# xcpEngine¶

xcpEngine is the pipeline system’s front end for image processing. This is the program that parses design and cohort files to deploy processing streams for different imaging modalities. This page documents command-line options and acceptable arguments for the front end. If Docker or Singularity images are used, the default command that is run is xcpEngine.

## -d: Design file¶

The Pipeline design file file parameterizes the processing stream. Standard design files are stored in \${XCPEDIR}/designs. New design files can be generated using the xcpConfig system or via manual editing.

## -c: Cohort file¶

The Pipeline cohort file file parameterizes the pipeline’s input sample, over which image processing is to be executed. Each row corresponds to a subject, and each column corresponds to a variable.

## -o: Output path¶

The output path specifies the parent directory, wherein all output from the instance of the XCP system will be written. The argument to -o must correspond to an valid path on the current filesystem. If the indicated directory does not exist, xcpEngine will attempt to create it. If xcpEngine cannot create the indicated directory, it will abort with an error.

## -i: Intermediate path¶

The intermediate path specifies a directory that the XCP instance will use as a scratch space for storage of temporary files. Some systems operate more quickly when temporary files are written in a dedicated scratch space. The -i option enables a dedicated scratch space for intermediates. Like -o, -i requires an argument that is a valid path on the current filesystem. If -i is unspecified, temporary files will be stored in the argument to -o.

## -r: Reference directory¶

Cohort files can be made more portable if the input paths defined therein are defined relative to some root directory. If input paths are defined relatively rather than absolutely, then xcpEngine requires knowledge of the reference directory relative to which all input paths are defined. This information is provided to xcpEngine as the argument to the -r option.

## -a: Direct assignment¶

The values of single variables in a design file can be overriden via direct assignment, which is discussed in the design file documentation. Direct assignment of variables is not generally advised. Direct assignment is enabled by passing, as the argument to -a, an equality with the variable to be assigned on the left-hand side and its new value on the right-hand side. For instance, to override the default value of the variable standard and replace it with MNI%2x2x2, use -a standard=MNI%2x2x2.

Other ways to override the default value of the variable especially in design files:

-a atlas=power264 # choose preferred atlases such as power264,schaefer

-a spatialsmooth=5 # spatial smoothing for regress (if needed), alff and reho, default is 6mm

-a temporalfilter=0.01,0.10 # temporal filter frequency range, the deafult is [0.01 0.08]

To select confound matrix by overriwriting design filter -a confound=24p # this equivalent to confound2_rps[cxt]=1; confound2_sq[cxt]=2; confound2_dx[cxt]=1

-a confound=36p # this equivalent to onfound2_rps[cxt]=1; confound2_sq[cxt]=2; confound2_dx[cxt]=1,confound2_wm[cxt]=1; confound2_csf[cxt]=1; confound2_gsr[cxt]=1

-a confound=aroma # confound2_wm[cxt]=1; confound2_csf[cxt]=1; confound2_aroma[cxt]=1;

-a confound=tcompcor # confound2_tcompcor[cxt] = 1

-a confound=acompcor # confound2_rps[cxt]=1; confound2_acompcor[cxt]=1; confound2_dx[cxt]=1;

-a fd_thresh=fds:0.083 # override the framewise threshold ( threshold will be 0.083*TRs in mm)

To select the regress option -a regress=despike # despiking, regress_process[cxt]=DMT-DSP-TMP-REG

-a regress=censor # censoring, regress_process[cxt]=DMT-TMP-REG ; censor=1

## -t: Trace (verbosity) level¶

The verbosity level instructs the XCP system and its child scripts whether it should print image processing commands as they are called. Error traces and diagnostics are printed regardless of verbosity level, but increasing the verbosity can make it easier to trace an error to its origin point.

• 0: The default verbosity level prints only human-readable, descriptive explanations of processing steps as they are executed.
• 1: Module-level trace. Any image processing commands that are called by pipeline modules are explicitly printed to the console or log. Calls to utility scripts are printed, but any subroutines of utilities are omitted. For module validation and enhanced diagnosis of errors at the module level.
• 2: Utility-level trace. Like 1, but subroutines of utility scripts are also explicitly printed. For utility validation and enhanced diagnosis of errors at the utility level.
• 3: Maximum verbosity. Absolutely every command is traced. We’ve found that this level of verbosity is almost never warranted and that it will usually make error diagnosis more difficult, not easier, because it’s easy to lose the most relevant information in the noise.