The expression after the equals must be a simple constant expression. Houdini will use the Cd (diffuse color) point attribute to color particles. Time in seconds since the particle was born. To get the relative time, useful for fading effects, use @nage.What is the Houdini Engine documentation?
The Houdini Engine Documentation gives you information about Houdini's compact Engine API which extracts Houdini's core technologies into a procedural engine for film and game studios to integrate into other applications. With this documentation you can create a plug-in for any number of different DCC apps.What is the VEX checkbox for in Houdini?
If the checkbox is off, the parameters use the normal Houdini expression language, but those values cannot vary per-particle. When this checkbox is on, a multi-line editor appears where you can write a snippet of VEX code to evaluate on each particle at each time step.What is the difference between expressvex and Houdini?
VEX expressions use a variation of the VEX language instead of the Houdini expression language. Many of the commonly used expression functions have equivalent VEX functions with the same name (for example, ch, chs, chramp, point, relbbox, rand, and so on).