ScaleVarianceTask¶
- class lsst.meas.algorithms.ScaleVarianceTask(*args, **kwargs)¶
Bases:
Task
Scale the variance in a MaskedImage
The variance plane in a convolved or warped image (or a coadd derived from warped images) does not accurately reflect the noise properties of the image because variance has been lost to covariance. This Task attempts to correct for this by scaling the variance plane to match the observed variance in the image. This is not perfect (because we’re not tracking the covariance) but it’s simple and is often good enough.
The task implements a pixel-based and an image-based correction estimator.
Methods Summary
computeScaleFactors
(maskedImage)Calculate and return both variance scaling factors without modifying the image.
Empty (clear) the metadata for this Task and all sub-Tasks.
Get metadata for all tasks.
Get the task name as a hierarchical name including parent task names.
getName
()Get the name of the task.
Get a dictionary of all tasks as a shallow copy.
imageBased
(maskedImage)Determine the variance rescaling factor from image statistics
makeField
(doc)Make a
lsst.pex.config.ConfigurableField
for this task.makeSubtask
(name, **keyArgs)Create a subtask as a new instance as the
name
attribute of this task.pixelBased
(maskedImage)Determine the variance rescaling factor from pixel statistics
run
(maskedImage)Rescale the variance in a maskedImage in place.
subtractedBackground
(maskedImage)Context manager for subtracting the background
timer
(name[, logLevel])Context manager to log performance data for an arbitrary block of code.
Methods Documentation
- computeScaleFactors(maskedImage)¶
Calculate and return both variance scaling factors without modifying the image.
- Parameters:
- maskedImage
lsst.afw.image.MaskedImage
Image for which to determine the variance rescaling factor.
- maskedImage
- Returns:
- getFullMetadata() TaskMetadata ¶
Get metadata for all tasks.
- Returns:
- metadata
TaskMetadata
The keys are the full task name. Values are metadata for the top-level task and all subtasks, sub-subtasks, etc.
- metadata
Notes
The returned metadata includes timing information (if
@timer.timeMethod
is used) and any metadata set by the task. The name of each item consists of the full task name with.
replaced by:
, followed by.
and the name of the item, e.g.:topLevelTaskName:subtaskName:subsubtaskName.itemName
using
:
in the full task name disambiguates the rare situation that a task has a subtask and a metadata item with the same name.
- getFullName() str ¶
Get the task name as a hierarchical name including parent task names.
- Returns:
- fullName
str
The full name consists of the name of the parent task and each subtask separated by periods. For example:
The full name of top-level task “top” is simply “top”.
The full name of subtask “sub” of top-level task “top” is “top.sub”.
The full name of subtask “sub2” of subtask “sub” of top-level task “top” is “top.sub.sub2”.
- fullName
- getName() str ¶
Get the name of the task.
- Returns:
- taskName
str
Name of the task.
- taskName
See also
getFullName
Get the full name of the task.
- getTaskDict() dict[str, weakref.ReferenceType[lsst.pipe.base.task.Task]] ¶
Get a dictionary of all tasks as a shallow copy.
- Returns:
- taskDict
dict
Dictionary containing full task name: task object for the top-level task and all subtasks, sub-subtasks, etc.
- taskDict
- imageBased(maskedImage)¶
Determine the variance rescaling factor from image statistics
We calculate average(SNR) = stdev(image)/median(variance), and the value should be unity. We use the interquartile range as a robust estimator of the stdev. The variance rescaling factor is the factor that brings this value to unity.
This may not work well if the pixels from which we measure the standard deviation of the image are not effectively the same pixels from which we measure the median of the variance. In that case, use an alternate method.
- Parameters:
- maskedImage
lsst.afw.image.MaskedImage
Image for which to determine the variance rescaling factor.
- maskedImage
- Returns:
- factor
float
Variance rescaling factor or 1 if all pixels are masked or non-finite.
- factor
- classmethod makeField(doc: str) ConfigurableField ¶
Make a
lsst.pex.config.ConfigurableField
for this task.- Parameters:
- doc
str
Help text for the field.
- doc
- Returns:
- configurableField
lsst.pex.config.ConfigurableField
A
ConfigurableField
for this task.
- configurableField
Examples
Provides a convenient way to specify this task is a subtask of another task.
Here is an example of use:
class OtherTaskConfig(lsst.pex.config.Config): aSubtask = ATaskClass.makeField("brief description of task")
- makeSubtask(name: str, **keyArgs: Any) None ¶
Create a subtask as a new instance as the
name
attribute of this task.- Parameters:
- name
str
Brief name of the subtask.
- **keyArgs
Extra keyword arguments used to construct the task. The following arguments are automatically provided and cannot be overridden:
config
.parentTask
.
- name
Notes
The subtask must be defined by
Task.config.name
, an instance ofConfigurableField
orRegistryField
.
- pixelBased(maskedImage)¶
Determine the variance rescaling factor from pixel statistics
We calculate SNR = image/sqrt(variance), and the distribution for most of the background-subtracted image should have a standard deviation of unity. We use the interquartile range as a robust estimator of the SNR standard deviation. The variance rescaling factor is the factor that brings that distribution to have unit standard deviation.
This may not work well if the image has a lot of structure in it, as the assumptions are violated. In that case, use an alternate method.
- Parameters:
- maskedImage
lsst.afw.image.MaskedImage
Image for which to determine the variance rescaling factor.
- maskedImage
- Returns:
- factor
float
Variance rescaling factor or 1 if all pixels are masked or non-finite.
- factor
- run(maskedImage)¶
Rescale the variance in a maskedImage in place.
- Parameters:
- maskedImage
lsst.afw.image.MaskedImage
Image for which to determine the variance rescaling factor. The image is modified in place.
- maskedImage
- Returns:
- factor
float
Variance rescaling factor.
- factor
- Raises:
- RuntimeError
If the estimated variance rescaling factor by both methods exceed the configured limit.
Notes
The task calculates and applies the pixel-based correction unless it is over the
config.limit
threshold. In this case, the image-based method is applied.
- subtractedBackground(maskedImage)¶
Context manager for subtracting the background
We need to subtract the background so that the entire image (apart from objects, which should be clipped) will have the image/sqrt(variance) distributed about zero.
This context manager subtracts the background, and ensures it is restored on exit.
- Parameters:
- maskedImage
lsst.afw.image.MaskedImage
Image+mask+variance to have background subtracted and restored.
- maskedImage
- Returns:
- contextcontext manager
Context manager that ensure the background is restored.