I have a volume object with a basic volume material. I want to fade it along the Y axis, getting thinner and less dense as it goes up. Or even better, as it ages.

How do I change the density? Is it something to do with the volume mix node?

Or could I transfer alpha attributes from points to a volume?

This will probably help: http://www.sidefx.com/index.php?option=com_forum&Itemid=172&page=viewtopic&t=15262&highlight=smoke+fade [sidefx.com]

I think he wants to alter an already generated volume.

You can use a Volume Mix set to user mode and use something like:

$V * (1-$BBY)

or add a ramp parm and index in to the chramp() function with $BBY to have a ramp control the fall-off

or set it to multiply and create a second volume filled with increasing density and composite the two.

or use a Volume VOP SOP and do it in there. Volume VOP SOP works on voxels instead of points. Just do your bby stuff in there.

jeff
$V * (1-$BBY)

How does this node know I am changing the opacity? Does any expression in here change opacity only?

Oh I thought you were in SOPs… In SOPs you work with a single field at a time. Opacity is usually a color which requies three volumes but sure you can apply a Volume Mix SOP to all three and scale the voxel values by that expression.

$V is the current voxel's value. By multiplying it by $BBY you are linearly reducing the voxel values as you move up in Y in the volume primitive(s).

I think you mean opacity from within a shader. You can read in any volume primitive in to the surface shader as a parameter by it's name in SOPs. It's probably best to create another volume that is the same size and location that has decreasing values from 1 to 0 then in your surface shader, multiply the volume that you are using to shade (probably density if it is smoke).

other thing you can do is $LIFE as global variable
you might want to try using it on Density.