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.