# 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))
# }
```

*Documentation reproduced from package spatstat, version 1.61-0, License: GPL (>= 2)*