Splits a pixel image into sub-images and applies a function to each sub-image.
# S3 method for im
by(data, INDICES, FUN, ...)
A pixel image (object of class "im"
).
Grouping variable.
Either a tessellation (object of class "tess"
) or
a factor-valued pixel image.
Function to be applied to each sub-image of data
.
Extra arguments passed to FUN
.
A list containing the results of each evaluation of FUN
.
This is a method for the generic function by
for
pixel images (class "im"
).
The pixel image data
is first divided into sub-images according
to INDICES
. Then the function FUN
is applied to each subset.
The results of each computation are returned in a list.
The grouping variable INDICES
may be either
a tessellation (object of class "tess"
). Each tile of
the tessellation delineates a subset of the spatial domain.
a pixel image (object of class "im"
) with factor
values. The levels of the factor determine subsets of the spatial
domain.
# NOT RUN {
W <- square(1)
X <- as.im(function(x,y){sqrt(x^2+y^2)}, W)
Y <- dirichlet(runifpoint(12, W))
# mean pixel value in each subset
unlist(by(X, Y, mean))
# trimmed mean
unlist(by(X, Y, mean, trim=0.05))
# }
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