Allow xcube serve to use SNAP specific color maps

[AliceBalfanz]

The user wants a non linear color display, to map specific values with a predefined color.
This could be solved through making xcube serve able to import SNAP specific color maps provided by the user.

[AliceBalfanz]

@forman
Should this be implemented by allowing the --styles parameter accept not only matplotlib colormap names, but also files containing the SNAP specific color mapping? Or rather pass the SNAP colorbar file as a new parameter within the xcube serve command? something like --snapcol:
xcube serve --config my_server_config.yml --snapcol snap_specific_color_file.cpd
or should the snap specific color mapping files be included within the source code?

[forman]

Should this be implemented by allowing the --styles parameter accept not only matplotlib colormap names, but also files containing the SNAP specific color mapping?

Yes. But there should also be a parameter, e.g. color_map_file, in the configuration file.

Or rather pass the SNAP colorbar file as a new parameter within the xcube serve command? something like --snapcol

I don't like exposing SNAP-specifcs in the xcube interface. Better make it an implementation detail: If any color map filename has extension .cpd and has valid SNAP-XML format then we read and apply it. In the future we should support a more flexible format allowing color mappings as described here.

[forman]

@AliceBalfanz, here is a new idea how to implement this feature:

We make sure that variables use special color table attributes. These attributes are already written by the SNAP NetCDF Writer anyway. For clarification what is meant here, have a look at our xcube demo cube generated from SNAP-written NetCDFs:

>> import xarray as xr
>> ds = xr.open_dataset('xcube/webapi/res/demo/cube.nc')
>> ds.conc_chl
<xarray.DataArray 'conc_chl' (time: 5, lat: 1000, lon: 2000)>
[10000000 values with dtype=float64]
Coordinates:
  * lat      (lat) float64 52.5 52.5 52.49 52.49 52.49 ... 50.01 50.01 50.0 50.0
  * lon      (lon) float64 0.00125 0.00375 0.00625 0.00875 ... 4.994 4.996 4.999
  * time     (time) datetime64[ns] 2017-01-16T10:09:21.834255872 ... 2017-01-30T10:46:33.836892416
Attributes:
    long_name:                  Chlorophylll concentration
    units:                      mg m^-3
    valid_pixel_expression:     c2rcc_flags.Valid_PE
    color_table_sample_values:  [ 0.   0.5  1.   2.   3.   4.5 13.  25.  30. ...
    color_table_red_values:     [  0   0  51   0   0   0 255 255 215 150]
    color_table_green_values:   [  0   0 102 204 255 128 255  95   0   0]
    color_table_blue_values:    [128 255 255 255 255   0   0   0   0   0]

If those need be overwritten, this would be best implemented in the specific SNAP input processor provided in the class SnapNetcdfInputProcessor that will be used with xcube gen. The input processor can then read any special color table files and set the above attributes accordingly.

Then we would have to change xcube serve to interprete the color table attributes and use them as default when no other style is configured for a given variable.

[AliceBalfanz]

Resolve in a more generic way

• extend xcube dataset specs to also consider color mappings as variable attributes
• allow xcube gen to configure color mappings written as variable attributes
• allow xcube serve to overwrite or specify color mappings
• let xcube serve exploit the color mappings given as variable attributes
• let xcube gen and xcube serve read color mappings from configuration using same syntax
• do not allow ValueRange from server config if a variable has specified colormap which already includes valid value ranges

Other issues

• Address error handling
• Revise CDP parsing code

[AliceBalfanz]

Allow xcube serve to use SNAP specific color maps

[forman]

Closed in favor of #661