# The plasma effect: correction

[This article was first published on

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There’s a kind of error in the previous post. In this formula: \[ \exp\Bigl(-\frac{{(i/n-0.5)}^2 + {(j/n-0.5)}^2}{0.025^2} \Bigr), \] the indices \(i\) and \(j\) should range from \(0\) to \(n\), and \(n\) must be even in order to get something centered. See the corrected code for details:

fplasma4 <- function(n = 400L, gaussianMean = -50, gaussianSD = 5) { n <- n + 1L M <- matrix( rnorm(n*n, gaussianMean, gaussianSD), nrow = n, ncol = n ) FT <- dft(M) n <- n - 1L for(i in seq(n+1L)) { for(j in seq(n+1L)) { FT[i, j] <- FT[i, j] * exp(-(((i-1L)/n - 0.5)^2 + ((j-1L)/n - 0.5)^2) / 0.025^2) } } IFT <- dft(FT, inverse = TRUE) colorMap1(IFT, reverse = c(FALSE, FALSE, TRUE)) }

The funny point is that the result is more interesting with the “error”. But it is not bad without it:

To

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