[CodeLab v6.12 for Paint.NET 5.0.12 (Updated February 11, 2024)]

Recently a very simple thought stoke me - we can't use different layers in scripting, but we can use different parts of the image with rectangular selection. Here is example of using 3 "layers".

    int w=(selection.Right-selection.Left)/3;
   for (int y = rect.Top; y < rect.Bottom; y++)
   {
       for (int x = rect.Left; x < rect.Right; x++)
       {
           if (x=selection.Left+2*w)
           {
               continue;
           }
           //here we can change only middle part of the image using left and right "layer".
      }
   }
}

For non-rectangular selections offsets of "layers" have to be passed via parameters.

[CodeLab v6.12 for Paint.NET 5.0.12 (Updated February 11, 2024)]

How to use several layers in the effect? For example, user must made selection in zero layer and effect adds first layer to it with respect to transparency. Or the effect blurs the first image with the radius determined by the first layer intensity. There are a lot of interesting ideas involving more than one layer.