# 2D Projectors¶

Projectors determine the (implicit) weight matrix of the geometries. All CPU reconstruction algorithms require a projector.

Each projector is only suitable for specific projection geometries.

## Creation¶

id = astra_create_projector(...);


Create a projector object. Projectors determine the (implicit) weight matrix of the geometries. All CPU reconstruction algorithms require a projector.

This script is a wrapper around astra_mex_projector(‘create’, ...) with a more convenient interface. See below for specifics.

The allocated object must be freed after use with a call to

astra_mex_projector('delete', id)


## Parallel beam¶

The projectors in this section can be used with the parallel projection geometry.

line

proj_id = astra_create_projector('line', proj_geom, vol_geom);


The weight of a ray/pixel pair is given by the length of the intersection of the pixel and the ray, considered as a zero-thickness line.

strip

proj_id = astra_create_projector('strip', proj_geom, vol_geom);


The weight of a ray/pixel pair is given by the area of the intersection of the pixel and the ray, considered as a strip with the same width as a detector pixel.

linear

proj_id = astra_create_projector('linear', proj_geom, vol_geom);


A ray is traced through successive columns or rows (depending on which are most orthogonal to the ray). The contribution of this column/row to this ray is then given by linearly interpolating between the two nearest volume pixels of the intersection of the ray and the column/row.

This is also known as the Joseph kernel, or a slice-interpolated kernel.

## Fan beam¶

line_fanflat

proj_id = astra_create_projector('line_fanflat', proj_geom, vol_geom);


The weight of a ray/pixel pair is given by the length of the intersection of the pixel and the ray, considered as a zero-thickness line. This projector can be used with the fanflat and fanflat_vec geometries.

strip_fanflat

proj_id = astra_create_projector('strip_fanflat', proj_geom, vol_geom);


The weight of a ray/pixel pair is given by the area of the intersection of the pixel and the ray. The ray is considered as a 2D cone from the source to the full width of the detector pixel. The projector can only be used with the fanflat geometry.

NB: This mathematical model does not properly take into account the fan beam magnification effect.

## Miscellaneous¶

sparse_matrix

proj_id = astra_create_projector('sparse_matrix', proj_geom, vol_geom);


This projector uses a sparse matrix projection geometry. See the documentation for that geometry for details.

cuda

proj_id = astra_create_projector('cuda', proj_geom, vol_geom);


This projector does not directly specify a weight matrix, but instead is intended to let algorithms use GPU/CUDA code. It can be used with parallel, fanflat and fanflat_vec projection geometries.

NB: This functionality has not yet been implemented everywhere. This projector is currently only usable in astra_create_sino and astra_create_backprojection.

## astra_mex_projector¶

astra_mex_projector is used to manage projector objects.

It is a wrapper around the MEX file astra_mex_projector_c.

astra_mex_projector contains the following commands.

• create
• matrix
• volume_geometry
• projection_geometry
• is_cuda
• delete
• clear
• info

create

id = astra_mex_projector('create', cfg);


Create a projector from a config object. This is called internally by the script [[astra_create_projector]], which is the recommended way to create most projectors.

matrix

matrix_id = astra_mex_projector('matrix', projector_id);


Create an explicit sparse matrix for the weight matrix encoded by this projector.

This is only implemented for 2D CPU projectors.

The returned matrix_id can be further manipulated with [astra_mex_matrix]. In particular, it can be retrieved as a Matlab sparse matrix with

S = astra_mex_matrix('get', matrix_id);


It has to be freed after use with

astra_mex_matrix('delete', matrix_id);


NB: Such a matrix can be very large for large geometries.

volume_geometry

vol_geom = astra_mex_projector('volume_geometry', id);


Get the volume geometry attached to the given projector object.

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

projection_geometry

proj_geom = astra_mex_projector('projection_geometry', id);


Get the projection geometry attached to the given projector object.

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

is_cuda

c = astra_mex_projector('is_cuda', id);


Return if the the projector is a CUDA projector.

delete

astra_mex_projector('delete', id)
astra_mex_projector('delete', id1, id2, ...)


Free a single or multiple projector(s).

clear

astra_mex_projector('clear')


Free all projectors.

info

astra_mex_projector('info')


Print basic information about all allocated projector objects.