Raytracing and alpha


I have many transparent (by ray tracing) objects in my scene and since they overlap I can see objects that are behind one another and that’s fine. However, when I render the scene it has an alpha channel only “around the edges” of objects but no shades of gray for their transparency to the background which I need to add and frequently change in After Effects for later purposes (I suppose this is much faster then rendering in Blender with the background). The manual wiki mentions something about an index pass but the information about this is very scarce and when I rendered this index pass I got the same black and white image. Is there a way at all to render an alpha map for raytraced transparency or am I missing something? Also is it possible to render a map of refractions (it would be nice but it is not essential at this moment) to be used as displacement map in AE?



Disable the “Sky” button on the render layer tab and these raytransp objects will render with their proper alpha. Just realize that anything placed behind these raytransp objects after the initial render will look rediculous should you choose to go this route.


You can not actually turn off the Sky button, do you mean to set it to Premul or should it be set to Key?

I have wondered about this as well, getting the alpha out of somethingt that is being viewed through an already transparent object.

Yes you can. If you disable the Sky button on the Render Layer tab, NOT the Render tab, you’ll get what he was wanting concerning raytrans objects with an alpha channel.

The premul button is basically a legacy item in Blender (well, not exactly but it’s no longer necessary either). Leaving the Sky enabled globally (on the render tab) but disabling it locally (on the render layer tab) results in an alpha channel that is premultiplied with black, just as if the premul button had been used.