Hi,

I'm writing a matte shader that (among other things) catches indirect light from surrounding objects. Since I want the resulting composite to be purely additive, I've got the shader outputting the indirect color, opacity of 1, but alpha of zero.

It seems that Mantra renders no color in this case, although if I bump the alpha up a tiny amount (say to 0.00001), the color shows up fine.

I made a quick test shader, shown in the screenshot. This renders black in Mantra, though if I bump the constant “ZERO” node up to a nominal amount it shows up fine.

What am I missing?

Thanks

johner
Hi,

I'm writing a matte shader that (among other things) catches indirect light from surrounding objects. Since I want the resulting composite to be purely additive, I've got the shader outputting the indirect color, opacity of 1, but alpha of zero.
Thanks

Not sure, this works for me.

Thanks for the reply Mark.

Whoops, looks like it's actually an issue with the built-in compositing within mplay. I've been testing my shader by loading a background image into mplay then rendering on top of that. When there's a background loaded, the colors from the shader don't show up if the alpha is zero.

Same thing happens with your example. A regular render into mplay works fine, but then load a background and the image disappears. Strangely that still seems to be the case even if you then unload the background. You have to restart mplay to get it working again. This is with 9.0.760 on OpenSUSE 10.2-64 by the way.

Anyway, obviously this is only a problem for testing, and I can easily work around it by testing with a real composite within Houdini.

Thanks again