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DM-52537: Raise acceptable algorithm error if WCS transform fails #433

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Merged
yalsayyad merged 2 commits into from
Sep 24, 2025
Merged

DM-52537: Raise acceptable algorithm error if WCS transform fails #433

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ab54d7e
Raise acceptable algorithm error if WCS transform fails
yalsayyad Sep 19, 2025
71eeae9
Retry warped PSF evaluation if transform fails
yalsayyad Sep 22, 2025
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75 changes: 63 additions & 12 deletions python/lsst/ip/diffim/modelPsfMatch.py
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Original file line number Diff line number Diff line change
Expand Up @@ -45,6 +45,55 @@
sigma2fwhm = 2.*np.sqrt(2.*np.log(2.))


def _safeEstimateRoughShape(psf, position, jitter=2.0, maxTries=5, log=None):

"""
Try to compute the shape of `psf` at `position`.

If it fails, perturb the position until success or maxTries.

Parameters
----------
psf : `lsst.afw.detection.Psf`
The PSF object.
position : `lsst.geom.Point2D`
The nominal position at which to evaluate the PSF shape.
jitter : `float`
Size of random offset in pixels to try when retrying.
maxTries : `int`
Maximum number of attempts (including the original).
log : `lsst.utils.logging.LsstLogAdapter` optional
logger to use for logging retries.

Returns
-------
shape : lsst.afw.geom.ellipses.Quadrupole
The shape of the PSF at the (possibly perturbed) position.
"""
try:
return psf.computeShape(position)
except pexExceptions.Exception as err:
if log:
log.info(f"safeEstimateRoughShape: initial evaluation of PSF failed with message: {str(err)}."
" Retrying nearby")
firstErr = err

# random offsets must be deterministic
offsets = np.random.default_rng(seed=40).uniform(-jitter, jitter, size=(maxTries - 1, 2))
candidates = [position + geom.Extent2D(dx, dy) for dx, dy in offsets]
for i, candidate in enumerate(candidates):
try:
shape = psf.computeShape(candidate)
if log:
log.info(f"safeEstimateRoughShape succeeded on try {i + 2} at position {candidate}")
return shape
except pexExceptions.Exception:
continue

# If nothing worked, raise AlgorithmError from original position
raise PsfComputeShapeError(position) from firstErr


def nextOddInteger(x):
nextInt = int(np.ceil(x))
return nextInt + 1 if nextInt%2 == 0 else nextInt
Expand All @@ -61,14 +110,16 @@ def metadata(self) -> dict:

class PsfComputeShapeError(pipeBase.AlgorithmError):
def __init__(self, position):
message = f"Unable to compute the FWHM of the science Psf at {position}"
message = f"Unable to evaluate Psf at ({float(position.getX()):.4f}, {float(position.getY()):.4f})"
super().__init__(message)
self.position = position

@property
def metadata(self) -> dict:
return {
"position": self.position,
# must have values `str`, `int`, `float`, `bool`or nested-dict
"position": {"x": float(self.position.getX()),
"y": float(self.position.getY())}
}


Expand Down Expand Up @@ -173,15 +224,12 @@ def run(self, exposure, referencePsfModel, kernelSum=1.0):
# exposure's bounding box in DM-32756.
sciAvgPos = exposure.getPsf().getAveragePosition()
modelAvgPos = referencePsfModel.getAveragePosition()
try:
fwhmScience = exposure.getPsf().computeShape(sciAvgPos).getDeterminantRadius()*sigma2fwhm
except pexExceptions.RangeError:
raise WarpedPsfTransformTooBigError(
f"Unable to compute the FWHM of the science Psf at {sciAvgPos}"
"due to an unexpectedly large transform."
)
Comment on lines -178 to -182
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@fred3m fred3m Sep 23, 2025

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You don't seem to be catching this exception anymore. Is there a reason that we don't need to worry about this? Is pexExcepetions.RangeError a subset of pexExceptions.Exception?

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@yalsayyad yalsayyad Sep 24, 2025

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pexExceptions.Exception IS a catch all:

>>> import lsst.pex.exceptions as pexExceptions
>>> 
>>> try:
...     raise pexExceptions.RangeError("blah")
... except pexExceptions.Exception as err:
...     print("caught it")
... 
caught it

One of the reasons I sent this to you to review is I was wondering if you were attached to WarpedPsfTransformTooBigError specifically.

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@fred3m fred3m Sep 24, 2025

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It's used in MakePsfMatchedWarpTask to catch errors and make them annotated errors (part of DM-50702). That's a ticket that I completed but without a lot of background. Your PR catches PsfComputeShapeError too so the same failure modes should still return an annotated error, the question is do we want to persist a more informative error about why it failed or not. I have no opinion on this.

except pexExceptions.Exception:
raise PsfComputeShapeError(sciAvgPos)

# TODO If DM-52537 eliminates the need for _safeEstimateRoughShape,
# the remove it, and replace next line with a direct call to
# computeShape in a try/except block.
fwhmScience = _safeEstimateRoughShape(exposure.getPsf(), sciAvgPos,
log=self.log).getDeterminantRadius()*sigma2fwhm
fwhmModel = referencePsfModel.computeShape(modelAvgPos).getDeterminantRadius()*sigma2fwhm

basisList = makeKernelBasisList(self.kConfig, fwhmScience, fwhmModel, metadata=self.metadata)
Expand Down Expand Up @@ -281,7 +329,10 @@ def _buildCellSet(self, exposure, referencePsfModel):
posY = sizeCellY*row + sizeCellY//2 + scienceY0
for col in range(nCellX):
posX = sizeCellX*col + sizeCellX//2 + scienceX0
widthS, heightS = sciencePsfModel.computeBBox(geom.Point2D(posX, posY)).getDimensions()
try:
widthS, heightS = sciencePsfModel.computeBBox(geom.Point2D(posX, posY)).getDimensions()
except pexExceptions.InvalidParameterError as err:
raise PsfComputeShapeError(geom.Point2D(posX, posY)) from err
widthList.append(widthS)
heightList.append(heightS)

Expand Down