Painting foliage through a script - what's wrong here?

I'm making a script that paints the detail layer of the terrain (aka foliage) within a given polygon projection. The polygon projection part does work, but what's bugged is the filling itself - the foliage appears striped - lines altering between overgrown and bare, like in this image:

Been sniffing around this for hours and can't really find what's wrong. Here's some relevant code:

 private void PaintFoliage(Polygon polygon, int layerIndex, int intensity)
         {
             int[,] splatmapData = activeTerrainData.GetDetailLayer(0, 0, activeTerrainData.detailWidth, activeTerrainData.detailHeight, layerIndex);
 
             
             for (int y = 0; y < activeTerrainData.detailHeight; y++)
             {
                 for (int x = 0; x < activeTerrainData.detailWidth; x++)
                 {
                     Vector3 worldSpacePosition = ConvertTerrainToWorldSpace(new Vector2(x, y),
                         new Vector2(activeTerrainData.detailWidth, activeTerrainData.detailHeight));
                     if (IsPointInPolygon(polygon, worldSpacePosition))
                     {
                         splatmapData[x, y] = intensity;
                     }
                 }
             }
             activeTerrainData.SetDetailLayer(0, 0, layerIndex, splatmapData);
         }
 
 bool IsPointInPolygon(Polygon polygon, Vector3 point) 
         {
             bool result = false;
             var a = polygon.Points[polygon.Points.Length - 1];
             foreach (var b in polygon.Points)
             {
                 if ((b.x == point.x) && (b.z == point.z))
                     return true;
 
                 if ((b.z == a.z) && (point.z == a.z) && (a.x <= point.x) && (point.x <= b.x))
                     return true;
 
                 if ((b.z < point.z) && (a.z >= point.z) || (a.z < point.z) && (b.z >= point.z))
                 {
                     if (b.x + (point.z - b.z) / (a.z - b.z) * (a.x - b.x) <= point.x)
                         result = !result;
                 }
                 a = b;
             }
             return result;
         }
 
 Vector3 ConvertTerrainToWorldSpace(Vector2 coords, Vector2 limits)
         {
             Vector3 answer = new Vector3(
                 Terrain.activeTerrain.transform.position.x + (coords.y / limits.x) * Terrain.activeTerrain.terrainData.size.x, 0,
                 Terrain.activeTerrain.transform.position.z + (coords.x / limits.y) * Terrain.activeTerrain.terrainData.size.z);
             return answer;
         }

Curiously enough, I've made a very similar script dealing with painting terrain and it works like a charm. I'd be very thankful for pointing out what's wrong here.