Multiply Vop transparency with particle life

[smithkj]

If you make a vop net with a texture with alpha, let's say an image of a single fire flame, with transparency around the sides. Than you also have a pop net exporting to sops and copies of planes on the particles, with shop from the vop net as the shader. What I want to do in addition to the current transparency in the vop net is fade out based on $LIFE. Or, in a perfect world, control point colors on either color, incandescence, etc, anything based on $LIFE, while multiplying by the vop net colors....transparency, etc.

I've tried experiments with opacity only so far, tried exporting the Of param in the vop net, making a color pop and making it access this parameter...no go, (maybe another color pop under with alpha based on life?) also point sop after the copy sop, with attribute sop, changing Alpha to Of, then passing point attributes based on multiply....add...you name it. Can't seem to get it to work.

All I want to do is have two seperate controls for transparency, one in the Vop net for the individuals, then another controling overall behavior based on $LIFE.

So now this is my first post on this forum so you rocket scientists be nice!

Happy to be beyond lurking but sick of grabbing at straws.....Can't seem to find a tutorial on this to save my $LIFE!

"Jo"

[edward]

Here's what I've found to work.

In SOPs:

- Append AttribCreate SOP to your Pop SOP and set these parameters:

Name: tlife

Local Variable: TLIFE

Class: Point

Type: Float

Value: $LIFE*100 (or some appropriate scale factor)

- Append Shader SOP and direct it to your shop

In VOPs (inside your surface shader):

- Create a Parameter VOP and set these parameters:

Parameter Name: tlife

Parameter Label: TLife

Export Parameter: turn on

That worked for me. For some effects, I further multiplied the tlife output with a constant which I then fed into the bias input of a Color Mix VOP.

So basically, you can create any type of custom attributes you want in SOPs and pass them to VOPs this way. To help visualize the effect in the viewport, you can append a Paint SOP and turn on Override Color. Change Cd to tlife. Turn on Visualize Attribute. You can't actually paint on points but it will be useful to visualize the attribute. Of course, it also helps to change your viewer background colour to Dark instead to be able to see all the coloured dots (see the Viewer Display Options).

Hope that helps.

[smithkj]

This worked really great!

Since my lifespan was only about a second with some variance, I ended up using 1-($LIFE*.7) which looked pretty good. The suggestion of the paint sop was beautiful as I could see in the viewport exactly what my changes to the expression were looking like. I also changed tlife to a vector as I was also plugging it in to my RGB and that way there are very few conversions. So now tlife multiplies with the alpha in the texture as well as keeping the color in the same bounds, looks really good.

Thanks for bringing my Vops to life!

"Jo"