[Sidefx-houdini-list] H9 : CVEX shader question

[guillaume laforge]

Hi,

I'm learning the new workflow with the CVEX shaders and fur (sop and
procedural) in Houdini 9.
I don't understand completely how it works. Sometime the CVEX shader can
modify an attribute and sometime it creates this attribute.

Example 1 : a CVEX applied in the skin shader parameter of a fur sop :

If I create a Cd attribute on the rest skin geometry and add this cvex
shader on the fur sop it changes the fur color based on a color map file.
If I remove the Cd attribute, it doesn't work anymore.

Code :

cvex
gl_cvex_s_colormap(
            vector uv = 0;
            export vector Cd = 1;
            string map = "";)
{
    Cd = colormap(map,uv);
}




Example 2 : a CVEX applied in the guide shader parameter of a fur sop:

If I add a scale attribute on the rest skin geometry, this code does
nothing. I must remove the scale attribute to see the effect.

Code :

cvex
gl_cvex_g_furrandomscale(
        export vector P=1;
        vector origin = 1;
        int fur_id = 0;
        float scale=0.5;
        )
{
    float r = random(fur_id);
    P += scale * r * (P - origin);
}


Why an existing  attribute is needed in example 1 and not example 2 ?
Any explanation on the CVEX shaders is welcome !

Cheers,

-- 
Guillaume Laforge | La Maison