# FunctorImageFilter¶

In image processing and remote sensing, it is very common to write custom pixel-based or neighborhood-based operations between one or several co-registered images. Starting OTB 7.0, there is now a unique filter otb::FunctorImageFilter that will handle most cases:

- Any number of input images, being either
`Image`

,`VectorImage`

or a mix of both, - An
`Image`

or`VectorImage`

output - Operation based on pixels, neighborhoods or a mix of both,
- Functor classes, function, lambdas.

With otb::FunctorImageFilter you only need to write the operation you want to perform, and the filter will take care of everything (including multi-threading and streaming).

## Quickstart¶

### Defining the operation to perform¶

The operation to perform can be defined as a free function:

```
double myFreeFunction(const double& in) {...}
```

It can also be defined as a functor (i.e. a class defining `operator()`

:

```
class MyFunctor
{
public:
double operator()(const double& in) {...}
};
```

It can also be defined as a lambda:

```
auto myLambda = [](const double& in) -> double {...}
```

### Creating a `FunctorImageFilter`

¶

Once the operation to perform has been implemented, it is very easy to get an instance of `FunctorImageFilter`

from it:

```
auto filterFromFreeFunction = NewFunctorFilter(myFreeFunction);
auto filterFromFunctor = NewFunctorFilter(MyFunctor());
auto filterFromLambda = NewFunctorFilter(myLambda);
```

And you can use it just like any other filter:

```
filterFromLambda->SetInput(upStreamFilter->GetOutput());
downstreamFilter->SetInput(filterFromLambda->GetOutput());
```

You can also directly define instances of `FunctorImageFilter`

with built-in math functions:

```
using CosType = double(double);
auto filterCos = NewFunctorFilter(static_cast<CosType*>(std::cos));
```

Note, the `static_cast`

trick, which allows to disambguiate between different prototypes of the `cos`

function.

## Automatic types deduction¶

You probably notice that, contrary to other filters in ITK and OTB, there is no
need to specify input and output image types. This is because
`FunctorImageFilter`

uses C++ metaprogramming to automatically derive the
input and output image types from the free function, functor or lambda, with
the following rules.

Let `R (T1 t1, T2 t2 ..., TN tn)`

be the signature of the free function, `operator()`

or lambda. Note that the filter conversely supports passing by value `TN tn`

or by const reference `const TN & tn`

.

First lets define basic types:

- Scalar type (double, float, unsigned int, short …)
`std::complex<T>`

with T a scalar type`itk::FixedArray<T,N>`

,`itk::RGBPixel<T>`

,`itk::RGBAPixel<T>`

with T a scalar type

### Automatic input type deduction¶

From the basic types, the following deduction rules apply:

- If
`TN`

is a basic type as defined above, the Nth input will be of type`otb::Image<TN>`

- If
`TN`

is of type`itk::VariableLengthVector<T>`

with T a basic type as defined above, the Nth input will be of type`otb::VectorImage<T>`

- If
`TN`

is of type`const itk::ConstNeighborhoodIterator<otb::Image<T>> &`

with T a basic type as defined above, the Nth input will be of type`otb::Image<TN>`

- If
`TN`

is of type`const itk::ConstNeighborhoodIterator<otb::VectorImage<T>> &`

with T a basic type as defined above, the Nth input will be of type`otb::VectorImage<TN>`

Note that this will work for any number of inputs.

### Automatic output type deduction¶

Rules for output type deduction are simpler:
- If `R`

is a basic type, output of the filter will be of type `otb::Image<R>`

- If `R`

is of type `itk::VariableLengthVector<T>`

with T a basic type as defined above, the output of the filter will be of type `otb::VectorImage<R>`

Note that if `R`

is of type `itk::VariableLengthVector<T>`

, you need extra steps so that the filter can allocate the correct number of output bands, as explained in NumberOfOutputBands section.

### Alternative prototype for performance¶

Automatic type deduction will also work with the following signature:
`void (R&, T1 t1, T2 t2 ..., TN tn)`

This will be more efficient when `R`

is of type `itk::VariableLengthVector<T>`

and should be preferred in this case.

### Automatic type deduction examples¶

Consider the following free function:

```
itk::VariableLengthVector<double> myFreeFunction(unsigned char a,
const std::complex<float>& b,
const itk::VariableLengthVector<short>& c,
const itk::ConstNeighborhoodIterator<otb::Image<double>>& d) {...}
```

When a `FunctorImageFilter`

is built from this function, the following types will be deduced:

- First input (corresponding to a) will be of type
`otb::Image<unsigned char>`

- Second input (corresponding to b) will be of type
`otb::Image<std::complex<float>>`

- Third input (corresponding to c) will be of type
`otb::VectorImage<short>`

- Fourth input (corresponding to d) will be of type
`otb::Image<double>`

- Output type will be of type
`otb::VectorImage<double>`

This is strictly equivalent to:

```
void myFreeFunction(itk::VariableLengthVector<double> & out ,
unsigned char a,
const std::complex<float> & b,
const itk::VariableLengthVector<short> &c,
const itk::ConstNeighborhoodIterator<otb::Image<double>> & d) {...}
```

Since the output type is of type `itk::VariableLengthVector<T>`

, the latter should be preferred.

## Using the filter¶

### Setting inputs¶

The Nth parameter can be set with the template `SetInput()`

method:

```
myFilter->SetInput<N>(imageN);
```

You can also set all inputs at once with the `SetInputs()`

method:

```
myFilter->SetInputs(image0,...,imageN);
```

If you only have one input, you can simply call:

```
myFilter->SetInput(image);
```

Of course, input types must match the types deducted from the operator(), free function or lambda!

### Accessing the function¶

If `FunctorImageFilter`

was built from a functor class, this class may have parameters that you wish to change or read.

You can call `GetFunctor()`

to access a const reference to the functor in order to read a parameter value:

```
auto a = myFilter->GetFunctor().GetParameterA();
```

If you wish to modify a parameter of the functor, you will have to call `GetModifiableFunctor()`

, which will return a non-const reference to the functor and ensure that the filter will be re-run when updated.

### Setting the neighborhood radius¶

If you have `itk::ConstNeighborhoodIterator<otb::Image<T>>`

or `itk::ConstNeighborhoodIterator<otb::VectorImage<T>>`

as input type, you can set the neighborhood radius when building the filter instance, with:

```
auto filterFromFunctor = NewFunctorFilter(MyFunctor,{{3,3}});
```

## Advanced use¶

### Number of output bands¶

If is of type `itk::VariableLengthVector<T>`

, then the functor class should provide an `OutputSize()`

method as follows.

If the number of output bands is fixed:

```
class MyFunctor {
public:
...
constexpr size_t OutputSize(...) const
{
// Return the number of output bands
return 3;
}
};
```

If the number of output bands depends on the number of bands in one or more input images:

```
class MyFunctor {
public:
...
size_t OutputSize(const std::array<size_t,N> & nbBands) const
{
// N Is the number of inputs
// nbBands is an array containing the number of bands for each input
...
return outputNumberOfBands;
}
};
```

In this case you can use the information provided by the `nbBands`

parameter
which contain the number of bands for each input, to derive and return the
output number of bands.

If you are using a lambda, a free function or an existing functor which does not
offer the `OutputSize()`

method, you can still use `FunctorImageFilter`

but
you need to provide the number of bands when constructing the filter instance:

```
// Specify that the lambda output has 3 bands
auto filterFromLambda = NewFunctorFilter(myLambda,3);
```