# Walkthrough on PCA through the command line

PCA computations through the command line are governed through *PCA workflow* objects. We describe here how to create and handle them:

# Creation of a synthetic data set

dtutorial ttest128 -M 64 -N 64 -linear_tags 1 -tight 1

This generates a set of 128 particles where 64 are slightly closer than the other 64. The particle subtomogram are randomly oriented, but the alignment parameters are known.

# Creation of a workflow

## Input elements

The input of a PCA workflow are:

- a set of particles (called
*data container*in this article) - a table that expreses the alignment
- a mask that indicates the area of each alignment particle that will be taken into account during the classification procedure.

### Data

dataFolder = 'ttest128/data';

### Table

tableFile = 'ttest128/real.tbl';

### Mask

We create a cylindrical mask with the dimensions of the particles (40 pixels) mask = dcylinder([20,20],40);

### Syntax

We decide a name for the workflow itself, for instance

name = 'classtest128';

Now we are ready to create the workflow:

wb = dpkpca.new(name,'t',tableFile,'d',dataFolder,'m',mask);

This creates an workflow object (arbitrarily called wb in the workspace during the current session). It also creates a folder called classtest128.PCA where results will be stored as they are produced.

## Mathematical parameters

The main parameters that can be chosen in this area are:

- bandpass
- symmetry
- binning level (to accelerate the computations)

## Computational parameters

The main burden of the PCA computation is the creation of the cross correlation matrix.

### Computing device

PCA computations can be run on GPUs of on CPUs, in both cases in parallel.

### Size of parallel blocks

The

# Running

In this workflow we run the steps one by one to discuss them. In real workflows, you can use the `run` methods to just launch all steps sequentially.

## Prealigning

wb.steps.items.prealign.compute();

## Correlation matrix

All pairs of correlations are computed in blocks, as described above

wb.steps.items.ccmatrix.compute();

## Eigentable

The correlation matrix is diagonalised. The eigenvectors are used to expressed as the particles as combinations of weights.

wb.steps.items.eigentable.compute();

These weights are ordered in descending order relative to their impact on the variance of the set, ideally a particle should be represented by its few components on this basis. The weights are stored in a regula *Dynamo* table. First eigencomponent of a particle goes into column 41.

## Eigenvolumes

The eigenvectors are expressed as three=dimensional volumes.

wb.steps.items.eigenvolumes.compute();

## TSNE reduction

TSNE remaps the particles into 2D maps which can be visualised and operated interactively.

wb.steps.items.tsene.compute();

# Visualization

## Correlation of tilts

It is a good idea to check if some eigenvolumes correlate strongly with the tilt.

wb.show.correlationEigenvectorTilt(1:10)

In this plot, each point represents a particle in your data set.