title: How to extract polygons from raster?
body: <p>The goal is to get the vector polygons from raster areas accordingly to their gray scale. For example: </p>
  
  <ul>
  <li>Dark pixels to dark_relief.shp   </li>
  <li>Gray pixels to gray_relief.shp   </li>
  <li>Light pixels to light_relief.shp</li>
  </ul>
  
  <p>First I got GeoTIFF from SRTM file using Raster – Analysis – DEM (Terrain models) in Hillshade mode:</p>
  
  <p><a href="https://i.stack.imgur.com/imGcs.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/imGcs.jpg" alt="enter image description here"></a></p>
  
  <p>Then I use Raster - Conversion - Polygonize (Raster to Vector), but result is too many small polygons. Obviously I should prepare the image before a conversion.</p>
  
  <p>Any hints what to do?
  May be other right way exists?</p>
  
  <p>Regards!</p>