I am using QGIS to create a layer of slope region polygons (e.g. less than 30% slope, 30-50%, more than 50% categories). My data comes from point elevations and I interpolated using SAGA b-spline approximation. I then used the Raster Terrain plugin to calculate slope. However, after using the raster calculator and polygonizing the categories of interest, I still have a problem in terms of how neat the borders are. I read about region growing, but am not sure if I have enough information to do so/how to do it.
This above image resembles what I'm aiming for. The bottom image is what I currently have.