scatter simulation/estimation recommendations for downsampling etc

[KrisThielemans]

It looks like our default settings could lead to suboptimal scatter estimates (definitely simulations). I haven't checked the documentation yet.

A potential issue is that, as we use ray-tracing for computing the line integrals (through both emission and attenuation), the image resolution needs to match (downsampled) detector size. This seems currently not implemented by default. Some pointers

scatter estimation

STIR/src/scatter_buildblock/ScatterEstimation.cxx

Lines 587 to 588 in 8ced2d7

	if (this->downsample_scanner_bool)
	this->scatter_simulation_sptr->downsample_scanner();

STIR/src/scatter_buildblock/ScatterEstimation.cxx

Lines 558 to 563 in 8ced2d7

	if (this->override_density_image)
	{
	info("ScatterEstimation: Over-riding attenuation image! (The file and settings set in the simulation par file are "
	"discarded)");
	this->scatter_simulation_sptr->set_density_image_sptr(this->atten_image_sptr);
	}

scatter simulation

STIR/src/scatter_buildblock/ScatterSimulation.cxx

Lines 320 to 326 in 8ced2d7

	if (downsample_scanner_bool)
	{
	if (this->_already_set_up)
	error("ScatterSimulation: set_up() called twice. This is currently not supported.");
	downsample_scanner();
	}

STIR/src/scatter_buildblock/ScatterSimulation.cxx

Lines 328 to 333 in 8ced2d7

	if (is_null_ptr(density_image_for_scatter_points_sptr))
	{
	if (this->_already_set_up)
	error("ScatterSimulation: set_up() called twice. This is currently not supported.");
	downsample_density_image_for_scatter_points(zoom_xy, zoom_z, zoom_size_xy, zoom_size_z);
	}

We do have this function which tries to set voxel sizes appropriately, but it seems it isn't called anywhere

STIR/src/scatter_buildblock/ScatterSimulation.cxx

Line 920 in 8ced2d7

ScatterSimulation::downsample_images_to_scanner_size()