Doodlez

You will probably use only a single shader for both materials. You can do it with two shaders but then it would become a compositing exercise (blending your images based on a mask).

Within a single shader you can mix two inputs using multiple nodes, the end result is usually the same. For the mix bias you can load in your attribute with the parameter node. This might still look a bit blurry because it is dependent on the base geometry resolution. I would suggest using point clouds to look up the mask attribute which creates a nice sharp and more natural transition since it does a weighted average.

You can take a look at the documentation on this topic here: http://www.sidefx.com/docs/houdini14.0/dopparticles/vexpressions [sidefx.com]

It states: “This is the same VEX expression syntax used in “wrangler” nodes such as the Point Wrangle SOP. However, whereas wranglers affect geometry by directly setting attribute values, particle nodes cannot change attribute values (usually below). Instead, they work by reading parameter and attribute values and then modifying the parameters values accordingly. This lets you change the values of the node's parameters for each particle.”

In your case you can set the forcescale to 1 in the parameter menu and use your old code but just use multiplication instead of directly setting the value:

float lenP = length(@P);

if (lenP > 1.2) {
forcescale *= 5;
}
if (lenP < 0.9) {
forcescale *= -5;
}

I also have the macpro 6core with 2xD700 but with bootcamp to run Windows because I'm stubborn Houdini generally runs fine although there are a few GPU bugs because the bootcamp drivers are apple's responsibility instead of AMD's (As far as I understand).