xmipp3.protocols.protocol_flexalign module
- class xmipp3.protocols.protocol_flexalign.XmippProtFlexAlign(**args)[source]
Bases:
ProtAlignMoviesWrapper protocol for Xmipp Movie Alignment using cross-correlation methods. It aligns movie frames to produce beam-induced motion corrected micrographs.
AI Generated
## Overview
The FlexAlign protocol aligns cryo-EM movie frames to correct for beam-induced motion and produce motion-corrected micrographs. During exposure, the electron beam causes the sample, ice, and support film to move. If this motion is not corrected, high-resolution information is blurred when the frames are summed.
FlexAlign estimates the shifts between movie frames using cross-correlation methods. It can perform both global movie alignment and local alignment, where different regions of the image are allowed to move slightly differently. This local correction is especially important for modern direct-electron detector data, where beam-induced motion may vary across the field of view.
The main outputs are aligned movies, averaged micrographs, motion-shift plots, and, optionally, power spectral density diagnostics before and after alignment. These outputs help the user assess whether the movie alignment has improved the data and whether the corrected micrographs are suitable for later CTF estimation, particle picking, and reconstruction.
## Inputs and General Workflow
The protocol takes a set of input movies. Each movie is processed independently. For each movie, FlexAlign reads the selected frame range, estimates frame shifts, optionally estimates local motion, applies the correction, and produces the requested output objects.
Depending on the selected options, the protocol may save:
an aligned average micrograph;
an aligned movie;
estimated frame shifts;
shift plots;
PSD images before and after alignment.
The averaged micrographs are usually the most important output for the standard single-particle workflow. They are used for CTF estimation and particle picking. The aligned movies are useful if further movie-level processing is needed.
## EER Movies
If the input movies are in EER format, the protocol allows the user to define the Number of EER frames. EER files contain very fine-grained subframes, which must be grouped into a practical number of frames before alignment.
A larger number of EER frames gives finer temporal sampling of the motion, but also increases computational cost and may reduce the signal available in each frame group. A smaller number of frames gives stronger signal per frame group, but may describe the motion less precisely.
The default value is a reasonable starting point for many datasets, but users may adjust it depending on dose, exposure time, and the expected amount of motion.
## Frame Ranges for Alignment and Summation
As in other Scipion movie-alignment protocols, the user can define which frames are used to estimate the alignment and which frames are used to generate the final summed micrograph.
The alignment frame range determines the frames used to estimate motion. The summation frame range determines the frames included in the final average.
These two ranges do not necessarily have to be identical. For example, very early frames may contain strong initial motion, and very late frames may contain more radiation damage. Depending on the dataset, users may exclude some frames from the final sum while still using enough frames to estimate a stable motion trajectory.
From a biological point of view, this choice controls the compromise between signal, radiation damage, and motion correction quality.
## Global and Local Alignment
FlexAlign can perform local alignment in addition to global alignment.
Global alignment estimates a single shift per frame. This corrects the average motion of the whole movie. It is often sufficient when the motion is small or spatially homogeneous.
Local alignment estimates position-dependent motion. In this mode, different regions of the micrograph can have different shifts. This is useful because beam-induced motion is frequently not uniform across the image.
The option Compute local alignment? enables this local correction. In most modern cryo-EM workflows, local alignment is recommended, especially for high-resolution single-particle analysis.
If local alignment is disabled, the protocol performs a simpler correction. This may be faster, but it can leave residual local blurring in the corrected micrograph.
## Control Points
For local alignment, FlexAlign models the motion field using control points. These control points define how flexibly the estimated deformation can vary across space and time.
The protocol can determine the number of control points automatically. This is the recommended option for most users. Automatic control-point selection uses the movie dimensions, sampling rate, and number of frames to choose values that are appropriate for the dataset.
Advanced users can disable automatic control-point selection and manually set the number of control points in X, Y, and time. Larger numbers allow a more flexible motion model, but they may also make the estimation less stable if the movie does not contain enough signal. Too few control points may underfit the motion and fail to correct local deformation.
At least three control points are required in each dimension.
## Patches for Local Alignment
Local alignment also uses image patches. These patches provide local regions from which the motion can be estimated.
The Auto patches option lets the protocol choose the number of patches automatically. This is recommended for routine use.
If the number of patches is set manually, the user controls how finely the micrograph is divided for local motion estimation. More patches can capture more local variation, but each patch contains less signal. Fewer patches are more stable but may miss spatially varying motion.
The correct balance depends on micrograph size, particle distribution, dose, ice quality, and the amount of beam-induced movement.
## Minimum Patch Size
The Min size of the patch parameter defines the minimum physical size, in angstroms, that each local-alignment patch should cover.
This parameter helps prevent the local-alignment model from using patches that are too small to provide reliable correlation signal. If patches are too small, their estimated shifts may become noisy. If they are too large, local motion may be averaged out.
For most users, the default value should be left unchanged unless there is a specific reason to tune local alignment behavior.
## Grouping Frames
The Group N frames parameter controls whether several consecutive frames are summed before estimating local alignment.
Grouping frames increases the signal-to-noise ratio of the images used for alignment, which can make shift estimation more stable. However, grouping also reduces temporal resolution. If too many frames are grouped together, rapid motion may be smoothed out and not fully corrected.
The default value is a practical compromise. Increasing this value may help for very noisy movies; decreasing it may help when motion changes rapidly across the exposure.
## Maximum Resolution for Correlation
The Maximum resolution parameter defines the highest-resolution information preserved during the correlation step.
Movie alignment is usually driven by relatively low- and medium-resolution features, which are more robust to noise. Very high-resolution information may be too noisy to help the correlation and can sometimes make alignment less stable.
The default value limits the correlation to a resolution range that is usually suitable for estimating motion. Users should be cautious when changing this parameter. Preserving too much high-resolution information during correlation does not necessarily improve the final micrograph.
## Maximum Shift
The Maximum shift parameter defines the maximum allowed displacement, in angstroms, between consecutive frames.
This parameter acts as a safeguard against unrealistic frame-to-frame jumps. If the value is too small, genuine motion may be artificially restricted. If it is too large, the algorithm may accept unstable or incorrect correlations.
The default value is suitable for many datasets. Datasets with very strong initial beam-induced motion may require a larger value, but such changes should be checked carefully using the resulting shift plots.
## Binning
If a binning factor is used, the movie is processed at reduced image size. This can speed up alignment and reduce memory requirements.
Binning may be useful for very large movies or for preliminary processing. However, excessive binning can remove information needed for accurate alignment and may reduce the quality of the final corrected micrographs.
The bin factor must be greater than or equal to 1. A value of 1 means that no binning is applied.
## Gain and Dark Correction
If the input movie set has associated gain or dark references, FlexAlign can use them during processing.
Gain correction compensates for pixel-to-pixel sensitivity differences in the detector. Dark correction compensates for detector background signal. These corrections are important because uncorrected detector artifacts can interfere with frame alignment, CTF estimation, and later particle processing.
The protocol includes a Gain orientation section that allows the user to rotate or flip the gain reference. This is useful when the gain image and the movie frames have different orientations.
For TIFF movies, the gain reference may require an automatic vertical flip. The protocol also allows explicit gain rotation by 90, 180, or 270 degrees, and flipping upside down or left-right.
Users should pay special attention to gain orientation. An incorrectly oriented gain reference can introduce strong artifacts into the corrected micrographs.
## PSD Computation
The option Compute PSD? makes the protocol compute power spectral density diagnostics before and after alignment.
PSD images are useful for assessing data quality and the effect of motion correction. After successful alignment, Thon rings and other frequency-domain features may become clearer, especially at higher resolution.
These diagnostics are particularly useful when checking whether motion correction has improved the data before proceeding to CTF estimation.
If PSD computation is enabled only for diagnostic purposes, temporary average micrographs used for PSD calculation may be removed automatically unless the user has also requested to save the average micrographs.
## Shift Plots
For each processed movie, FlexAlign stores a plot of the estimated motion trajectory. This plot shows the cumulative shifts in X and Y across frames.
Shift plots are one of the most useful practical diagnostics of movie alignment. Smooth trajectories usually indicate stable alignment. Very abrupt jumps may indicate poor correlation, very low signal, incorrect gain correction, bad frames, or excessive motion.
Users should inspect representative shift plots, especially when processing a new dataset or when changing alignment parameters.
## GPU and CPU Execution
FlexAlign can use either the GPU or CPU implementation, depending on the available installation and selected options.
Local alignment requires the GPU implementation. If local alignment is enabled, the protocol must be run with GPU support.
The protocol checks that the required Xmipp binaries are available. If the GPU version is selected but the CUDA-enabled binary is not present, the protocol will report an error. In that case, the Xmipp installation should be checked.
When using GPUs, the number of Scipion threads should be consistent with the number of selected GPU devices. In typical use, the protocol expects the number of threads to correspond to the number of GPUs plus one.
## Outputs and Their Interpretation
Depending on the selected options, the protocol can produce aligned micrographs, aligned movies, shift metadata, alignment plots, and PSD diagnostics.
The aligned average micrographs are generally used as input for CTF estimation and particle picking. They represent the motion-corrected sum of the selected movie frames.
The aligned movies preserve the corrected frame sequence and may be useful for additional movie-level processing.
The shift metadata and plots describe the estimated motion. They should be used to evaluate whether the correction behaved sensibly.
The PSD diagnostics help assess whether the alignment improves the frequency content of the micrograph and whether the corrected images are suitable for subsequent processing.
## Practical Recommendations
For most modern single-particle cryo-EM datasets, local alignment should be enabled and automatic control points and patches should be used.
Keep PSD computation enabled when processing a new dataset, because it provides a useful diagnostic of alignment quality.
Use the default maximum resolution for correlation unless there is a specific reason to change it. Movie alignment does not usually benefit from using very high-resolution noisy information during correlation.
Inspect the shift plots for several movies. Smooth, physically plausible motion trajectories are a good sign. Sudden jumps or erratic behavior should be investigated.
Check gain orientation carefully. Many alignment problems are caused not by the motion algorithm itself, but by incorrect gain correction.
Use frame ranges thoughtfully. Excluding damaged late frames or unstable early frames from the final sum can improve the quality of the averaged micrographs.
## Final Perspective
FlexAlign is an early but crucial step in the cryo-EM image-processing workflow. It converts raw detector movies into motion-corrected images that can be reliably used for CTF estimation, particle picking, classification, and reconstruction.
Good movie alignment preserves high-resolution information that would otherwise be blurred by beam-induced motion. Poor alignment, incorrect gain correction, or inappropriate frame selection can limit the achievable resolution of the entire project.
For biological users, the main goal is to obtain corrected micrographs with stable motion trajectories, clear diagnostic PSDs, and no obvious artifacts. These corrected micrographs form the foundation for the rest of the single-particle analysis workflow.
- NO_FLIP = 0
- NO_ROTATION = 0