# by.im

From spatstat v1.20-1
by Adrian Baddeley

##### 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 class '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

```
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 1.20-1, License: GPL (>= 2)*

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