I have done a classification using the semi-automatic classification plugin, and now, I would like to express each class in percentage of the study area (e.g. forest represent 20% of the study area and so one). Does anyone know how to get that ?
Try the GRASS module
r.stats. It allows to print out either area totals, or percents or area for each category. (Available in the Processing Toolbox)
In raster structure you can count number of pixel which fall into a class, multiply for area of pixel then area_class_forest/area_of_raster. With vector structure you must have polygon then with a SQL query is possible obtain a table with class and percent area.