It works! :D

worldeditadditions/lib/conv/convolution.lua

@@ -41,6 +41,15 @@ end
 function worldeditadditions.convolve(pos1, pos2, kernel, kernel_size)
 	pos1, pos2 = worldedit.sort_pos(pos1, pos2)
 	
+	local border_size = {}
+	border_size[0] = (kernel_size[0]-1) / 2		-- height
+	border_size[1] = (kernel_size[1]-1) / 2		-- width
+	
+	pos1.z = pos1.z - border_size[0]
+	pos2.z = pos2.z + border_size[0]
+	pos1.x = pos1.x - border_size[1]
+	pos2.x = pos2.x + border_size[1]
+	
 	local manip, area = worldedit.manip_helpers.init(pos1, pos2)
 	local data = manip:get_data()
 	
@@ -52,8 +61,8 @@ function worldeditadditions.convolve(pos1, pos2, kernel, kernel_size)
 	local heightmap_conv = worldeditadditions.shallowcopy(heightmap)
 	
 	local heightmap_size = {}
-	heightmap_size[0] = pos2.z - pos1.z
-	heightmap_size[1] = pos2.x - pos1.x
+	heightmap_size[0] = (pos2.z - pos1.z) + 1
+	heightmap_size[1] = (pos2.x - pos1.x) + 1
 	
 	worldeditadditions.conv.convolve(
 		heightmap_conv,
@@ -61,25 +70,46 @@ function worldeditadditions.convolve(pos1, pos2, kernel, kernel_size)
 		kernel,
 		kernel_size
 	)
+	
+	-- print("original")
+	-- worldeditadditions.print_2d(heightmap, (pos2.z - pos1.z) + 1)
+	-- print("transformed")
+	-- worldeditadditions.print_2d(heightmap_conv, (pos2.z - pos1.z) + 1)
+	-- It seems to be convolving as intended, but something's probably getting lost in translation below
+	
 	for z = heightmap_size[0], 0, -1 do
 		for x = heightmap_size[1], 0, -1 do
 			local hi = z*heightmap_size[1] + x
-			local diff = heightmap_conv[hi] - heightmap[hi]
+			
+			
+			local height_old = heightmap[hi]
+			local height_new = heightmap_conv[hi]
+			-- print("[conv/save] hi", hi, "height_old", heightmap[hi], "height_new", heightmap_conv[hi], "z", z, "x", x, "pos1.y", pos1.y)
 			
 			-- Lua doesn't have a continue statement :-/
-			if diff ~= 0 then
+			if height_old == height_new then
+				-- noop
+			elseif height_new < height_old then
+				stats.removed = stats.removed + (height_old - height_new)
+				local y = height_new
+				while y < height_old do
+					local ci = area:index(pos1.x + x, pos1.y + y, pos1.z + z)
+					-- print("[conv/save] remove at y", y, "→", pos1.y + y, "current:", minetest.get_name_from_content_id(data[ci]))
+					data[ci] = node_id_air
+					y = y + 1
+				end
+			else -- height_new > height_old
 				-- We subtract one because the heightmap starts at 1 (i.e. 1 = 1 node in the column), but the selected region is inclusive
-				local node_id = data[area:index(pos1.x + x, pos1.y + (heightmap[hi] - 1), pos1.z + z)]
-				if diff > 0 then
-					stats.added = stats.added + diff
-					for y = pos1.y + heightmap[hi], pos1.y + heightmap_conv[hi], 1 do
-						data[area:index(pos1.x + x, y, pos1.z + z)] = node_id
-					end
-				else
-					stats.removed = stats.removed + math.abs(diff)
-					for y = pos1.y + heightmap_conv[hi], pos1.y + heightmap[hi], 1 do
-						data[area:index(pos1.x + x, y, pos1.z + z)] = node_id_air
-					end
+				local node_id = data[area:index(pos1.x + x, pos1.y + (height_old - 1), pos1.z + z)]
+				-- print("[conv/save] filling with ", node_id, "→", minetest.get_name_from_content_id(node_id))
+				
+				stats.added = stats.added + (height_new - height_old)
+				local y = height_old
+				while y < height_new do
+					local ci = area:index(pos1.x + x, pos1.y + y, pos1.z + z)
+					-- print("[conv/save] add at y", y, "→", pos1.y + y, "current:", minetest.get_name_from_content_id(data[ci]))
+					data[ci] = node_id
+					y = y + 1
 				end
 			end
 		end

worldeditadditions/lib/conv/convolve.lua

@@ -18,31 +18,40 @@ function worldeditadditions.conv.convolve(heightmap, heightmap_size, matrix, mat
 	border_size[1] = (matrix_size[1]-1) / 2		-- width
 	print("[convolve] matrix_size", matrix_size[0], matrix_size[1])
 	print("[convolve] border_size", border_size[0], border_size[1])
+	print("[convolve] heightmap_size: ", heightmap_size[0], heightmap_size[1])
+	
+	print("[convolve] z: from", (heightmap_size[0]-border_size[0]) - 1, "to", border_size[0], "step", -1)
+	print("[convolve] x: from", (heightmap_size[1]-border_size[1]) - 1, "to", border_size[1], "step", -1)
 	
 	-- Convolve over only the bit that allows us to use the full convolution matrix
-	for y = (heightmap_size[0]-border_size[0]) - 1, border_size[0], -1 do
+	for z = (heightmap_size[0]-border_size[0]) - 1, border_size[0], -1 do
 		for x = (heightmap_size[1]-border_size[1]) - 1, border_size[1], -1 do
 			local total = 0
 			
 			
-			for my = matrix_size[0]-1, 0, -1 do
+			local hi = (z * heightmap_size[1]) + x
+			-- print("[convolve/internal] z", z, "x", x, "hi", hi)
+			
+			-- No continue statement in Lua :-/
+			if heightmap[hi] ~= -1 then
+				for mz = matrix_size[0]-1, 0, -1 do
 					for mx = matrix_size[1]-1, 0, -1 do
-					local mi = (my * matrix_size[1]) + mx
-					local cy = y + (my - border_size[0])
+						local mi = (mz * matrix_size[1]) + mx
+						local cz = z + (mz - border_size[0])
 						local cx = x + (mx - border_size[1])
 						
-					local i = (cy * heightmap_size[1]) + cx
+						local i = (cz * heightmap_size[1]) + cx
 						
-					-- print("[convolve] i", i, "mi", mi, "matrix[mi]", matrix[mi], "heightmap[i]", heightmap[i])
 						-- A value of -1 = nothing in this column (so we should ignore it)
 						if heightmap[i] ~= -1 then
 							total = total + (matrix[mi] * heightmap[i])
 						end
 					end
 				end
-			
 				-- Rounding hack - ref https://stackoverflow.com/a/18313481/1460422
-			heightmap[(y * heightmap_size[1]) + x] = math.floor(total + 0.5)
+				-- heightmap[hi] = math.floor(total + 0.5)
+				heightmap[hi] = math.ceil(total)
+			end
 		end
 	end
 	

worldeditadditions/utils/strings.lua

@@ -95,7 +95,7 @@ function worldeditadditions.print_2d(tbl, width)
 	for i=0, #tbl do
 		table.insert(next, worldeditadditions.str_padstart(tostring(tbl[i]), display_width))
 		if #next == width then
-			print(table.concat(next, "\t"))
+			print(table.concat(next, ""))
 			next = {}
 		end
 	end