xmipp3.protocols.protocol_reconstruct_fourier module
- class xmipp3.protocols.protocol_reconstruct_fourier.XmippProtReconstructFourier(**kwargs)[source]
Bases:
ProtReconstruct3DReconstruct a volume using Xmipp_reconstruct_fourier from a given set of particles. The alignment parameters will be converted to a Xmipp xmd file and used as direction projections to reconstruct.
AI Generated
## Overview
The Reconstruct Fourier protocol reconstructs a 3D volume from a set of particles with known projection-alignment parameters.
In single-particle cryo-EM, each particle image is interpreted as a 2D projection of the 3D structure from a particular direction. Once particles have projection angles and shifts, those images can be inserted into Fourier space and combined to produce a 3D map. This protocol performs that reconstruction using Xmipp Fourier reconstruction programs.
The protocol can optionally correct the CTF before reconstruction, impose symmetry, limit the maximum resolution used in Fourier space, and generate independent half-map reconstructions for resolution assessment.
The main output is a reconstructed volume. If half maps are requested, the output volume also keeps the two half-map file names associated with it.
## Inputs and General Workflow
The main input is a set of particles with projection-alignment information.
The protocol first converts the input particle set into Xmipp metadata format. If CTF information is available and CTF correction is requested, the particles are corrected by Wiener filtering before reconstruction. Otherwise, the original particle metadata are used directly.
If the Use halves option is enabled, the corrected particle metadata are split into two random subsets. Each subset is reconstructed independently, and the two resulting half maps are averaged to produce the final output volume.
If halves are not requested, all particles are reconstructed together into a single output volume.
Finally, the reconstructed volume is registered in Scipion with the sampling rate of the input particles.
## Input Particles
The Input particles parameter should point to a SetOfParticles with 3D projection alignment.
This is essential. The protocol does not determine particle orientations from scratch. It assumes that each particle already has projection angles and shifts from a previous refinement, angular assignment, classification, or imported alignment.
If the angular assignments are poor, the reconstruction will also be poor. The output volume should therefore be interpreted in relation to the quality of the input particle alignment.
The input particles should also have a consistent box size, sampling rate, and contrast convention.
## Symmetry Group
The Symmetry group parameter defines the symmetry imposed during reconstruction.
For asymmetric particles, use c1. If the particle has known point-group symmetry, the corresponding Xmipp symmetry group can be specified.
Correct symmetry can improve the reconstruction by averaging equivalent views and increasing signal. However, incorrect symmetry can introduce artificial density, blur asymmetric features, or obscure real biological differences.
Users should impose symmetry only when it is justified by prior structural or biological knowledge.
## Maximum Resolution
The Maximum resolution parameter limits the highest-resolution information used during Fourier reconstruction.
The value is given in angstroms. If it is set to -1, the protocol uses the Nyquist limit.
Limiting the maximum resolution can be useful when the input particles or angular assignments are not reliable at high frequency. It can reduce the influence of high-frequency noise during reconstruction.
Internally, the angstrom value is converted to a digital frequency using the particle sampling rate. The reconstruction program then uses this value as the maximum Fourier-space resolution.
## CTF Correction
The Correct CTF option applies Wiener-filter CTF correction to the particles before reconstruction, when CTF metadata are available.
CTF correction compensates for the contrast transfer effects introduced by the microscope. This can make the reconstructed map more physically meaningful, especially when particles come from different defocus values.
If the particles already have CTF information, the protocol writes a corrected stack and metadata file before reconstruction. If the particles are marked as phase-flipped, this information is passed to the CTF-correction step.
If CTF information is not present, the protocol simply reconstructs from the input particle metadata without CTF correction.
## Correct CTF Envelope
The Correct CTF envelope option is available when CTF correction is enabled.
The CTF envelope models additional attenuation of signal at higher spatial frequencies. Correcting it may be useful when the envelope has been estimated reliably.
This option should be used with caution. If the envelope estimate is poor, correcting it may amplify noise or introduce artifacts.
Most users should enable it only when they understand how the envelope was estimated and why it is appropriate for the dataset.
## Use Halves
The Use halves option creates two independent reconstructions from two random subsets of the input particles.
This is useful for resolution estimation and validation. Half maps are commonly used to compute FSC curves and to assess reproducibility of structural signal.
When this option is enabled, the protocol:
splits the particle metadata into two subsets;
reconstructs half map 1;
reconstructs half map 2;
averages the two half maps to create the final output volume;
stores the half-map file names in the output volume metadata.
The final averaged map can be used for visualization or downstream processing, while the half maps can be used for validation.
## Padding Factor
The Padding factor parameters control padding of the input projections and the reconstructed volume during Fourier reconstruction.
There are two values:
Projection padding;
Volume padding.
Padding can improve interpolation accuracy in Fourier space, but it increases memory use and computation time. Larger padding values may therefore be more accurate but slower and more demanding.
The default values are a practical compromise for many datasets. Advanced users may adjust them when reconstruction accuracy or performance needs to be tuned.
## Approximative Version
The Approximative version option enables a faster approximation of the Fourier reconstruction algorithm.
When enabled, reconstruction is faster but may be slightly less precise than the full version.
This option is useful for routine reconstruction or exploratory workflows where speed is important. If maximum numerical precision is required, advanced users may disable the approximation.
The approximative version is not compatible with the legacy CPU implementation.
## Legacy Version
The Legacy version option uses the original CPU implementation of the Fourier reconstruction algorithm.
This option is provided mainly for backward compatibility. In routine use, it should usually not be necessary.
The legacy version cannot be used with GPU execution and is not compatible with the approximative version.
## Extra Parameters
The Extra parameters field allows advanced users to pass additional options to the underlying Xmipp Fourier reconstruction program.
This can be useful for specialized workflows that require options not exposed directly in the graphical form.
Most users should leave this field empty. Incorrect extra parameters may cause the reconstruction to fail or produce unexpected results.
## GPU and CPU Execution
The protocol supports GPU and CPU execution.
GPU execution is enabled by default and uses the Xmipp CUDA Fourier reconstruction program when available. GPU execution is usually faster and is recommended for large datasets.
If GPU execution is requested but the required Xmipp CUDA programs are not available, the protocol reports a validation error.
The CPU version has limitations. In particular, the non-GPU version can use only a single thread; MPI should be used instead for CPU parallelism. The legacy version is CPU-only.
## Output Volume
The main output is outputVolume.
This volume is reconstructed from the input particles and registered with the same sampling rate as the input particle set.
If Use halves is disabled, the output volume is reconstructed directly from all particles.
If Use halves is enabled, the output volume is the average of the two independent half-map reconstructions, and the half-map file names are stored with the output volume.
The output volume can be used for visualization, post-processing, FSC calculation, refinement assessment, or as input to other 3D analysis protocols.
## Interpreting the Reconstruction
The reconstructed volume reflects the input particles and their assigned orientations.
Good input alignments, appropriate CTF handling, and correct symmetry usually lead to a more interpretable map. Poor alignments, incorrect CTF metadata, wrong symmetry, or inconsistent particle populations can produce blurred or distorted density.
If half maps are generated, they should be used for validation. Agreement between half maps provides evidence that the reconstructed features are supported reproducibly by independent subsets of particles.
## Practical Recommendations
Use this protocol after particles have reliable 3D projection alignment.
Enable CTF correction when reliable CTF metadata are available and the downstream reconstruction strategy requires corrected particles.
Use half maps when you plan to estimate resolution or validate the map with FSC.
Use symmetry only when it is biologically justified.
Keep the default padding values unless memory use, speed, or reconstruction accuracy requires adjustment.
Use GPU execution when available. It is usually the practical choice for large particle sets.
Inspect the output volume and, when generated, compare the two half maps. Poor agreement between half maps may indicate overfitting, bad alignments, heterogeneity, or insufficient particle quality.
## Final Perspective
Reconstruct Fourier is a core 3D reconstruction protocol. It converts a set of projection-aligned particles into a 3D map using Fourier-space reconstruction.
For biological users, the key point is that the protocol does not solve the orientation problem; it uses orientations that already exist in the input particles. Therefore, the quality of the reconstructed volume depends strongly on the quality of the previous alignment or refinement step.
Used with reliable particle alignments, appropriate CTF correction, and well-chosen symmetry, the protocol produces maps and half maps that can support subsequent validation, post-processing, interpretation, and refinement.