An isotropic image gradient operator using a 3x3 window
The Sobel-Feldmanh operator is a discrete differentiation operator, deriving an approximation of the gradient of the intensity function. abrupt discontinuity in the gradient function represents edges, making this a common approach for edge detection. The Sobel-Feldman operator is based on convolving the image with a small, separable, and integer matrix in the horizontal and vertical directions. The operator uses two 3x3 kernels which are convolved with the original image to calculate approximations of the derivatives - one for horizontal changes, and one for vertical. Where x is defined here as increasing in the right-direction, and y as increasing in the down-direction. At each pixel in the raster, the resulting gradient can be combined to give the gradient intensity, using: SQRT( Gx^2 Gy^2 ). This can be expanded into the gradient direction using atan(Gx/Gy)
sobal(x, method = "intensity", ...)
Arguments
| x | A raster class object |
|---|---|
| method | Type of operator ("intensity", "direction", "edge") |
| ... | Additional arguments passed to raster::overlay or, if method="edge", raster::focal (if you want a file written to disk use filename = "" argument) |
Value
A raster class object or raster written to disk
References
Sobel, I., & G. Feldman, (1969) A 3x3 Isotropic Gradient Operator for Image Processing, presented at the Stanford Artificial Intelligence Project (SAIL).
Author
Jeffrey S. Evans <jeffrey_evans@tnc.org>
Examples
library(raster) r <- brick(system.file("external/rlogo.grd", package="raster")) s.int <- sobal(r[[1]]) s.dir <- sobal(r[[1]], method = "direction") s.edge <- sobal(r[[1]], method = "edge") opar <- par(no.readonly=TRUE) par(mfrow=c(2,2)) plot(r[[1]]) plot(s.int, main="intensity") plot(s.dir, main="direction") plot(s.edge, main="edge")
