# Scattering from Arbitrary Structures with DDA¶

The discrete dipole approximation (DDA) lets us calculate scattering
from any arbitrary object by representing it as a closely packed array
of point dipoles. In HoloPy you can make use of the DDA by specifying
a general `Scatterer`

with an indicator function (or set of
functions for a composite scatterer containing multiple media).

HoloPy uses ADDA to do the actual DDA calculations, so you will need to install ADDA and be able to run:

```
adda
```

at a terminal for HoloPy DDA calculations to succeed. To install ADDA, first download or clone the code from GitHub. In a terminal window, go to the directory ’adda/src’ and compile using one of three options:

```
make seq
```

or:

```
make
```

or:

```
make OpenCL
```

`make seq`

will not take advantage of any parallel processing of the cores
on your computer. `make`

uses mpi for parallel processing. `make OpenCL`

uses
OpenCL for parallel processing. If the make does not work due to missing packages,
you will have to download the specified packages and install them.

Next, you must modify your path in your .bashrc or /bash_profile (for mac). Add the line:

```
export PATH=$PATH:userpath/adda/src/seq
```

or:

```
export PATH=$PATH:userpath/adda/src/mpi
```

or:

```
export PATH=$PATH:userpath/adda/src/OpenCL
```

where you should use the path that matches the make you chose above.

A lot of the code associated with DDA is fairly new so be careful; there are probably bugs. If you find any, please report them.

## Defining the geometry of the scatterer¶

To calculate the scattering pattern for an arbitrary object, you first need an indicator function which outputs ‘True’ if a test coordinate lies within your scatterer, and ‘False’ if it doesn’t. The indicator function is an argument of the constructor of your scatterer.

For example, if you wanted to define a dumbbell consisting of the union of two overlapping spheres you could do so like this:

```
import holopy as hp
from holopy.scattering import Scatterer, Sphere, calc_holo
import numpy as np
s1 = Sphere(r = .5, center = (0, -.4, 0))
s2 = Sphere(r = .5, center = (0, .4, 0))
detector = hp.detector_grid(100, .1)
dumbbell = Scatterer(lambda point: np.logical_or(s1.contains(point), s2.contains(point)),
1.59, (5, 5, 5))
holo = calc_holo(detector, dumbbell, medium_index=1.33, illum_wavelen=.66, illum_polarization=(1, 0))
```

Here we take advantage of the fact that Spheres can tell us if a point
lies inside them. We use `s1`

and `s2`

as purely geometrical
constructs, so we do not give them indices of refraction, instead
specifying n when defining `dumbbell`

.

HoloPy contains convenient wrappers for many built-in ADDA constructions.
The dumbbell defined explicitly above could also have been defined with the HoloPy `Bisphere`

class instead.
Similar classes exist to define an `Ellipsoid`

, `Cylinder`

, or `Capsule`

.

## Mutiple Materials: A Janus Sphere¶

You can also provide a set of indicators and indices to define a scatterer containing multiple materials. As an example, lets look at a janus sphere consisting of a plastic sphere with a high index coating on the top half:

```
from holopy.scattering.scatterer import Indicators
import numpy as np
s1 = Sphere(r = .5, center = (0, 0, 0))
s2 = Sphere(r = .51, center = (0, 0, 0))
def cap(point):
return(np.logical_and(np.logical_and(point[...,2] > 0, s2.contains(point)),
np.logical_not(s1.contains(point))))
indicators = Indicators([s1.contains, cap],
[[-.51, .51], [-.51, .51], [-.51, .51]])
janus = Scatterer(indicators, (1.34, 2.0), (5, 5, 5))
holo = calc_holo(detector, dumbbell, medium_index=1.33, illum_wavelen=.66, illum_polarization=(1, 0))
```

We had to manually set up the bounds of the indicator functions here because the automatic bounds determination routine gets confused by the cap that does not contain the origin.

We also provide a `JanusSphere`

scatterer which is very
similar to the scatterer defined above, but can also take a rotation
angle to specify other orientations:

```
from holopy.scattering import JanusSphere
janus = JanusSphere(n = [1.34, 2.0], r = [.5, .51], rotation = (-np.pi/2, 0),
center = (5, 5, 5))
```