Composites are defined as arrays of data that are created by processing and/or combining one or multiple data arrays (prerequisites) together.
Composites are generated in satpy using Compositor classes. The attributes of the resulting composites are usually a combination of the prerequisites' attributes and the key/values of the DataID used to identify it.
There are many built-in compositors available in Satpy. The majority 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 ~satpy.scene.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
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
.
Single channel composites can also be generated with the GenericCompositor
, but in some cases, the SingleBandCompositor
may be more appropriate. For example, the GenericCompositor
removes attributes such as units
because they are typically not meaningful for an RGB image. Such attributes are retained in the SingleBandCompositor
.
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
:: 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
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
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. Three arguments are used to generate the image (default values shown in the example below). They can be defined when initializing the compositor:
- lim_low (float): lower limit of Sun zenith angle for the
blending of the given channels
- lim_high (float): upper limit of Sun zenith angle for the
blending of the given channels
Together with `lim_low` they define the width
of the blending zone
- day_night (string): "day_night" means both day and night portions will be kept
"day_only" means only day portion will be kept
"night_only" means only night portion will be kept
Usage (with default values):
>>> from satpy.composites import DayNightCompositor
>>> compositor = DayNightCompositor("dnc", lim_low=85., lim_high=88., day_night="day_night")
>>> composite = compositor([local_scene['true_color'],
... local_scene['night_fog']])
As above, with day_night flag it is also available to use only a day product or only a night product and mask out (make transparent) the opposite portion of the image (night or day). The example below provides only a day product with night portion masked-out:
>>> from satpy.composites import DayNightCompositor
>>> compositor = DayNightCompositor("dnc", lim_low=85., lim_high=88., day_night="day_only")
>>> composite = compositor([local_scene['true_color'])
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
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
SelfSharpenedRGB
sharpens the RGB with ratio of a band with a strided version of itself.
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])
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])
StaticImageCompositor
can be used to read an image from disk and used just like satellite data, including resampling and using as a part of other composites.>>> from satpy.composites import StaticImageCompositor >>> compositor = StaticImageCompositor("static_image", filename="image.tif") >>> composite = compositor()
BackgroundCompositor
can be used to stack two composites together. If the composites don't have alpha channels, the background is used where foreground has no data. If foreground has alpha channel, the alpha values are used to weight when blending the two composites.>>> from satpy import Scene >>> from satpy.composites import BackgroundCompositor >>> compositor = BackgroundCompositor() >>> clouds = local_scene['ir_cloud_day'] >>> background = local_scene['overview'] >>> composite = compositor([clouds, background])
CategoricalDataCompositor
can be used to recategorize categorical data. This is for example useful to combine comparable categories into a common category. The category remapping from data to composite is done using a look-up-table (lut):
composite = [[lut[data[0,0]], lut[data[0,1]], lut[data[0,Nj]]],
[[lut[data[1,0]], lut[data[1,1]], lut[data[1,Nj]],
[[lut[data[Ni,0]], lut[data[Ni,1]], lut[data[Ni,Nj]]]
Hence, lut must have a length that is greater than the maximum value in data in orer to avoid an IndexError. Below is an example on how to create a binary clear-sky/cloud mask from a pseodu cloud type product with six categories representing clear sky (cat1/cat5), cloudy features (cat2-cat4) and missing/undefined data (cat0):
>>> cloud_type = local_scene['cloud_type'] # 0 - cat0, 1 - cat1, 2 - cat2, 3 - cat3, 4 - cat4, 5 - cat5,
# categories: 0 1 2 3 4 5
>>> lut = [np.nan, 0, 1, 1, 1, 0]
>>> compositor = CategoricalDataCompositor('binary_cloud_mask', lut=lut)
>>> composite = compositor([cloud_type]) # 0 - cat1/cat5, 1 - cat2/cat3/cat4, nan - cat0
To save the custom composite, follow the component_configuration
documentation. Once your component configuration directory is created you can create your custom composite YAML configuration files. Compositors that can be used for multiple instruments can be placed in the generic $SATPY_CONFIG_PATH/composites/visir.yaml
file. Composites that are specific to one sensor should be placed in $SATPY_CONFIG_PATH/composites/<sensor>.yaml
. Custom enhancements for your new composites can be stored in $SATPY_CONFIG_PATH/enhancements/generic.yaml
or $SATPY_CONFIG_PATH/enhancements/<sensor>.yaml
.
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.
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.
In many cases the basic datasets that go into the composite 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:
- channels with modifiers need to have either name or wavelength added in front of the channel name or wavelength, respectively
- 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.
More examples can be found in Satpy source code directory satpy/etc/composites.
See the modifiers
documentation for more information on available built-in modifiers.
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 three 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: 88.0
day_night: "day_night"
standard_name: natural_with_night_fog
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
Below is an example composite config using StaticImageCompositor
, DayNightCompositor
, CloudCompositor
and BackgroundCompositor
to show how to create a composite with a blended day/night imagery as background for clouds. As the images are in PNG format, and thus not georeferenced, the name of the area definition for the background images are given. When using GeoTIFF images the area parameter can be left out.
Note
The background blending uses the current time if there is no timestamps in the image filenames.
clouds_with_background:
compositor: !!python/name:satpy.composites.BackgroundCompositor
standard_name: clouds_with_background
prerequisites:
- ir_cloud_day
- compositor: !!python/name:satpy.composites.DayNightCompositor
prerequisites:
- static_day
- static_night
static_day:
compositor: !!python/name:satpy.composites.StaticImageCompositor
standard_name: static_day
filename: /path/to/day_image.png
area: euro4
static_night:
compositor: !!python/name:satpy.composites.StaticImageCompositor
standard_name: static_night
filename: /path/to/night_image.png
area: euro4
To ensure that the images aren't auto-stretched and possibly altered, the following should be added to enhancement config (assuming 8-bit image) for both of the static images:
static_day:
standard_name: static_day
operations:
- name: stretch
method: !!python/name:satpy.enhancements.stretch
kwargs:
stretch: crude
min_stretch: [0, 0, 0]
max_stretch: [255, 255, 255]
Explain how composite names, composite standard_name, enhancement names, and enhancement standard_name are related to each other
Explain what happens when no enhancement is configured for a product (= use the default enhancement).
Explain that the methods are often just a wrapper for XRImage methods, but can also be something completely custom.
List and explain in detail the built-in enhancements:
- stretch
- gamma
- invert
- crefl_scaling
- cira_stretch
- lookup
- colorize
- palettize
- three_d_effect
- btemp_threshold
Should this be in another file/page?
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 ~trollimage.xrimage.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 $SATPY_CONFIG_PATH/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]
Warning
If you define a composite with no matching enhancement, Satpy will by default apply the ~trollimage.xrimage.XRImage.stretch_linear
enhancement with cutoffs of 0.5% and 99.5%. If you want no enhancement at all (maybe you are enhancing a composite based on DayNightCompositor
where the components have their own enhancements defined), you need to define an enhancement that does nothing:
- enhancements:
- day_x:
standard_name: day_x operations: []
It is recommended to define an enhancement even if you intend to use the default, in case the default should change in future versions of Satpy.
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.