transforms.py

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#!/usr/bin/env python
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# LSST Data Management System
# Copyright 2008-2015 AURA/LSST.
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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 transformaing (all fields in the input
table produced by that plugin will have their field names prefixed by the
plugin name) and a `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 `SourceCatalog` containing the
measurements to be transformed, a `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 .baseLib import CentroidResultKey

__all__ = ("NullTransform", "PassThroughTransform", "SimpleCentroidTransform")

class MeasurementTransform(object):
    """!
    Base class for measurement transformations.

    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, calib):
        raise NotImplementedError()

    @staticmethod
    def _checkCatalogSize(cat1, cat2):
        if len(cat1) != len(cat2):
            raise LengthError("Catalog size mismatch")


class NullTransform(MeasurementTransform):
    """!
    The null transform transfers no data from input to output.

    This is intended as the default for measurements for which no other
    transformation is specified.
    """
    def __call__(self, inputCatalog, outputCatalog, wcs, calib):
        self._checkCatalogSize(inputCatalog, outputCatalog)


class PassThroughTransform(MeasurementTransform):
    """!
    Copy all fields named after the measurement plugin from input to output, without transformation.
    """
    def __init__(self, config, name, mapper):
        MeasurementTransform.__init__(self, config, name, mapper)
        for key, field in mapper.getInputSchema().extract(name + "*").itervalues():
            mapper.addMapping(key)

    def __call__(self, inputCatalog, outputCatalog, wcs, calib):
        self._checkCatalogSize(inputCatalog, outputCatalog)


class SimpleCentroidTransform(MeasurementTransform):
    """!
    Transform a pixel centroid, excluding uncertainties, to celestial coordinates.
    """
    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, calib):
        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()))