Hello everyone! In a recent SSCAIT broadcast, there was a weird situation in which a Protoss AI Cannon-contained a Terran AI on La Mancha. This alone is unusual enough, but what made it even stranger was that one of the Cannons was freely shooting at the Barracks which was part of the Terran's high ground wall. Please see in the video below (timestamp 1:18:52 if the link does not automatically move you there):
Here is a still shot of the Cannon firing on the Barracks:
This sparked a discussion in the SSCAIT Discord about what the possible reasons for this are. One theory is that this has to do with the modified ramps on La Mancha's top right spawning position. This map, and SSCAIT, were made for/operate on 1.16, so reverse ramps had to be manually made by mapmakers, which frequently leads to strange behavior such as the fact that in some positions on reverse ramps, units have nearly 0 vision. The hypothesis was that the Barracks is actually standing on a low ground tile, which means that the Cannon can see it despite it theoretically being on the "high" ground.
My theory is a bit different. I know from experience that units can frequently see and attack buildings on the high ground when they are right up against the edge of the map. This has happened to me many times because I am a scrub that goes 1 base Reaver/Sair PvZ, and the best way that I've found to transition out of this into a more standard game is to place a Pylon on the edge of the high ground, and then use the support area on the low ground to make my first few Cannons in order to establish an expansion. Hydralisks have frequently fired uphill on these Pylons/Cannons without having vision.
Some had not seen/heard of this before in the SSCAIT Discord, so I used an unlocked version of FS to test this. Here are some of the Pylon locations I found which can be freely seen and attacked from the low ground:
As you can see, most of these locations are not near a ramp - and thus, should have no low ground tiles used in that area. My theory is that the edge of "rough" tilesets like FS's jungle high ground and La Mancha's desert high ground are as uneven in terms of vision as they appear visually, so in some rare cases, a building may "overlap" with the vision permitted by this uneven terrain. Of course, it could be that both theories are correct, and that vision is provided based on whichever one applies.
This raises a lot of questions:
1. Is this a well-known phenomenon? Searching on Google and TL did not give me any relevant results.
2. Have pros have abused this "feature," or know where not to place their buildings to avoid being vulnerable?
3. Why do it be the way that it is, doe?
If you know of any relevant discussions/threads or if you have any answers, please post them here I pinged Freakling on Discord who I suspect would be the best resource concerning such issues, and if he doesn't post in response here and responds there, I will update the thread with his analysis.
I know the problem and read about it a few years ago. Unfortunately I don't know exactly whether here or in a Mapmaker forum. I think the reason was that the cliffs weren't always closed properly (vision wise) and that you have the opportunity to look up, depending on your unit position. Not 100% sure tho, I think Freakling knows the correct answer.
There is probably low ground tiles at the top of the reverse ramp. This is a big issue with most reverse ramps, even the new SC:R doodads and need to be manually fixed.
On February 09 2021 08:04 MineraIs wrote: There is probably low ground tiles at the top of the reverse ramp. This is a big issue with most reverse ramps, even the new SC:R doodads and need to be manually fixed.
This is not the issue in this case but is an issue, yes.
I'll just write down the gist of the answer as I've already given it on the BWAI discord:
At game start all tiles are classified by terrain level (and some other properties): Each tile is made up of 4×4 minitiles, each of which has flags for medium ground, high ground and pure vision block (and, of course, walkability, but that is not strictly relevant here) If at least 12 minitiles are flagged as medium or high ground then the tile is considered medium ground. If at least 12 minitiles are flagged as high ground then the tile is considered high ground. If at least 1 minitile is flagged as vision blocking then the tile is considered vision blocking (meaning it will always block vision for ground units, no matter what any other tiles in the vicinity are).
Lower/higher ground henceforth refer to the vision blocking hierarchy of low ground < medium ground < high ground < vision blocking. The terrain level of the minitile at the unit position (position of the centre point of the unit) determines the unit's terrain level. A unit has vision of all tiles of the same or lower terrain level as well as any higher ground tile that is adjacent, including diagonally, to a lower ground tile and in the unit's line of sight. Any unit with position on a visible tile will also be visible to the first unit. For buildings to be visible it it suffices if any tile of their building area is visible.
Many cliff tiles (particularly in the BW tilesets Desert, Ice and Twilight) and some doodads (including the vast majority of the new ramps introduced by Blizzard in patch 1.22!!!!!!!!!!!!!!!!!!) have faultily or haphazardly set terrain level flags that do not meat the tile flag conditions stated at the beginning of this post, leading to some significant cliff vision issues.
EPT![[image loading]](https://i.imgur.com/wJgAe10.png)
![[image loading]](https://i.imgur.com/LBuEiuN.png)
![[image loading]](https://i.imgur.com/gKOu0bb.png)
![[image loading]](https://i.imgur.com/pfmbPrc.png)
![[image loading]](https://i.imgur.com/vxekWA2.png)
I pinged Freakling on Discord who I suspect would be the best resource concerning such issues, and if he doesn't post in response here and responds there, I will update the thread with his analysis. 
