I'm keying a lot of clips from After Effects and I export them all as DNxHD with uncompressed alpha. And I spent HOURS trying figuring out why my files where so huge as the clips were all DNxHD before key as well but after rendering the same sequences were 4x the size. I found a thread somewhere that alpha just takes up insane amounts of space and I guess that is just how it is? Is there some work around I'm not aware of? And perhaps could you explain the technical stuff behind it? I'm keying away in the area of 70% of the image so it just baffles me that the alpha data takes up so much more than 1080p picture data.
And when I choose my codec settings I have the option for "compressed" alpha render and it does in fact take up 40%-ish less space than uncompressed but I don't seem to get any transparency from it when I take it back into AE?
Even with 20 clips to key I could live with the HDD usage, as I just would delete them when I'm done but I'm going to have several clips simultaneously. I'm making a music video with instruments and vocalists in a sorta collage style and I'm sure my harddisk/RAM (whatever is doing it) surely can't keep up with 6-7 files at a time each with such a high mbps as it is with these DNxHD-alpha files which is easily 15+ GB for a 3½ minute long video..??
"Uncompressed alpha" means exactly that: the alpha channel is not compressed. Whether it's very simple, full on or off, or very complicated, it will always take up the same amount of space: the number of bits per pixel (the bit depth of the alpha channel) times the number of pixels in the raster, for every frame of video.
There is a bug where compressed alpha isn't properly supported.
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If you are in fact keying 70% of the frame I might recommend trying a render in QuickTime Animation at 100% quality.
This uses a form of Run Length Encoding that saves a lot of space when the same values appear over and over again in footage (solid colors like alphas work especially well). Render your footage with Alpha and Premultiplied and the transparent portions of your image should be compressed very well.
The files are lossless so they won't be super small, but the use case you describe sounds almost ideal for the Animation codec.