using terra::focal() with even sided weight matrix

Question

i want to use the terra::focal() with an even sided matrix (e.g. to apply "Roberts cross" edge detection; https://en.wikipedia.org/wiki/Roberts_cross).

However, focal() only accepts windows with odd sides. On the other hand, the focal3D() documentation states "w =window. [...] If you desire to use even sides, you can use an array, and pad the values with rows and/or columns that contain only NAs."

Applying this "trick" for 2D focal does not work:

r <- rast(matrix(rnorm(1000),ncol=100,nrow=100))
robx <- matrix(c(NA,1,0,NA,0,1),nrow=3)
plot(focal(r,w=robx,fun="sum")

while (of course), a 3 by 3 matrix without NAs works:

sobx <- matrix(c(-1,-2,-1,0,0,0,1,2,1) / 4, nrow=3)
plot(focal(r,w=sobx,fun="sum")

Are there any known workarounds? How can i get focal() to accept even sided windows (or at least trick it)?

Cheers.

Allan Cameron · Accepted Answer · 2023-10-05 14:24:08Z

2

The matrix robx in your question doesn't meet the requirements described in the instructions; it is a 3 x 2 matrix.

matrix(c(NA, 1, 0, NA, 0, 1), nrow = 3)
#>      [,1] [,2]
#> [1,]   NA   NA
#> [2,]    1    0
#> [3,]    0    1

Obviously, one of its sides has an even number of entries, so you get the w must be odd sized error.

If you give a 3x3 matrix where the top row and last column are NA, then you get the desired result:

library(terra)
#> terra 1.7.3

r <- rast(matrix(rnorm(1000), ncol = 100, nrow = 100))

robx <- matrix(c(NA, 1, 0, NA, 0,1, NA, NA, NA), nrow = 3)

robx
#>      [,1] [,2] [,3]
#> [1,]   NA   NA   NA
#> [2,]    1    0   NA
#> [3,]    0    1   NA

No we can run focal without issues:

plot(focal(r, w = robx, fun = "sum"))

^{Created on 2023-10-05 with reprex v2.0.2}

edited Oct 5, 2023 at 14:24

answered Oct 5, 2023 at 14:18

Allan Cameron

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3 Comments

JFV Over a year ago

Wow. How stupid i am. Time for a break. Thank you, Allan!

Allan Cameron Over a year ago

@JFV not at all. It's easy to overlook such things - sometimes it just takes a fresh pair of eyes. Since your question was reproducible and easy to understand, there will be several fresh pairs of eyes right here. Nice first question - welcome to Stack Overflow!

JFV Over a year ago

Well, i would have preferred a first question that is not purely based on my own lack of attention. At least someone trying to do this, who didn't see the info in the focal3D() can gain something from it. Thank you once again.

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