Alpha channel question

[Eric2575]

I've got a simple scene with a central object receiving reflections from a dome that has an HDRI image applied via a projection map. It renders very nicely, but I want to use an alpha channel to composite the object into another scene without the dome background. I tried tga output with the alpha channel enabled, but the resultant alpha is the whole tga, not just the central object. What am I missing here?

[HomeSlice]

I think you may need to apply an environment map on the central object, using the HDRI image for the environment map. The environment map will fake reflections on the object. Then you can get rid of the dome and render just the object with an alpha channel.

[robcat2075]

The environment map solution will work in many situations.

 

If you absolutely need accurate, interacting reflections from the scene, a little picking and choosing may be needed.

 

To get just the central object you will need to turn off everything except that object when you render. But then I expect you wont' get the reflections on it you need. So...

 

 

Render it once, full scene (with alpha). this gets you your object, the surroundings and the reflections of the surroundings and the alpha you dont' want.

 

then do another render of just the object with alpha ON. This will get the alpha shape you really want.

 

Then in Photoshop (or any paint app that lets you edit alpha channels), copy the alpha from the second render onto the alpha of the first. Resave the first

 

AfterEffects can also do this trick of using the alpha for one source as the alpha for a second source, but I forget the exact steps.

 

 

It may even be possible to do this in A:M Composite. There is a Post effect called "Channel Select" that gives you access to the R, G, B or alpha channel of an image. I haven't investigated that much.

[Eric2575]

Thanks again guys, both ideas will work and I will try them both to see which fits the bill better.