A level terrace cut into a slope always has a steep uphill bank, and no blend shape avoids it

Fact

Flatten a strip of a slope g by blending the height field toward a constant over a half-width W. The terrace holds height while the ground keeps rising, so the g·W it cut away has to be given back within the blend. The average slope across the give-back is therefore g no matter what, and because the blend is flat in the middle the peak is worse than the average.

Measured across cut depths from 60% to 95% and window shapes from a bare cosine to a wide flat-topped one, the peak bank never came out below about 1.3x the slope of the face it is cut into, and a cosine window put it near 1.6x. A comfortable trail on a 40 degree face has a 55-60 degree bank above it.

Widening the blend does not help: the cut depth g·W grows with W at the same rate the distance to recover it does.

Why it matters

It is normally discovered as a bug — an invisible wall along a path, an agent that cannot leave a road, a character stuck in a trench — and then chased through the blend function, where there is nothing to find. Knowing the bound turns it into a design decision instead: a terrace is a corridor you leave downhill, and leaving it uphill costs something.

The only real lever is how much you flatten. Keeping 30% of the cross slope makes the path a graded traverse rather than a bench and takes roughly 30% off the bank, at the cost of a path that is no longer level.

How to apply

• Budget the bank before building the path: bank ≈ 1.4 · g. If that exceeds what the character can climb, the terrace will wall them in.
• Decide deliberately whether crossing the bank should be possible, and check it against the movement limits rather than assuming.
• Where the corridor must be enterable from above, do not flatten — mark a line that is already walkable and leave the surface alone.