How to damage units under dark swarm

There are three terrain levels in Brood War: low, medium and high ground. These are determined by minitile flags (small areas of 8x8 pixels). A unit's terrain level is determined by the flag of the minitile at its exact position, below the centre point of its collision box, coordinates rounded up to full pixels. Miss chance is the same from low to medium, low to high and medium to high ground.

Units do not actually have to be positioned visibly behind the doodad sprite, in fact for most doodads this will have no effect at all, except for visibly obscuring the unit. To gain cover they need to be placed on one of the doodad tiles.
A tile is a basic terrain block of 32x32 pixels. Unit position is determined as below the centre point of its collision box, coordinates rounded up to full pixels.
Cover providing doodads roughly encompass the following :

• tree doodads in Badlands
  
• tree doodads in Jungle
  
• most of the hut doodads in Desert as well as most of the bulkier plant doodads and the big derelict CC and derelict factory doodads.
  
• the more bulky building on snow doodads in Ice.
  
• none of the doodads in any other tileset provide cover.

Again, what determines whether a unit is considered under Dark Swarm cover is not any kind of overlap of the visible sprites but whether the unit's collision box overlaps the Dark Swarm's one, which is a square of 160x160 pixels.