It is possible to manipulate Creep in a Use Map Settings map by using specific triggers. Creep can be forced to decay or even (in limited circumstances) become permanent.
How Creep Decays
Though the exact process for Creep decay is unknown, it appears to work in two stages:
- Kill any Creep tile that has less than four adjacent Creep tiles.
- Kill any Creep tile that has less than three adjacent Creep tiles.
This cycle appears to be map-wide: only 1 piece of Creep on the map can decay during any stage, with generally the Creep that was most recently created being preferred. The two stages and possibly the most-recent-creation clause combine to ensure that when a Creep colony is destroyed, the Creep will decay from the edges inwards. Another result of these rules is that the rate of decay is constant so the total area of Creep that is to decay has a linear relationship with decay time: 20 tiles will decay twice as slowly as 10 tiles, and 100 tiles will decay ten times as slowly as 10 tiles.
To remove Creep with triggers, simply kill a Zerg Extractor, and then remove the underlying Vespene Geyser.
Sacrificial Creep Method
The sacrificial creep method was first described on SEN by Lanthanide. The first method discovered required the use of the creep terrain tiles found in all tilesets, but did not produce optimal results and did not allow all possible configurations of permanent creep. The creep-tile method was superseded by the sacrificial creep method that was briefly explained at the end of Lanthanide's last post in the thread. The sacrificial method prevents any Creep on the map from decaying without regard to when (or by whom) the Creep was created, by creating a special patch of sacrificial creep that will decay during both stages 1 and 2 of the decay cycle, and by constantly refreshing this sacrificial Creep the apparent recent-Creep clause will ensure that the sacrificial Creep decays in preference to any other Creep on the map.
Start by creating two areas that are just large enough to hold Zerg Hatcheries. These regions should be surrounded by a one-tile-thick border of terrain that cannot hold Creep (magma, water, etc.). That border should in turn be surrounded by at least a one-tile-thick border of terrain that can hold Creep. The way this works is that when the hatcheries are created in the central islands, a small border of Creep will be created on the other side of the moat, which is the sacrificial Creep that will decay for us.
Create two Locations over the spots that can hold Zerg Hatcheries. (We shall call these locations "Hatchery A" and "Hatchery B".)
Next, add some pre-placed buildings where you want the permanent Creep to appear. (Note that this method essentially turns all creep into permanent Creep. Here, we are just pre-placing some of that Creep.) In our example, we shall use Zerg Greater Spires owned by Player 1.
Finally, apply these triggers (along with Hyper Triggers).
Here is a run-down of what those triggers do.
Every trigger cycle, we alternate between two Zerg Hatcheries. We create a Hatchery at a spot (creating its Creep in the process), and immediately remove it -- leaving only its Creep behind. The very next trigger cycle, we repeat the process at a second spot. This recreation of Creep-generating Zerg Buildings seems to cause their new Creep to preferentially decay at the first stage of the decay process. This protects all other Creep on the map from decaying at any stage of the decay process, making it permanent. The hatcheries must be created in locations, so that when hatchery B is present, the creep leftover from hatchery A can decay, and vice versa. If you just create the hatchery in the same spot, the sacrificial Creep will never get a chance to decay.
There are some interesting consequences of this system: all existing Creep on the map is made permanent during the time when the hatcheries are being created and removed. This means you can turn the Creep decay on or off at will. If you want to create a new permanent creep patch somewhere, creating Zerg buildings there, remove them and begin the hatchery creation/removal cycle. Then to have the creep decay, stop the hatchery cycle. Note that because all Creep is made permanent, this applies to player created Creep also, so if you have Zerg players in your map, using sacrificial Creep their Creep will never decay (you could potentially build some kind of 'infection' map gameplay around this idea).
The mineral method for creating permanent Creep was first described on SEN by Aristocrat. It has the advantage that it does not affect all Creep on the map (it only affects the targeted creep). However, it can only be pre-placed (before the map loads), and it will be disrupted if a player builds a structure whose Creep overlaps the permanent Creep.
In a map editor, place a neutral (Player 12) Sunken Colony (as either a Unit or a Unit Sprite). Place Minerals over any Creep that you want to become permanent. In-game, with triggers, kill or remove the Sunken Colonies, and then (after at least one trigger loop has passed) remove the Minerals. The Creep from the removed Colonies will start to decay, but the Creep that was beneath the removed Minerals will remain even after the rest of the Creep is gone.
Here is a basic implementation of the effect, as seen in the map editor.
Here is the finished product, after the excess Creep has decayed.
And here are the triggers.
(Because Command continues to detect removed units for the remainder of the trigger cycle in which they were removed, the second trigger only runs on the trigger cycle after the Sunkens are removed.)