I would like to run a 5e encounter in a grassy plains, with tall grass about 3–4ft high. Based on the RAW in the PHB, I am taken to thinking that small creatures (goblins) are effectively under heavy obscurement. That is, creatures trying to see the goblins are looking through thick grass. Normal medium sized creatures, however, are standing about waist high, and would be lucky to be lightly obscured.
Concealment is explained in the RAW on page 183:
"A given area might be lightly or heavily obscured. In a lightly obscured area, such as dim light, patchy fog, or moderate foliage, creatures have disadvantage on Wisdom (Perception) checks that rely on sight. A heavily obscured area-such as darkness, opaque fog, or dense foliage-blocks vision entirely. A creature in a heavily obscured area effectively suffers from the blinded condition (see appendix A)."
My problem is that the RAW designate an area as under concealment. In a case such as this, where there is a clear line between obscurement (grass) and no obscurement (no grass), there should be some deliberation here. It seems the lines of sight should differ for different size groups as a result of treating those groups as obscured or not. For example, these size groups in 3–4ft grass. (Read ‘→’ as “looking at”.)
Small → Small (Small creatures would be heavily obscured. Is the grass interfering with the perception roll of the perceiver?)
Small → Medium (Medium creatures, it would seem, should not be considered under the effects of obscurement, but would the small creatures still have trouble seeing over the grass? i.e. Are they under the effects of heavy obscurement by nature of being in the grass?)
Medium → Small (Small creatures would be heavily obscured, so a medium creature should have trouble spotting them, but the perceiver itself isn't in heavy obscurement.)
Medium → Medium (Neither creature should gain any obscurement, but maybe this case could be interesting if there were stealth checks involved or prone creatures.)
My actual question is if we can treat obscurement conditionally, with different sized creatures being a factor, instead of "always on" or "always off" in a given area. It would seem errata v1.0 says:
A heavily obscured area doesn't blind you, but you are effectively blinded when you try to see something obscured by it
Which is decidedly different, turning the condition to the area doing the obscuring to your target, rather than creatures being in a state of obscurement.
I am aware of a lengthy discussion about cloud of fog and how attacks work, so I'm pretty clear on that aspect, but I'm mostly asking about what the state of vision is like here.
Additionally, the only possible thing that was left unanswered in that discussion is about positions and whether sight is required to know a target's position, considering creatures making noise from attacks and movement. Of note, in RAW it says heavily obscured areas block vision but not any other senses. It would be interesting if someone had input here, but in this particular case, since they're goblins, they will be using bonus actions to hide.