Color Composite Images

A color composite image is created from three different variables in a data file.  For example, try rendering radiance data from the Sentinel-3 Ocean and Land Colour Instrument (OLCI) data file.  Use cwrender with the default range, function, and palette settings to create a color enhancement of each band as follows (we use --nolegends to simplify the images):

cwrender --nolegends --enhance EV_BandOa07 example_olci_rad_sep_2022.hdf olci_band7.png
cwrender --nolegends --enhance EV_BandOa06 example_olci_rad_sep_2022.hdf olci_band6.png
cwrender --nolegends --enhance EV_BandOa04 example_olci_rad_sep_2022.hdf olci_band4.png

You should get images that look like those in the table below.

Table 3-3: Sentinel-3 OLCI band radiance data images from cwrender

Band 7 (620 nm)	Band 6 (560 nm)	Band 4 (442 nm)

Now try running cwrender to create a composite image with these three same variables using the --composite option instead of the --enhance option, with legends:

cwrender --verbose --composite EV_BandOa07/EV_BandOa06/EV_BandOa04 example_olci_rad_sep_2022.hdf olci_band764.png

Your output image should look like this:

You'll notice that we also used the --verbose option in cwrender, which shows informational messages while cwrender is running, along with what scaling was computed and used for each variable:

Each pixel in the output image was computed by scaling the data from the three input variables and combining the scaled data into the red, green, and blue (RGB) color components of the pixel.  For example, a pixel in the center of the data was scaled and converted to an RGB color value as shown in Table 3-4.

The resulting RGB value of [163,147,90] shown would create a pixel with this color:         Just as cwrender takes the --range and  --function options for a color enhancement as shown in the previous section, the range and function for scaling the three variables for a color composite can also be controlled using individual options:

--redrange --redfunction
--greenrange --greenfunction
--bluerange --bluefunction

Since these can be cumbersome to type, the --compositehint option is a shortcut — try this and see what image it produces:

cwrender --verbose --composite EV_BandOa07/EV_BandOa06/EV_BandOa04 --compositehint true_color_uncorr example_olci_rad_sep_2022.hdf olci_band764_hint.png

The cwrender tool color composite mode can also be used with variable data from non-visible spectrum sensor bands to create false color composites, and accepts all the overlay options like --coast and --grid shown in the previous section.

Bonus exercises:

• Look at the manual page for cwrender and see what other values --compositehint can take.
• Render the data from the GOCI-II data file to a true color composite. (Hint: the data is in reflectance units in the range [0..1], not radiance units).
• Try rendering the OLCI data using one of the command lines above but at full resolution using a magnification option, for example --magnify -16.5/124/1.