Representing forked WCS trees in ASDF using GWCS

ast_to_gwcs.py

# This is to show how one might represent two linear WCS pipelines that share a step in ASDF using GWCS

# The transform comes from JWST NIRCam imaging.

from jwst import datamodels
import asdf
from copy import deepcopy


# Use some existing NIRCam Imaging data which has a GWCS object in it
model = datamodels.open("/Users/jdavies/Downloads/jw42424001001_01101_00001_nrca5_cal.fits")

# Pull out the GWCS object and make a deep copy of it
wcs = model.meta.wcs
wcs.name = "Primary WCS"
wcs2 = deepcopy(wcs)
wcs2.name = "Secondary WCS"

# Make the first step in the pipeline not just a copy, but the *same* object, i.e.
# one points the other
wcs2.pipeline[0] = wcs.pipeline[0]

# Store the WCSs as a list and serialize
tree = dict(wcs=[wcs, wcs2])
with asdf.AsdfFile(tree=tree) as af: 
    af.write_to("test.asdf", auto_inline=65000)
    
# The resulting serialized ASDF is below.
#
# Notice the Primary WCS transform:
#
#     transform: &id002 !transform/compose-1.2.0
#
# has a reference "&id002" there.  This allows it to be referenced by the Secondary
# WCS as:
#
#     transform: *id002
#
# Voila!

# --------------------

# test.asdf

#ASDF 1.0.0
#ASDF_STANDARD 1.5.0
%YAML 1.1
%TAG ! tag:stsci.edu:asdf/
--- !core/asdf-1.1.0
asdf_library: !core/software-1.0.0 {author: Space Telescope Science Institute, homepage: 'http://github.com/spacetelescope/asdf',
  name: asdf, version: 2.6.0}
history:
  extensions:
  - !core/extension_metadata-1.0.0
    extension_class: asdf.extension.BuiltinExtension
    software: !core/software-1.0.0 {name: asdf, version: 2.6.0}
  - !core/extension_metadata-1.0.0
    extension_class: jwst.transforms.jwextension.JWSTExtension
    software: !core/software-1.0.0 {name: jwst, version: 0.16.3.dev28+gc6204a4b.d20200611}
  - !core/extension_metadata-1.0.0
    extension_class: astropy.io.misc.asdf.extension.AstropyAsdfExtension
    software: !core/software-1.0.0 {name: astropy, version: 4.1rc1}
  - !core/extension_metadata-1.0.0
    extension_class: astropy.io.misc.asdf.extension.AstropyExtension
    software: !core/software-1.0.0 {name: astropy, version: 4.1rc1}
  - !core/extension_metadata-1.0.0
    extension_class: gwcs.extension.GWCSExtension
    software: !core/software-1.0.0 {name: gwcs, version: 0.13.0}
wcs:
- !<tag:stsci.edu:gwcs/wcs-1.0.0>
  name: Primary WCS
  steps:
  - !<tag:stsci.edu:gwcs/step-1.0.0>
    frame: !<tag:stsci.edu:gwcs/frame2d-1.0.0>
      axes_names: [x, y]
      axes_order: [0, 1]
      axis_physical_types: ['custom:x', 'custom:y']
      name: detector
      unit: [!unit/unit-1.0.0 pixel, !unit/unit-1.0.0 pixel]
    transform: &id002 !transform/compose-1.2.0
      bounding_box:
      - [-0.5, 2047.5]
      - [-0.5, 2047.5]
      forward:
      - !transform/compose-1.2.0
        forward:
        - !transform/compose-1.2.0
          forward:
          - !transform/concatenate-1.2.0
            forward:
            - !transform/shift-1.2.0 {offset: 1.0}
            - !transform/shift-1.2.0 {offset: 1.0}
          - !transform/concatenate-1.2.0
            forward:
            - !transform/shift-1.2.0 {offset: -1024.5}
            - !transform/shift-1.2.0 {offset: -1024.5}
        - !transform/compose-1.2.0
          forward:
          - !transform/compose-1.2.0
            forward:
            - !transform/remap_axes-1.2.0
              mapping: [0, 1, 0, 1]
            - !transform/concatenate-1.2.0
              forward:
              - !transform/polynomial-1.2.0
                coefficients: !core/ndarray-1.0.0
                  data:
                  - [-0.0, 0.0, -9.721046535057192e-08, -2.164006028817789e-13, -3.590620404845533e-16,
                    4.354424173002358e-20]
                  - [0.06277674557948036, -7.244807505775117e-07, 9.66518115581259e-11,
                    -3.191824242088728e-15, 4.71684808024088e-18, 0.0]
                  - [1.198364733530203e-07, -5.782468597491751e-12, 2.117937146007029e-16,
                    5.143162171129567e-19, 0.0, 0.0]
                  - [9.094661041997164e-11, -2.949576253844291e-15, 9.531355664635004e-18,
                    0.0, 0.0, 0.0]
                  - [6.449038115869934e-16, 4.879315376187496e-19, 0.0, 0.0, 0.0,
                    0.0]
                  - [4.706941538958384e-18, 0.0, 0.0, 0.0, 0.0, 0.0]
                  datatype: float64
                  shape: [6, 6]
                domain:
                - &id001 [-1, 1]
                - *id001
                window:
                - *id001
                - *id001
              - !transform/polynomial-1.2.0
                coefficients: !core/ndarray-1.0.0
                  data:
                  - [0.0, 0.0630762699429303, -4.239854922865822e-07, 9.762139471915262e-11,
                    -2.015274358881797e-15, 4.410393661077231e-18]
                  - [-9.102681200381172e-05, 2.287097298253153e-07, -6.481047139577153e-12,
                    1.0853593918469e-15, 4.941571953195922e-19, 0.0]
                  - [3.086258377388918e-07, 8.751405385759113e-11, -5.078890561912942e-16,
                    9.736970732740994e-18, 0.0, 0.0]
                  - [-6.696189796258203e-13, 1.146884680565914e-15, 5.986044579428994e-19,
                    0.0, 0.0, 0.0]
                  - [1.454596109813538e-15, 5.27551747031014e-18, 0.0, 0.0, 0.0, 0.0]
                  - [9.296183515265389e-20, 0.0, 0.0, 0.0, 0.0, 0.0]
                  datatype: float64
                  shape: [6, 6]
                domain:
                - *id001
                - *id001
                window:
                - *id001
                - *id001
          - !transform/compose-1.2.0
            forward:
            - !transform/remap_axes-1.2.0
              mapping: [0, 1, 0, 1]
            - !transform/concatenate-1.2.0
              forward:
              - !transform/polynomial-1.2.0
                coefficients: !core/ndarray-1.0.0
                  data:
                  - [0.0, -0.0012889474613725963]
                  - [-0.9999991693068759, 0.0]
                  datatype: float64
                  shape: [2, 2]
                domain:
                - *id001
                - *id001
                window:
                - *id001
                - *id001
              - !transform/polynomial-1.2.0
                coefficients: !core/ndarray-1.0.0
                  data:
                  - [0.0, 0.9999991693068759]
                  - [-0.0012889474613725963, 0.0]
                  datatype: float64
                  shape: [2, 2]
                domain:
                - *id001
                - *id001
                window:
                - *id001
                - *id001
      - !transform/concatenate-1.2.0
        forward:
        - !transform/shift-1.2.0 {offset: 86.039011}
        - !transform/shift-1.2.0 {offset: -493.385704}
      inverse: !transform/compose-1.2.0
        forward:
        - !transform/concatenate-1.2.0
          forward:
          - !transform/shift-1.2.0 {offset: -86.039011}
          - !transform/shift-1.2.0 {offset: 493.385704}
        - !transform/compose-1.2.0
          forward:
          - !transform/compose-1.2.0
            forward:
            - !transform/compose-1.2.0
              forward:
              - !transform/remap_axes-1.2.0
                mapping: [0, 1, 0, 1]
              - !transform/concatenate-1.2.0
                forward:
                - !transform/polynomial-1.2.0
                  coefficients: !core/ndarray-1.0.0
                    data:
                    - [0.0, -0.0012889474613725963]
                    - [-0.9999991693068759, 0.0]
                    datatype: float64
                    shape: [2, 2]
                  domain:
                  - *id001
                  - *id001
                  window:
                  - *id001
                  - *id001
                - !transform/polynomial-1.2.0
                  coefficients: !core/ndarray-1.0.0
                    data:
                    - [0.0, 0.9999991693068759]
                    - [-0.0012889474613725963, 0.0]
                    datatype: float64
                    shape: [2, 2]
                  domain:
                  - *id001
                  - *id001
                  window:
                  - *id001
                  - *id001
            - !transform/compose-1.2.0
              forward:
              - !transform/remap_axes-1.2.0
                mapping: [0, 1, 0, 1]
              - !transform/concatenate-1.2.0
                forward:
                - !transform/polynomial-1.2.0
                  coefficients: !core/ndarray-1.0.0
                    data:
                    - [0.001606802685174102, 1.072121003490439e-05, 0.0003899472961152239,
                      1.741491572219416e-07, -1.730326086686541e-10, -1.214407164376713e-12]
                    - [15.92985674331845, 0.002932499409606826, -5.492406520770091e-06,
                      -3.48506269432795e-09, -6.053447585998923e-11, 0.0]
                    - [-0.0004827156577026869, -1.704997124234288e-07, 4.500242110641451e-10,
                      -7.573478292656556e-12, 0.0, 0.0]
                    - [-6.181790050341375e-06, -3.7994370832503e-09, -1.214837102200545e-10,
                      0.0, 0.0, 0.0]
                    - [6.600092079203932e-10, -6.325812350975392e-12, 0.0, 0.0, 0.0,
                      0.0]
                    - [-5.99901579643954e-11, 0.0, 0.0, 0.0, 0.0, 0.0]
                    datatype: float64
                    shape: [6, 6]
                  domain:
                  - *id001
                  - *id001
                  window:
                  - *id001
                  - *id001
                - !transform/polynomial-1.2.0
                  coefficients: !core/ndarray-1.0.0
                    data:
                    - [-0.006780939009559006, 15.85435606632966, 0.001699308339193117,
                      -5.931127954103713e-06, -2.505780246788893e-09, -5.582941651153512e-11]
                    - [0.02314264781384863, -0.0009119151347307442, -1.968853925968155e-07,
                      9.314930747598225e-10, -6.10620822524126e-12, 0.0]
                    - [-0.001244711452021884, -6.450451269943015e-06, -2.485642883412269e-09,
                      -1.192566344000974e-10, 0.0, 0.0]
                    - [1.499394209694464e-07, 1.029780982881483e-09, -7.045014588881225e-12,
                      0.0, 0.0, 0.0]
                    - [3.124699039332876e-10, -6.284891892869384e-11, 0.0, 0.0, 0.0,
                      0.0]
                    - [-9.811118954903338e-13, 0.0, 0.0, 0.0, 0.0, 0.0]
                    datatype: float64
                    shape: [6, 6]
                  domain:
                  - *id001
                  - *id001
                  window:
                  - *id001
                  - *id001
          - !transform/compose-1.2.0
            forward:
            - !transform/concatenate-1.2.0
              forward:
              - !transform/shift-1.2.0 {offset: 1024.5}
              - !transform/shift-1.2.0 {offset: 1024.5}
            - !transform/concatenate-1.2.0
              forward:
              - !transform/shift-1.2.0 {offset: -1.0}
              - !transform/shift-1.2.0 {offset: -1.0}
  - !<tag:stsci.edu:gwcs/step-1.0.0>
    frame: !<tag:stsci.edu:gwcs/frame2d-1.0.0>
      axes_names: [x, y]
      axes_order: [0, 1]
      axis_physical_types: ['custom:x', 'custom:y']
      name: v2v3
      unit: [!unit/unit-1.0.0 arcsec, !unit/unit-1.0.0 arcsec]
    transform: !transform/compose-1.2.0
      forward:
      - !transform/concatenate-1.2.0
        forward:
        - !transform/scale-1.2.0 {factor: 0.0002777777777777778}
        - !transform/scale-1.2.0 {factor: 0.0002777777777777778}
      - !<tag:stsci.edu:jwst_pipeline/v23tosky-0.7.0>
        angles: [-0.02389972527777778, -0.13705158444444443, -0.005076934167039675,
          -11.99875540218638, 22.02351763251896]
        axes_order: zyxyz
        name: v23tosky
  - !<tag:stsci.edu:gwcs/step-1.0.0>
    frame: !<tag:stsci.edu:gwcs/celestial_frame-1.0.0>
      axes_names: [lon, lat]
      axes_order: [0, 1]
      axis_physical_types: [pos.eq.ra, pos.eq.dec]
      name: world
      reference_frame: !<tag:astropy.org:astropy/coordinates/frames/icrs-1.1.0>
        frame_attributes: {}
      unit: [!unit/unit-1.0.0 deg, !unit/unit-1.0.0 deg]
- !<tag:stsci.edu:gwcs/wcs-1.0.0>
  name: Secondary WCS
  steps:
  - !<tag:stsci.edu:gwcs/step-1.0.0>
    frame: !<tag:stsci.edu:gwcs/frame2d-1.0.0>
      axes_names: [x, y]
      axes_order: [0, 1]
      axis_physical_types: ['custom:x', 'custom:y']
      name: detector
      unit: [!unit/unit-1.0.0 pixel, !unit/unit-1.0.0 pixel]
    transform: *id002
  - !<tag:stsci.edu:gwcs/step-1.0.0>
    frame: !<tag:stsci.edu:gwcs/frame2d-1.0.0>
      axes_names: [x, y]
      axes_order: [0, 1]
      axis_physical_types: ['custom:x', 'custom:y']
      name: v2v3
      unit: [!unit/unit-1.0.0 arcsec, !unit/unit-1.0.0 arcsec]
    transform: !transform/compose-1.2.0
      forward:
      - !transform/concatenate-1.2.0
        forward:
        - !transform/scale-1.2.0 {factor: 0.0002777777777777778}
        - !transform/scale-1.2.0 {factor: 0.0002777777777777778}
      - !<tag:stsci.edu:jwst_pipeline/v23tosky-0.7.0>
        angles: [-0.02389972527777778, -0.13705158444444443, -0.005076934167039675,
          -11.99875540218638, 22.02351763251896]
        axes_order: zyxyz
        name: v23tosky
  - !<tag:stsci.edu:gwcs/step-1.0.0>
    frame: !<tag:stsci.edu:gwcs/celestial_frame-1.0.0>
      axes_names: [lon, lat]
      axes_order: [0, 1]
      axis_physical_types: [pos.eq.ra, pos.eq.dec]
      name: world
      reference_frame: !<tag:astropy.org:astropy/coordinates/frames/icrs-1.1.0>
        frame_attributes: {}
      unit: [!unit/unit-1.0.0 deg, !unit/unit-1.0.0 deg]
...

One thing I'm not clear on is why the internal reference points to the transform and not the step itself.

One thing I'm not clear on is why the internal reference points to the transform and not the step itself.

The WCSType class creates new objects for each step when serializing the tree:

https://github.com/spacetelescope/gwcs/blob/0.13.0/gwcs/tags/wcs.py#L48

So the tagged steps end up with different ids, and pyyaml can't use anchors/aliases with the steps because it doesn't know that they're identical.