---
title: "A (kind of) plasma effect in R"
author: "Stéphane Laurent"
date: '2023-02-10'
tags: R, graphics
rbloggers: yes
output:
  md_document:
    variant: markdown
    preserve_yaml: true
  html_document:
    highlight: kate
    keep_md: no
highlighter: pandoc-solarized
---

I found such an algorithm on Paul Bourke's website:

-   take a random matrix $M$ of size $n \times n$ (we'll take $n=400$),
    real or complex;

-   compute the discrete Fourier transform of $M$, this gives a complex
    matrix $FT$ of size $n \times n$;

-   for each pair $(i,j)$ of indices, multiply the entry $FT_{ij}$ of
    $FT$ by\
    $$
    \exp\Bigl(-\frac{{(i/n-0.5)}^2 + {(j/n-0.5)}^2}{0.025^2} \Bigr);
    $$

-   finally, take the inverse discrete Fourier transform of the obtained
    matrix, and map the resulting matrix to an image by associating a
    color to each complex number.

Here is some code producing the above algorithm:

``` r
library(cooltools)  # for the dft() function (discrete Fourier transform)
library(RcppColors) # for the colorMap1() function

fplasma1 <- function(n = 400L, gaussianMean = -50, gaussianSD = 5) {
  M <- matrix(
    rnorm(n*n, gaussianMean, gaussianSD), 
    nrow = n, ncol = n
  )
  FT <- dft(M)
  for(i in seq(n)) {
    for(j in seq(n)) {
      FT[i, j] <- FT[i, j] * 
        exp(-((i/n - 0.5)^2 + (j/n - 0.5)^2) / 0.025^2)
    }
  }
  IFT <- dft(FT, inverse = TRUE)
  colorMap1(IFT, reverse = c(FALSE, FALSE, TRUE))
}
```

Let's see a first image:

``` r
img <- fplasma1()
opar <- par(mar = c(0, 0, 0, 0))
plot(
  NULL, xlim = c(0, 1), ylim = c(0, 1), asp = 1, 
  xlab = NA, ylab = NA, axes = FALSE, xaxs = "i", yaxs = "i"
)
rasterImage(img, 0, 0, 1, 1)
```

`<img src="figures/plasmaFourier_fig1-1.png" width="50%" style="display: block; margin: auto;" />`{=html}

``` r
par(opar)
```

And more images:

`<img src="figures/plasmaFourier_fourfigs-1.png" width="50%" style="display: block; margin: auto;" />`{=html}

You can play with the parameters to obtain something different.

Below I take the first image and I alter the colors by exchanging the
green part with the blue part and then by darkening:

``` r
library(colorspace) # for the darken() function
alterColor <- function(col) {
  RGB <- col2rgb(col)
  darken(
    rgb(RGB[1, ], RGB[3, ], RGB[2, ], maxColorValue = 255),
    amount = 0.5
  )
}

img      <- alterColor(img)
dim(img) <- c(400L, 400L)
```

`<img src="figures/plasmaFourier_camouflage-1.png" width="50%" style="display: block; margin: auto;" />`{=html}

Looks like a camouflage.

Note that the images are doubly periodic, so you can map them to a
torus.

Now let's do an animation. The `fplasma2` function below does the same
thing as `fplasma1` after adding a number to the matrix $M$, which will
range from $-1$ to $1$.

``` r
fplasma2 <- function(M, t) {
  M <- M + sinpi(t / 64) # t will run from 1 to 128
  FT <- dft(M)
  n <- nrow(M)
  for(i in seq(n)) {
    for(j in seq(n)) {
      FT[i, j] <- FT[i, j] * 
        exp(-((i/n - 0.5)^2 + (j/n - 0.5)^2) / 0.025^2)
    }
  }
  IFT <- dft(FT, inverse = TRUE)
  colorMap1(IFT, reverse = c(FALSE, FALSE, TRUE))
}
```

Here is how to use this function to make an animation:

``` r
n <- 400L
M <- matrix(rnorm(n*n, -50, 5), nrow = n, ncol = n)

for(t in 1:128) {
  img <- fplasma2(M, t)
  fl <- sprintf("img%03d.png", t)
  png(file = fl, width = 400, height = 400)
  par(mar = c(0, 0, 0, 0))
  plot(
    NULL, xlim = c(0, 1), ylim = c(0, 1), asp = 1,
    xlab = NA, ylab = NA, axes = FALSE, xaxs = "i", yaxs = "i"
  )
  rasterImage(img, 0, 0, 1, 1)
  dev.off()
}

library(gifski)
pngFiles <- Sys.glob("img*.png")
gifski(
  png_files = pngFiles,
  gif_file  = "plasmaFourier_anim1.gif",
  width = 400, height = 400,
  delay  = 1/10
)
file.remove(pngFiles)
```

![](./figures/plasmaFourier_anim1.gif)

Observe the black and blue background: it does not move. If instead of
adding a number in the interval $[-1, 1]$, we add a number in the
complex interval $[-i, i]$, then we observe the opposite behavior:

``` r
fplasma3 <- function(M, t) {
  M <- M + 1i * sinpi(t / 64) # t will run from 1 to 128
  FT <- dft(M)
  n <- nrow(M)
  for(i in seq(n)) {
    for(j in seq(n)) {
      FT[i, j] <- FT[i, j] * 
        exp(-((i/n - 0.5)^2 + (j/n - 0.5)^2) / 0.025^2)
    }
  }
  IFT <- dft(FT, inverse = TRUE)
  colorMap1(IFT, reverse = c(FALSE, FALSE, TRUE))
}
```

![](./figures/plasmaFourier_anim2.gif)