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From Odwiki

Renderman Shading Language can be translated to VEX quite easily.

Syntactical Differences

There are a few syntactical differences to be aware of when you're translating from RSL to VEX. Here is a few of them:

• dot product. eg. RSL uses 'N.L' and VEX uses 'dot(N,L)'. In VEX the '.' is strictly used for subscripting an array element
• array syntax. Please see VEX's array syntax. Help > VEX > Arrays (http://www.sidefx.com/docs/houdini9.5/vex/arrays)

Mappings

Here are some equivalence tables for mapping expressions from VEX to RSL and vise-versa. Their values are understood to be functionally equivalent, but not necessarily "identical". Multiple expressions inside a single cell represent different ways of expressing the same value in a given language.

Many of these are defined in the include file shipped with Houdini here: $HIH/houdini/vex/include/prman.h. You can include these mapping in your source code but it is good to be aware of them.

#include "$HIH/houdini/vex/include/prman.h"

An official version of this table exists in the Houdini documentation under Help > Shading > Transitioning Between VEX and RSL (http://www.sidefx.com/docs/houdini9.5/shade/transitioning)

Partial Derivatives and Differentials

VEX	RSL
dPds Du(P)	dPdu * du Du(P) * du
dPds / Du(s) Du(P) / Du(s)	dPdu Du(P)
Du(s)	du
s	u s (at default)
dPdt Dv(P)	dPdv * dv Dv(P) * dv
dPdt / Dv(t) Dv(P) / Dv(t)	dPdv Dv(P)
Dv(t)	dv
t	v t (at default)
not available	Deriv(a,b)
[H9] dPdTime	dPdtime * dtime (?)
[H9] not available (?)	dtime
[H9] Time	time