Composites

Built-in Compositors

There are several built-in compositors available in SatPy. All of them use the GenericCompositor base class which handles various image modes (L, LA, RGB, and RGBA at the moment) and updates attributes.

The below sections summarize the composites that come with SatPy and show basic examples of creating and using them with an existing Scene object. It is recommended that any composites that are used repeatedly be configured in YAML configuration files. General-use compositor code dealing with visible or infrared satellite data can be put in a configuration file called visir.yaml. Composites that are specific to an instrument can be placed in YAML config files named accordingly (e.g., seviri.yaml or viirs.yaml). See the satpy repository for more examples.

GenericCompositor

GenericCompositor class can be used to create basic single channel and RGB composites. For example, building an overview composite can be done manually within Python code with:

>>> from satpy.composites import GenericCompositor
>>> compositor = GenericCompositor("overview")
>>> composite = compositor([local_scene[0.6],
...                         local_scene[0.8],
...                         local_scene[10.8]])

One important thing to notice is that there is an internal difference between a composite and an image. A composite is defined as a special dataset which may have several bands (like R, G and B bands). However, the data isn’t stretched, or clipped or gamma filtered until an image is generated. To get an image out of the above composite:

>>> from satpy.writers import to_image
>>> img = to_image(composite)
>>> img.invert([False, False, True])
>>> img.stretch("linear")
>>> img.gamma(1.7)
>>> img.show()

This part is called enhancement, and is covered in more detail in Enhancements.

DifferenceCompositor

DifferenceCompositor calculates a difference of two datasets:

>>> from satpy.composites import DifferenceCompositor
>>> compositor = DifferenceCompositor("diffcomp")
>>> composite = compositor([local_scene[10.8], local_scene[12.0]])

FillingCompositor

FillingCompositor:: fills the missing values in three datasets with the values of another dataset::

>>> from satpy.composites import FillingCompositor
>>> compositor = FillingCompositor("fillcomp")
>>> filler = local_scene[0.6]
>>> data_with_holes_1 = local_scene['ch_a']
>>> data_with_holes_2 = local_scene['ch_b']
>>> data_with_holes_3 = local_scene['ch_c']
>>> composite = compositor([filler, data_with_holes_1, data_with_holes_2,
...                         data_with_holes_3])

PaletteCompositor

PaletteCompositor creates a color version of a single channel categorical dataset using a colormap:

>>> from satpy.composites import PaletteCompositor
>>> compositor = PaletteCompositor("palcomp")
>>> composite = compositor([local_scene['cma'], local_scene['cma_pal']])

The palette should have a single entry for all the (possible) values in the dataset mapping the value to an RGB triplet. Typically the palette comes with the categorical (e.g. cloud mask) product that is being visualized.

DayNightCompositor

DayNightCompositor merges two different composites. The first composite will be placed on the day-side of the scene, and the second one on the night side. The transition from day to night is done by calculating solar zenith angle (SZA) weighed average of the two composites. The SZA can optionally be given as third dataset, and if not given, the angles will be calculated. Width of the blending zone can be defined when initializing the compositor (default values shown in the example below).

>>> from satpy.composites import DayNightCompositor
>>> compositor = DayNightCompositor("dnc", lim_low=85., lim_high=95.)
>>> composite = compositor([local_scene['true_color'],
...                         local_scene['night_fog']])

RealisticColors

RealisticColors compositor is a special compositor that is used to create realistic near-true-color composite from MSG/SEVIRI data:

>>> from satpy.composites import RealisticColors
>>> compositor = RealisticColors("realcols", lim_low=85., lim_high=95.)
>>> composite = compositor([local_scene['VIS006'],
...                         local_scene['VIS008'],
...                         local_scene['HRV']])

CloudCompositor

CloudCompositor can be used to threshold the data so that “only” clouds are visible. These composites can be used as an overlay on top of e.g. static terrain images to show a rough idea where there are clouds. The data are thresholded using three variables:

- `transition_min`: values below or equal to this are clouds -> opaque white
- `transition_max`: values above this are cloud free -> transparent
- `transition_gamma`: gamma correction applied to clarify the clouds

Usage (with default values):

>>> from satpy.composites import CloudCompositor
>>> compositor = CloudCompositor("clouds", transition_min=258.15,
...                              transition_max=298.15,
...                              transition_gamma=3.0)
>>> composite = compositor([local_scene[10.8]])

Support for using this compositor for VIS data, where the values for high/thick clouds tend to be in reverse order to brightness temperatures, is to be added.

RatioSharpenedRGB

RatioSharpenedRGB

SelfSharpenedRGB

SelfSharpenedRGB sharpens the RGB with ratio of a band with a strided version of itself.

LuminanceSharpeningCompositor

LuminanceSharpeningCompositor replaces the luminance from an RGB composite with luminance created from reflectance data. If the resolutions of the reflectance data _and_ of the target area definition are higher than the base RGB, more details can be retrieved. This compositor can be useful also with matching resolutions, e.g. to highlight shadowing at cloudtops in colorized infrared composite.

>>> from satpy.composites import LuminanceSharpeningCompositor
>>> compositor = LuminanceSharpeningCompositor("vis_sharpened_ir")
>>> vis_data = local_scene['HRV']
>>> colorized_ir_clouds = local_scene['colorized_ir_clouds']
>>> composite = compositor([vis_data, colorized_ir_clouds])

SandwichCompositor

Similar to LuminanceSharpeningCompositor, SandwichCompositor uses reflectance data to bring out more details out of infrared or low-resolution composites. SandwichCompositor multiplies the RGB channels with (scaled) reflectance.

>>> from satpy.composites import SandwichCompositor
>>> compositor = SandwichCompositor("ir_sandwich")
>>> vis_data = local_scene['HRV']
>>> colorized_ir_clouds = local_scene['colorized_ir_clouds']
>>> composite = compositor([vis_data, colorized_ir_clouds])

Creating composite configuration files

To save the custom composite, the following procedure can be used:

  1. Create a custom directory for your custom configs.
  2. Set the environment variable PPP_CONFIG_DIR to this path.
  3. Write config files with your changes only (see examples below), pointing to the (custom) module containing your composites. Generic compositors can be placed in $PPP_CONFIG_DIR/composites/visir.yaml and instrument- specific ones in $PPP_CONFIG_DIR/composites/<sensor>.yaml. Don’t forget to add changes to the enhancement/generic.yaml file too.
  4. If custom compositing code was used then it must be importable by python. If the code is not installed in your python environment then another option it to add it to your PYTHONPATH.

With that, you should be able to load your new composite directly. Example configuration files can be found in the satpy repository as well as a few simple examples below.

Simple RGB composite

This is the overview composite shown in the first code example above using GenericCompositor:

sensor_name: visir

composites:
  overview:
    compositor: !!python/name:satpy.composites.GenericCompositor
    prerequisites:
    - 0.6
    - 0.8
    - 10.8
    standard_name: overview

For an instrument specific version (here MSG/SEVIRI), we should use the channel _names_ instead of wavelengths. Note also that the sensor_name is now combination of visir and seviri, which means that it extends the generic visir composites:

sensor_name: visir/seviri

composites:

  overview:
    compositor: !!python/name:satpy.composites.GenericCompositor
    prerequisites:
    - VIS006
    - VIS008
    - IR_108
    standard_name: overview

In the following examples only the composite receipes are shown, and the header information (sensor_name, composites) and intendation needs to be added.

Using modifiers

In many cases the basic datasets need to be adjusted, e.g. for Solar zenith angle normalization. These modifiers can be applied in the following way:

overview:
  compositor: !!python/name:satpy.composites.GenericCompositor
  prerequisites:
  - name: VIS006
    modifiers: [sunz_corrected]
  - name: VIS008
    modifiers: [sunz_corrected]
  - IR_108
  standard_name: overview

Here we see two changes:

  1. channels with modifiers need to have either name or wavelength added in front of the channel name or wavelength, respectively
  2. a list of modifiers attached to the dictionary defining the channel

The modifier above is a built-in that normalizes the Solar zenith angle to Sun being directly at the zenith.

Using other composites

Often it is handy to use other composites as a part of the composite. In this example we have one composite that relies on solar channels on the day side, and another for the night side:

natural_with_night_fog:
  compositor: !!python/name:satpy.composites.DayNightCompositor
  prerequisites:
    - natural_color
    - night_fog
  standard_name: natural_with_night_fog

This compositor has two additional keyword arguments that can be defined (shown with the default values, thus identical result as above):

natural_with_night_fog:
  compositor: !!python/name:satpy.composites.DayNightCompositor
  prerequisites:
    - natural_color
    - night_fog
  lim_low: 85.0
  lim_high: 95.0
  standard_name: natural_with_night_fog

Defining other composites in-line

It is also possible to define sub-composites in-line. This example is the built-in airmass composite:

airmass:
  compositor: !!python/name:satpy.composites.GenericCompositor
  prerequisites:
  - compositor: !!python/name:satpy.composites.DifferenceCompositor
    prerequisites:
    - wavelength: 6.2
    - wavelength: 7.3
  - compositor: !!python/name:satpy.composites.DifferenceCompositor
    prerequisites:
      - wavelength: 9.7
      - wavelength: 10.8
  - wavelength: 6.2
  standard_name: airmass

Enhancing the images

After the composite is defined and created, it needs to be converted to an image. To do this, it is necessary to describe how the data values are mapped to values stored in the image format. This procedure is called stretching, and in SatPy it is implemented by enhancements.

The first step is to convert the composite to an XRImage object:

>>> from satpy.writers import to_image
>>> img = to_image(composite)

Now it is possible to apply enhancements available in the class:

>>> img.invert([False, False, True])
>>> img.stretch("linear")
>>> img.gamma(1.7)

And finally either show or save the image:

>>> img.show()
>>> img.save('image.tif')

As pointed out in the composite section, it is better to define frequently used enhancements in configuration files under $PPP_CONFIG_DIR/enhancements/. The enhancements can either be in generic.yaml or instrument-specific file (e.g., seviri.yaml).

The above enhancement can be written (with the headers necessary for the file) as:

enhancements:
  overview:
    standard_name: overview
    operations:
      - name: inverse
        method: !!python/name:satpy.enhancements.invert
        args: [False, False, True]
      - name: stretch
        method: !!python/name:satpy.enhancements.stretch
        kwargs:
          stretch: linear
      - name: gamma
        method: !!python/name:satpy.enhancements.gamma
        kwargs:
          gamma: [1.7, 1.7, 1.7]

More examples can be found in SatPy source code directory satpy/etc/enhancements/generic.yaml.

See the Enhancements documentation for more information on available built-in enhancements.