Problem with Rendering Alpha

[hcs]

I have a particle sim in H12 with sphere's copied onto the particles and set up to fade in and out instead of appearing with full alpha. It shows up great in the viewport, but I can't get the alpha change to render. When I look at the alpha of the render, the spheres are solid white the entire time they exist. I tried fussing around with the gamma and gain but it didn't help at all so the render really isn't picking up on the change. Any ideas what I'm missing here?

Thanks

fireflies_v01.hipnc

[Netvudu]

I posted this at SESI´s forum. I´m gonna risk and say it will be helpful for you

Alpha workflow changes

[hcs]

Thank you Netvudu, that was exactly what I needed. Seems like an odd way to do things, but whatever works.

[bloomendale]

I'm very new to Houdini and maybe i didn't understand problem but i just assigned your Constant shader on obj level and got it working.

[Netvudu]

bloomendale, basically you got lucky. Meaning the constant shader is old, so it still understands the alpha attribute. If you use any modern material such as Mantra Surface or even Clay it won´t read alpha anymore, and wants "opac_int" instead.

[bloomendale]

bloomendale, basically you got lucky. Meaning the constant shader is old, so it still understands the alpha attribute. If you use any modern material such as Mantra Surface or even Clay it won´t read alpha anymore, and wants "opac_int" instead.

opac_int is Opacity multiplier in surface model. But Alpha is not opacity. What should i do to render for example semi-transparent glass but fully opaque in alpha channel (or vise versa)? How can i unlink alpha and opacity?

thanks

UPDATE:

I looked through Clay Shader and it seems it evaluates Alpha point attrib and renders corect alpha channel but doesn't premult with opacity. If you want you can premult it and get premultiplied diffuse and corect alpha.

and here is fireflies with clay shader usin alpha point attrib:

alpha chanel:

Edited May 31, 2012 by bloomendale