by.im
Apply Function to Image Broken Down by Factor
Splits a pixel image into sub-images and applies a function to each sub-image.
Usage
# S3 method for im
by(data, INDICES, FUN, ...)Arguments
- data
A pixel image (object of class
"im").- INDICES
Grouping variable. Either a tessellation (object of class
"tess") or a factor-valued pixel image.- FUN
Function to be applied to each sub-image of
data.- …
Extra arguments passed to
FUN.
Details
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.
Value
A list containing the results of each evaluation of FUN.
See Also
Examples
# 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))
# }