Well, I think I was not clear enough.
In a Grib file, you have various wave datas. Say wind
wave, swell, primary wave, secondary wave and sometime, significant wave or combined wave data, most of them with height and direction values.
You can easily read all these values with zygrib for example.
display only ONE wave data with height and direction(not always).
What value(s) is/are read and/or computed by OpenCPN
to fill the "wave" values in OpenCPN grib_plugin, say height and direction when applicable ?
I could not find a clear response comparing all values in Zygrib and THE data in OpenCPN.