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

guillaume laforge guillaume.laforge.3d at gmail.com
Thu Oct 11 12:00:50 EDT 2007


Thanks Calin,

I understand it since ... today :).

CVEX is a great tool to deal with each rendered strands (fur_ID).

Cheers

Guillaume



On 10/11/07, calin casian <calin at sidefx.com> wrote:
>
> I use cvex mainly to tweak geometry, fur in our case, on a strand by
> strand base. For example if you want to apply noise on the whole fur,
> not just the one you see in the viewport you'll use a cvex shader.
> If you want to change the color of the fur based on a map you can pass
> the UV attribute to the SHOP shader and apply a normal SHOP shader.
> You can pass attributes to the shader in the 2 corresponding fields from
> the fur sop. If you don't pass anything all the attributes that goes
> into the fur sop will be deleted.
> Some common attributes you might want to pass are, "width" for the
> guides guideUV if your shader changes color from root to tip and UV for
> the skin.
> You need to pass UV to the skin so that the fur knows it's position
> relative to the skin and it can pick up color from the texture map.
> If you want to see the color feedback in the viewer you have to
> implement some of the shader capabilities in the cvex shader and export
> as a "cd" attribute.
>
> I hope this mekes sense
>
> cheers
> calin
>
>
>
>
> guillaume laforge wrote:
> > Sorry for the noise, I think I understand my mistake now...
> >
> > In example 2, scale is just a parameter used for the result (P). It
> looks
> > like the standard houdini behaviour to "override" this parameter if an
> > attribute got the same name and type.
> > CVEX is not so easy to understand for me :P.
> >
> > Cheers
> >
> > Guillaume.
> >
> > On 10/11/07, guillaume laforge <guillaume.laforge.3d at gmail.com> wrote:
> >
> >> 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
> >>
> >>
> >
> >
> >
> >
>
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-- 
Guillaume Laforge



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