Rendering only within specified radius of camera?

[PabloMack]

Is it possible to render only the geometry (that is) within a specified radius of the camera? This would include terrain and objects (as well as their populations) etc. I am trying to do an animation shot where an object that is to be created in another SW package is obscured from view only by Terragen-created geometry that is in the foreground. The Terragen-created geometry becomes background when the object is farther away. I want to create an alpha image for the geometry that is only in the foreground. What would be even better is to render all geometry that is inside a specified sphere that can be placed anywhere within TG coordinate space. I want to avoid having to do "rotoscoping" for a CG object against a CG background. I can see this feature being very useful in the future if it is not already in TG.

[Dune]

My idea would be to use a distance shader to feed the opacity of that object.

[TheBadger]

Is it possible to render only the geometry (that is) within a specified radius of the camera?

I thought that was what render layers were for?

[Matt]

The render layer has distance settings that are supposed to do this, but they don't work with terrain yet.

Another solution is to change the radius of the background sphere (part of the default project). Just make sure it still has a negative number, which is needed because we're looking at it from the inside. There may be some overdraw inefficiencies, but it should produce the end result that you want. You can also set most objects to "Holdout" mode, and this way you can use other objects as background clipping surfaces. Holdout mode is only in the Professional edition, but you can create a holdout effect in any edition of Terragen 3 by copying the internal shader from the default background sphere.

Matt

[PabloMack]

I played around with the default scene and I think that the background sphere Matt talks about is going to do what I need. At this point all I need is the alpha shot.  I noticed that there is no atmosphere when I have the radius set to -1000. Is this because little of the atmosphere is enclosed within this sphere?

In this particular situation I don't think that the terrain is doing any of the obscuring, only trees. So render layers might do what I need in this case as well. But it seems that the background sphere technique is trivial to use. I need to study up on how render layers works. I'll read what is in the wiki about it.

Thanks all.