With the Zonal statistics plugin, you can analyze the results of a thematic classification. It allows you to calculate several values of the pixels of a raster layer with the help of a polygonal vector layer (see figure_zonal_statistics). Choosing a color band, the plugin generates output columns in the vector layer with an user-defined prefix and calculates for each polygon, statistics on pixels that are within. The available statistics are :
Sum: para somar os valores de pixel
Figure Zonal Statistics