I tried using that method in my map, but it didn't work. I did some investigation and found something very interesting. The null terrain being used in that map is essentially regular null terrain, but also has a creep-type property. That is, if you place a unit on that terrain, they can walk around and build on it as normal, but if the unit is standing next to that terrain on regular walkable terrain, they will not be able to walk onto the null terrain or build on it. The creep-type property is that creep that is present on the terrain will not start decaying until after all creep has decayed off all other regular tiles in the map. In the test map in the 2nd thread above, there are no non-null tiles present, so for some reason the creep never decays. This doesn't seem particularly useful unless you want your entire map to be creep.
Turns out that the creep tiles share the same properties as the null tiles, except that they are normally walkable and zerg players can build hatcheries on them. The decay properties of these tiles can therefore be used to provide permanent creep.
So the idea goes like this:
1. Place the creep-looking tiles in the area you want permanent creep
2. Cover these tiles with zerg buildings, preferrably things like defiler mound and spires to get the particular creep coverage that you want
3. When the map starts, remove the buildings placed at step 2 (so no dead zerg building sprite is created)
4. Create a zerg building for any player at some spot on the map, remove the building immediately (leaves creep behind)
5. 1 trigger cycle after step 4, create another zerg building at a different spot in the map, remove the building immediately (leaves creep behind)
6. Repeat steps 4 and 5 on alternating trigger cycles.
It is important that you create the buildings in step 4 and 5 in different locations. If you simply create the buildings on the same spot and remove them every trigger cycle, the creep will never decay as it is being replenished, and therefore the creep you placed in steps 1 and 2 will decay instead. For this reason it is better to use one of the zerg buildings that has creep only directly under the building, as then you can have both buildings next to each other. Alternatively you could create a hatchery at step 4 and then wait until all of its creep has decayed (probably about 15-20 seconds) before creating another one, however you have to be sure that during the trigger cycle in which you created the hatchery that there is still creep somewhere on the map that is decaying, which is why the 2 building approach works well.
One odd thing is that the creep created by the buildings in step 2 will decay at the boundaries between the creep tiles from step 1 and regular tiles next to them, in sort of a diagonal-death pattern (game of life, anyone?). This is kinda tricky to explain, so I incuded several different patterns of crep tiles in my test map so you can see what the resulting permanent patches look like.
In my test map, you need to wait 15-20 seconds until all of the excess creep has died, and after that you will see the permanent creep remaining. You can build buildings on it all without issue, and use the drones to destroy the buildings and put new ones in their place if you want.
Edit: I've done some more playing around with different shapes, and it seems the the location of the buildings removed will affect the shape of the final permanent creep. I tried a small 2x2 creep tile spot in the center of a 4x4 tile area. I placed 2 defiler mounds (and also tried 4 spires) to cover the entire 4x4 area. Once the excess creep has decayed, the resulting shape is a fat cross 2 tiles wide on each arm (eg, the 4 corners of the 4x4 square are gone), whereas if you just place a single spire on the original 2x2 creep tiles, it will decay away to nothing. I have not updated the test map with this example.
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Post has been edited 2 time(s), last time on Feb 12 2010, 9:13 am by Lanthanide.
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