code: public static void filterWalkableLowHeightSpans(Context ctx, int walkableHeight, Heightfield solid) {
          ctx.startTimer("FILTER_WALKABLE");
  
          int w = solid.width;
          int h = solid.height;
          int MAX_HEIGHT = 0xffff;
  
          // Remove walkable flag from spans which do not have enough
          // space above them for the agent to stand there.
          for (int y = 0; y < h; ++y) {
              for (int x = 0; x < w; ++x) {
                  for (Span s = solid.spans[x + y * w]; s != null; s = s.next) {
                      int bot = (s.smax);
                      int top = s.next != null ? s.next.smin : MAX_HEIGHT;
                      if ((top - bot) <= walkableHeight)
                          s.area = RecastConstants.RC_NULL_AREA;
                  }
              }
          }
          ctx.stopTimer("FILTER_WALKABLE");
      }
docs: / @see rcHeightfield, rcConfig