Strange alphas from render elements

[Matt]

So the image map by default creates an alpha whereas before it was just black with a constant colour? How comes the constant does not also create an alpha?

The Constant Shader has a parameter called "alpha" which you can set to 0. It's the only shader that can do this at the moment. The only other way to render an object with 0 alpha is to set its render mode to Holdout (or use a Render Layer to do the same thing), but Holdout also makes the object black so that's not useful here.

Its a place holder. I am using it for lighting and reflections.

Ah, in that case you could change its render mode to Invisible while keeping "Visible to other rays" enabled for reflections and lighting.

[james adamson]

Wonderfull. And I would set that up in my render layer? And I will now get a proper surface alpha?
Will my render be any quicker?
 I think I know the answer to that one!
Cheers.
James.

[james adamson]

On the almost the same subject how would I get an alpha for my tree population? Do I do the above and this in the render layer node?

[james adamson]

Group 1 has the background sphere node in it. Will this do what I am after?

[james adamson]

oops uploaded same png twice.

[Matt]

If you already have an object group set up for the background, yes that will work.

But there's a simpler way. You don't need Render Layers, you can do it in the object itself.

[james adamson]

There I go again! Complicating things. Like i'ts not complicated enough already.
Thanks Matt.

[james adamson]

Last post on this honest!
So if I am comping a new sky but I want tgs atmos and I need my tg clouds over the surface and over the background I need to check 
atmos/cloud over background on?

I now have the correct alpha for the surface almost. It still has faint colour in the very far distance but I can isolate and grade that out.

Got my render times down from 135 mins a frame (no noise) to 60 but with some noise that I guess I can deal with layer by layer in the comp.
4 layers v3 clouds at 0.5 march. Voxel scattering 150  AA10  1/64 first samples PNT .02  Atmos AA 16  resolution 960/540 for test.
Final will be 1920/1080.

[Matt]

So if I am comping a new sky but I want tgs atmos and I need my tg clouds over the surface and over the background I need to check
atmos/cloud over background on?

If you want clouds and atmosphere to be in the sky, then yes. If you turn this off you'll get a hard line at the edge of the landscape. The only reason TG gives you the option to turn this off is so you can render more than one Terragen layer (e.g. terrain and sky in one layer, trees in another layer) and comp them together without doubling up the atmosphere. You'd turn this option off for the trees layer so that everything that's not a tree is black. But if you only have one Terragen render, you probably want atmosphere to be included.

[james adamson]

Great. 
Thanks again.