# 2D Data Objects¶

## astra_mex_data2d¶

astra_mex_data2d is used to manage 2D data objects. It is a wrapper around the MEX file astra_mex_data2d_c.

2D data objects come in two varieties: volume data and projection data (sinograms).

astra_mex_data2d contains the following commands.

- create
- get
- get_single
- set / store
- get_geometry
- change_geometry
- delete
- clear
- info

**create**

```
id = astra_mex_data2d('create', '-vol', vol_geom);
id = astra_mex_data2d('create', '-vol', vol_geom, initializer);
```

This creates an initialized 2D volume data object for the geometry vol_geom.

Initializer may be:

- a scalar: the object is initialized with this constant value.
- a matrix: the object is initialized with the contents of this matrix. The matrix must be of size (y,x) as defined in the volume geometry. It must be of class single, double or logical.

If an initializer is not present, the volume is initialized to zero.

```
id = astra_mex_data2d('create', '-sino', proj_geom);
id = astra_mex_data2d('create', '-sino', proj_geom, initializer);
```

This creates an initialized 2D projection data object for the geometry proj_geom.

Initializer may be:

- a scalar: the object is initialized with this constant value.
- a matrix: the object is initialized with the contents of this matrix. The matrix must be of size (angles,u), where u is the number of detector pixels as defined in the projection geometry. It must be of class single, double or logical.

If an initializer is not present, the volume is initialized to zero.

**get**

```
A = astra_mex_data2d('get', id);
```

This fetches the data object as a 2D matrix of class double.

**get_single**

```
A = astra_mex_data2d('get_single', id);
```

This fetches the data object as a 2D matrix of class single.

**set / store**

```
astra_mex_data2d('set', id, A)
astra_mex_data2d('store', id, A)
```

This stores the matrix A in the data object. The dimensions of A must be the same as when used as an initializer in astra_mex_data2d(‘create’).

Set and store are synonyms.

**get_geometry**

```
geom = astra_mex_data2d('get_geometry', id);
```

This gets the (volume or projection) geometry attached to the object.

NB: This is not fully implemented yet and the return value may not accurately represent the geometry.

**change_geometry**

```
astra_mex_data2d('change_geometry', id, geom);
```

This changes the (volume or projection) geometry attached to the object. It may not change the actual dimensions of the data object. This can be used to change the pixel dimensions or projection angles, for example.

**delete**

```
astra_mex_data2d('delete', id);
```

Free the memory of a data object.

**clear**

```
astra_mex_data2d('clear');
```

Free all data objects.

**info**

```
astra_mex_data2d('info')
```

Print basic information about all allocated data objects.