Strange alphas from render elements

[james adamson]

Hi all.
I have just rendered a test frame with some alphas. The alphas do not look as expected. The cloud alpha looks fine and the atmos I can use. I would just expect them all to have no colour information.
Any ideas anyone? I have posted stills. Also how would I go about getting a separate alpha for the population?
Cheers.
James.

[james adamson]

That was unhelpful. I have renamed the files incorrectly. The one named surf alpha is the atmos and the atmos is the surface.

[Hetzen]

Digitalguru put together this very good tutorial on how to render and composite elements here https://www.youtube.com/watch?v=ihY_kp2rzxg&feature=youtu.be

In a nutshell, TG renders these elements into linear colour space. The alphas are 3 channel to allow for light transmission, which you tend to invert and Multiply blend over an image, to then Add blend your other elements on top.

[james adamson]

Hi Hetzen. Thanks for your response. I am comping in Nuke so everything is linear and I have rendered exr's from Terragen so did not expect the alphas to look the way they do and yep that elements guide is the one I followed so I must have missed a trick.
I will take another look at digital gurus guide.
Cheers.
James.

[james adamson]

As the guy said in the thread I have just read. OOPS. I had atmos visible in the render layer so I reckon thats what is giving me strange alphas. And like the previous guy Im gonna blame lack of sleep!

[james adamson]

Nope, that did not work. It turned atmos off for main render.

[digitalguru]

Matt posted a very comprehensive post about TG's alphas here:

https://planetside.co.uk/forums/index.php/topic,23724.msg239993.html#msg239993

[james adamson]

Awesome. Thanks.

[Matt]

In tgSurfAlpha the sky should be black. Do you have an object in the background or is it just the default background sphere? The background sphere in the default scene uses a Constant Shader, and the Constant Shader has its alpha set to 0. If you replace this with another shader you may have reset its alpha to 1.

[james adamson]

Hi matt. I have a starfield image.

[james adamson]

I have used an image map connected to a default shader. Default shader settings are diffuse o luminsity 2. ( i think its its 2.) 
Where would I change the alpha?
Cheers.
James.

[james adamson]

There is no sphere in the background. Have I deleted it by accident?

[james adamson]

So I am not sure what to do about the alpha issue there are no obvious inputs or settings on the default shader. Also I will be replacing the background with an updated version of the same image so have unchecked atmos/cloud on background.

[Matt]

Unchecking "Atmo/cloud on background" will have no effect if the background has an alpha of 1. "Background" means any pixels with 0 alpha. With your image map shader connected to the background, this is now a surface just like any other, and the renderer does not know that it's meant to be the background. It may be possible to set the alpha to 0 by chaining a Constant Shader before the Image Map Shader but I forget whether this works. I'll try it later.

Since you're replacing the background, isn't it better to disable this object/shader anyway?

[james adamson]

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