Enable the transparency preview option at the bottom of the comp viewer (checkerboard icon) to preview transparencies. Still, I think you ahve a fundamental misunderstanding here. Support for transparencies is format dependent and how the receiving apps interpret that information. Just previewing it in AE doesn't make it happen and is actualyl completely unrelated to the creation of transparency information.
Thanks for the info. I do understand alpha in render formats, etc. What I didn't understand was AE's perspective that there's always a solid background and the choice is whether you want to see it while working on the project. In Photoshop, everything is transparent. You have a layer that creates a solid background if you want to put one in (or leave the default one in). But the project is inherently transparent as opposed to always a background that you can choose not to look at. I know there are probably reasons.... but I would have expected AE to be transparent and you could add a solid layer if you wanted a background. But... they didn't ask me.... :-)
What led me down this path.... I pretty much knew that I was stuck with a background of some color no matter what. So I figured perhaps if I rendered with renderer that offers alpha option, the renderer would just discard the background. So I tried rendering with the default AVI renderer. On the options panel I selected RGB+Alpha. Output still had the background. Hence, I figured there must be something I needed to do in order to remove the background.
If I'm understanding you correctly, 'removing the background' with the checkerboard icon doesn't have a thing to do with render. And the renderer will remove the background if it's told to do alpha channel (and it supports alpha channel), correct? So I guess my question has changed.... what else do I need to set in the AVI renderer (and/or any other renderers that I might need in the future w/ alpha), in addition to setting RGB+Alpha in the channels dropdown, in order to have the renderer drop out the background?
what else do I need to set in the AVI renderer (and/or any other renderers that I might need in the future w/ alpha), in addition to setting RGB+Alpha in the channels dropdown, in order to have the renderer drop out the background?
That will do it.
Hmm, I'll try it again. But that's what I thought I did last night. Background was still in the rendered output. Does it matter for renders if the preview-transparency button is on? I didn't have transparency preview set on last night. Only thing I know that would be different, unless somehow I messed up on the settings. I'll do another test.
There's nothing to remove. There is no genuine transparency with any video format (or on a more generic level not even exactly in any sort of computer graphics). As I wrote, whether or not Alpha channels are used solely depends on which program you use and whether it interprets this information and when they do, they replace it with their background color, another pattern, your desktop or whatever. That's all transparency means in a computer - to replace one color with another based on a specific channel that modulates this substitution. This is no different than Photoshop files - the only real difference is that PS uses associated transparency rather than separate channels, but outside PS (or programs that read PS files with transparency) all you ever will see is a flattened image. So to sum it up: Everything you see is perfectly normal. Outside AE or other programs handling transparencies all you will ever see is the RGB colors or the Alpha channel's greyscale representation. You can only verify the usability of your Alpha channel by re.importing your clips into any such program.
I haven't seen the question of codecs come up. You have to select a codec that will support an alpha channel. Not all do. Personally, I use Quicktime's PNG codec a lot. Also Quicktime's Animation codec.