Right, without firing up my other PC & grabbing screenshots, a step filter is a filter that has zero value along the bottom till halfway along, then will suddenly jump up to full value (one) in one step, so that half the filter is grey and the second half is white. Imagine a grey scale along the bottom of the filter, going from black on the left through all the shades of grey to white at the far right hand side. These are your grey shades in your terrain image. Now imagine, on the other axis (the "y" axis) another scale, except that this one is "amount of trees". Zero value (bottom edge of filter) represents no trees, while the top edge of the filter (maximum value) represents lots of trees. The step filter will generate no trees for all image values from black to mid-grey, after that any grey tone up to white will generate trees. I will put up some pictures explaining more if anyone really wants them.