Shade volume attribute based on Volume Size

[CinnamonMetal]

I'm using a basic smoke shader to shade volumes. I want to shade the scattering of the volumes perimeter depending on it's size; if the size of the volume is less then a specific size, then don't apply the scattering to the volume and only shade the volume using another type of basic smoke shader ?

Edited July 22, 2019 by CinnamonMetal

[3dome]

not sure if i understood correctly, but I'd try a switch SOP with an expression like sizeOfVolume < threshold to switch between 2 material SOPs

edit: or store the size of the volume in attribute that you read (bind) inside of the material followed by two-way switch, etc to drive whatever material parameters you want to drive based on that

Edited July 22, 2019 by 3dome

[CinnamonMetal]

Can a volume have a perimeter; and can I store the perimeter to an attribute ?

[Andrea]

It doesn't have polygons so no, you can't calculate the perimeter of a volume. You can store the perimeter of the volume bounds, but it doesn't make much sense.

 

[CinnamonMetal]

@Andrea Sbabo What do you mean it doesn't make much sense ? 

The volume bounds is what I'm after, but it doesn't create an attribute which I can use in materials. I need to iterate over only volumes clusters which are within a specific size, and get only the perimeters or size of those volumes; that is what I'm not sure how ? 

If some volume pieces are small, I want to ignore those, if the volume size is large; those are of which I want the size.

Edited July 22, 2019 by CinnamonMetal

[3dome]

It doesn't create the attribute by default, but what stops you from doing it yourself?

Also you might leverage the intrinsic attributes measuredvolume and measuredarea

[CinnamonMetal]

@3dome Yes, those intrinsic attributes are what I need; but, how do I access them if they are not available, since I created my volume with the volumeRasterizeAttribute SOP ? 

[3dome]

don't have H17 at hand but since volumeRasterizeAttribute uses volumeRasterizeParticles internally and at least in H16.5 volumeRasterizeParticles has those intrinsics I don't know why they shouldn't be available to you. I don't think SideFX removed them in 17, need to check later though

 

edit: in any case, this is Houdini so there's a million way to do things. How about convert your volume (to SDF) to polygons, measure area, sum up area -> get some value that way

Edited July 23, 2019 by 3dome

[CinnamonMetal]

4 minutes ago, 3dome said:

edit: in any case, this is Houdini so there's a million way to do things. How about convert your volume (to SDF) to polygons, measure area, sum up area -> get some value that way

VolumeSDF SOP is what you mean by converting your volume to SDF, you can convert an SDF to polygons, I thought only if it's a VDB ? 

I don't have a previous version of Houdini installed, if you can verify if they removed the intrinsic attributes ?

[3dome]

can't upload files and i usually work straight with VDBs but let's assume

1) you have a sphere -> isooffset

    Do you see the measured* intrinsics? (in H16.5 they are available)

2) convertvdb (to VDB, class Fog to SDF)

3) convertvdb (to Polygons)

4) from there all the measure and sum stuff (measure + attribpromote SOP)

 

will check this evening if I can see intrinsics after 1) in H17

 

Edited July 23, 2019 by 3dome

[CinnamonMetal]

6 minutes ago, 3dome said:

1) you have a sphere -> isooffset

    Do you see the measured* intrinsics? (in H16.5 they are available)

Yes.  Although converting it to polygons from an SDF removes all the intrinsic measure* attributes. 

[3dome]

1 minute ago, CinnamonMetal said:

Yes.  Although converting it to polygons from an SDF removes all the intrinsic measure* attributes. 

of course it removes them, because they are volume related intrinsic attributes. But if you see them, there's no need to convert to polys anyway. Just use them in your shader

[CinnamonMetal]

Sorry, I located the Intrinsic attributes for the volumeRasterizeAttributes SOP  Thanks