# filters.python

The Python Filter allows Python software to be embedded in a Pipeline that allows modification of PDAL points through a NumPy array. Additionally, some global Metadata is also available that Python functions can interact with.

The function must have two NumPy arrays as arguments, `ins` and `outs`. The `ins` array represents the points before the `filters.python` filter and the `outs` array represents the points after filtering.

Warning

Make sure NumPy is installed in your Python environment.

    $ python3 -c "import numpy; print(numpy.__version__)"
    1.18.1

Warning

Each array contains all the Dimensions of the incoming `ins` point schema. Each array in the `outs` list matches the NumPy array of the same type as provided as `ins` for shape and type.

Dynamic Plugin

This stage requires a dynamic plugin to operate

    import numpy as np

    def multiply_z(ins,outs):
        Z = ins['Z']
        Z = Z * 10.0
        outs['Z'] = Z
        return True

1. The function must always return True upon success. If the function returned False, an error would be thrown and the Pipeline exited.

2. If you want write a dimension that might not be available, you can specify it with the add_dimension option:

       "add_dimension": "NewDimensionOne"

   To create more than one dimension, this option also accepts an array:

       "add_dimension": [ "NewDimensionOne", "NewDimensionTwo", "NewDimensionThree" ]

   You can also specify the type of the dimension using an `=`.

       "add_dimension": "NewDimensionOne=uint8"

## Modification Example

    [
        "file-input.las",
        {
            "type":"filters.smrf"
        },
        {
            "type":"filters.python",
            "script":"multiply_z.py",
            "function":"multiply_z",
            "module":"anything"
        },
        {
            "type":"writers.las",
            "filename":"file-filtered.las"
        }
    ]

The JSON pipeline file referenced the external multiply_z.py Python script, which scales the `Z` coordinate by a factor of 10.

    import numpy as np

    def multiply_z(ins,outs):
        Z = ins['Z']
        Z = Z * 10.0
        outs['Z'] = Z
        return True

## Predicates

Points can be retained/removed from the stream by setting true/false values into a special “Mask” dimension in the output point array.

The example above sets the “mask” to true for points that are in classifications 1 or 2 and to false otherwise, causing points that are not classified 1 or 2 to be dropped from the point stream.

    import numpy as np

    def filter(ins,outs):
       cls = ins['Classification']

       keep_classes = [1, 2]

       # Use the first test for our base array.
       keep = np.equal(cls, keep_classes[0])

       # For 1:n, test each predicate and join back
       # to our existing predicate array
       for k in range(1, len(keep_classes)):
           t = np.equal(cls, keep_classes[k])
           keep = keep + t

       outs['Mask'] = keep
       return True

Note

filters.range is a specialized filter that implements the exact functionality described in this Python operation. It is likely to be much faster than Python, but not as flexible. filters.python is the tool you can use for prototyping point stream processing operations.

See also

If you want to read a Pipeline of operations into a numpy array, the PDAL Python extension is available.

### Example pipeline

    [
        "file-input.las",
        {
            "type":"filters.smrf"
        },
        {
            "type":"filters.python",
            "script":"filter_pdal.py",
            "function":"filter",
            "module":"anything"
        },
        {
            "type":"writers.las",
            "filename":"file-filtered.las"
        }
    ]

## Module Globals

Three global variables are added to the Python module as it is run to allow you to get Dimensions, Metadata, and coordinate system information. Additionally, the `metadata` object can be set by the function to modify metadata for the in-scope filters.python `pdal::Stage`.

    def myfunc(ins,outs):
        print('schema: ', schema)
        print('srs: ', spatialreference)
        print('metadata: ', metadata)
        outs = ins
        return True

### Updating metadata

The filter can update the global `metadata` dictionary as needed, define it as a global Python variable for the function’s scope, and the updates will be reflected back into the pipeline from that stage forward.

    def myfunc(ins,outs):
      global metadata
      metadata = {'name': 'root', 'value': 'a string', 'type': 'string', 'description': 'a description', 'children': [{'name': 'filters.python', 'value': 52, 'type': 'integer', 'description': 'a filter description', 'children': []}, {'name': 'readers.faux', 'value': 'another string', 'type': 'string', 'description': 'a reader description', 'children': []}]}
      return True

### Passing Python objects

An JSON-formatted option can be passed to the filter representing a Python dictionary containing objects you want to use in your function. This feature is useful in situations where you wish to call pipeline with substitutions.

If we needed to be able to provide the Z scaling factor of Example Pipeline with a Python argument, we can place that in a dictionary and pass that to the filter as a separate argument. This feature allows us to be able easily reuse the same basic Python function while substituting values as necessary.

    [
        "input.las",
        {
            "type":"filters.python",
            "module":"anything",
            "function":"filter",
            "script":"arguments.py",
            "pdalargs":"{\"factor\":0.3048,\"an_argument\":42, \"another\": \"a string\"}"
        },
        "output.las"
    ]

With that option set, you can now fetch the pdalargs dictionary in your Python script and use it:

    import numpy as np

    def multiply_z(ins,outs):
        Z = ins['Z']
        Z = Z * float(pdalargs['factor'])
        outs['Z'] = Z
        return True

### Standard output and error

A `redirector` module is available for scripts to output to PDAL’s log stream explicitly. The module handles redirecting `sys.stderr` and `sys.stdout` for you transparently, but it can be used directly by scripts. See the PDAL source code for more details.

## Options

script

  When reading a function from a separate Python file, the file name to read from.

source

  The literal Python code to execute, when the script option is not being used.

module

  The Python module that is holding the function to run. [Required]

function

  The function to call. [Required]

add_dimension

  A dimension name or an array of dimension names to add to the pipeline that do not already exist.

pdalargs

  A JSON dictionary of items you wish to pass into the modules globals as the `pdalargs` object.

where

  An expression that limits points passed to a filter. Points that don’t pass the expression skip the stage but are available to subsequent stages in a pipeline. [Default: no filtering]

where_merge

  A strategy for merging points skipped by a ‘where’ option when running in standard mode. If `true`, the skipped points are added to the first point view returned by the skipped filter. If `false`, skipped points are placed in their own point view. If `auto`, skipped points are merged into the returned point view provided that only one point view is returned and it has the same point count as it did when the filter was run. [Default: `auto`]