... no practical impact on typical colorimetric calculations".

If the zonohedron $Z$ is called the _Optimal Color Solid_ (OCS),
and the inside of the 2-transition surface is called the 
_Schr&ouml;dinger Color Solid_ (SCS),
we see that the OCS is obtained by adding a very thin "skin"
on some regions of the SCS.


<br><br>

# Inside or Outside ?

Consider the midpoint of $XYZ_{\text{LPsoln}}$ and $XYZ_{\text{two-trans}}$.
It lies on the same ray as these 2 points,
so it must be *inside* the zonohedron, but *outside* the 2-transition surface.
We can verify this easily:
```{r, echo=TRUE,  message=FALSE}
xyz_mid = (xyz_opt + xyz_2trans) / 2
inside( zono, xyz_mid )
inside2trans( zono, xyz_mid )
```



<br><br>

# References

<div id="refs"></div>




<br><br>




# Session Information

This document was prepared
`r format(Sys.Date(), "%a %b %d, %Y")`
with the following configuration:
<pre>
```{r, echo=FALSE, results='asis'}
options(old_opt)
sessionInfo()
```
</pre>
