After computing the slope direction, rotate it by this amount. By setting to 90 you will get directions that avoid going up slope and instead circumnavigate hills.

Heights are often not to scale, so this adjusts the direction’s magnitude globally to get into a useful value.

Determine if derivatives at the boundary should read outside the image or restrict to the image range. Reading outside is useful for building seamless textures with wrap-mode. But restricting to the image range avoids artificial flattening of the derivative at the boundary in clamp modes, as it acts more like an extrapolation.

Usually a derivative is computed with adjacent pixels, but if the values are slowly varying there may not be enough bit-detph to get smooth derivatives. This will expand the distance, in buffer elements, to compute the derivative, possibly removing stepping artifacts from the derivative.