What is the purpose of the outlier check?

[monolith4007]

The Sonic Physics Guide on Sonic Retro states that the player's current "mode" of collision is determined by the angle ranges listed in the following page:
https://info.sonicretro.org/SPG:Slope_Collision

This can be replicated by setting the player's mask direction as their angle rounded to 90 degrees:
mask_direction = round(direction / 90) * 90;
(Since GameMaker uses banker's rounding, the mask will not rotate at an angle of 45 degrees.)

Why doesn't the template use this?

[VectorSatyr]

The purpose of the outlier check is to anticipate when the mask should actually rotate in order to avoid two pitfals with the naive approach:

1. At an exact 45 degree angle, the mask angle could end up rapidly swapping back and forth as multiple mask orientations are valid simultaneously.
2. Rotating, then pushing out of the terrain from the new terrain angle can be a jarring and dramatic change if done late, especially at higher speeds.