transforms.py

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# This file is part of meas_base.
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"""Measurement transformations.
 
When a measurement plugin is run, it provides raw, uncalibrated outputs such
as pixel positions. A transformation may be run as a post-processing step to
convert those outputs to calibrated quantities, such as celestial coordinates.
 
At construction, the transformation is passed the configuration and name of
the plugin whose outputs it will be transforming (all fields in the input
table produced by that plugin will have their field names prefixed by the
plugin name) and a `~lsst.afw.table.SchemaMapper` which holds the schemata for
the input and output catalogs and which may be used to directly map fields
between the catalogs.
 
When a transformer is called, it is handed a `~lsst.afw.table.SourceCatalog`
containing the measurements to be transformed, a `~lsst.afw.table.BaseCatalog`
in which to store results, and information about the WCS and calibration of
the data. It may be safely assumed that both are contiguous in memory, thus a
``ColumnView`` may be used for efficient processing. If the transformation is
not possible, it should be aborted by throwing an exception; if this happens,
the caller should assume that the contents of the output catalog are
inconsistent.
 
Transformations can be defined in Python or in C++. Python code should inherit
from `MeasurementTransform`, following its interface.
"""
 
from lsst.afw.table import CoordKey
from lsst.pex.exceptions import LengthError
from ._measBaseLib import CentroidResultKey
 
__all__ = ("MeasurementTransform", "NullTransform", "PassThroughTransform", "SimpleCentroidTransform")
 
 
class MeasurementTransform:
    """Base class for measurement transformations.
 
    Parameters
    ----------
    config : subclass of `BasePluginConfig`
        The configuration of the measurement plugin whose outputs are being
        transformed.
    name : `str`
        The name of the measurement plugin whose outputs are being
        transformed.
    mapper : `lsst.afw.table.SchemaMapper`
        Mapping between the input (pre-transformation) and output
        (transformed) catalogs.
 
    Notes
    -----
    Create transformations by deriving from this class, implementing
    `__call__()` and (optionally) augmenting `__init__()`.
    """
 
    def __init__(self, config, name, mapper):
        self.name = name
        self.config = config
 
    def __call__(self, inputCatalog, outputCatalog, wcs, photoCalib):
        raise NotImplementedError()
 
    @staticmethod
    def _checkCatalogSize(cat1, cat2):
        if len(cat1) != len(cat2):
            raise LengthError("Catalog size mismatch")
 
 
class NullTransform(MeasurementTransform):
    """Null transform which transfers no data from input to output.
 
    This is intended as the default for measurements for which no other
    transformation is specified.
 
    Parameters
    ----------
    config : subclass of `BasePluginConfig`
        The configuration of the measurement plugin whose outputs are being
        transformed.
    name : `str`
        The name of the measurement plugin whose outputs are being
        transformed.
    mapper : `lsst.afw.table.SchemaMapper`
        Mapping between the input (pre-transformation) and output
        (transformed) catalogs.
    """
 
    def __call__(self, inputCatalog, outputCatalog, wcs, photoCalib):
        self._checkCatalogSize(inputCatalog, outputCatalog)
 
 
class PassThroughTransform(MeasurementTransform):
    """Copy fields from input to output without transformation.
 
    Parameters
    ----------
    config : subclass of `BasePluginConfig`
        The configuration of the measurement plugin whose outputs are being
        transformed.
    name : `str`
        The name of the measurement plugin whose outputs are being
        transformed.
    mapper : `lsst.afw.table.SchemaMapper`
        Mapping between the input (pre-transformation) and output
        (transformed) catalogs.
    """
 
    def __init__(self, config, name, mapper):
        MeasurementTransform.__init__(self, config, name, mapper)
        for key, field in mapper.getInputSchema().extract(name + "*").values():
            mapper.addMapping(key)
 
    def __call__(self, inputCatalog, outputCatalog, wcs, photoCalib):
        self._checkCatalogSize(inputCatalog, outputCatalog)
 
 
class SimpleCentroidTransform(MeasurementTransform):
    """Transform pixel centroid, without uncertainty, to celestial coordinates.
 
    Parameters
    ----------
    config : subclass of `BasePluginConfig`
        The configuration of the measurement plugin whose outputs are being
        transformed.
    name : `str`
        The name of the measurement plugin whose outputs are being
        transformed.
    mapper : `lsst.afw.table.SchemaMapper`
        Mapping between the input (pre-transformation) and output
        (transformed) catalogs.
    """
 
    def __init__(self, config, name, mapper):
        MeasurementTransform.__init__(self, config, name, mapper)
        self.coordKey = CoordKey.addFields(mapper.editOutputSchema(), name, "Position from " + name)
 
    def __call__(self, inputCatalog, outputCatalog, wcs, photoCalib):
        self._checkCatalogSize(inputCatalog, outputCatalog)
        centroidResultKey = CentroidResultKey(inputCatalog.schema[self.name])
        for inSrc, outSrc in zip(inputCatalog, outputCatalog):
            self.coordKey.set(outSrc, wcs.pixelToSky(centroidResultKey.get(inSrc).getCentroid()))