What exact version of After Effects are you using (including the decimals)?
What are your memory and multiprocessing settings?
What are your cache settings?
You have very little RAM for working in After Effects, I would highly recommend getting more, if possible.
Side note: I never use the spacebar in AE.
If you apply any effects or transformations to your movie, it must be rendered.
If your AVI is being used in a comp that uses a different compressor or pixel aspect ratio, it will need to be rendered.
If your movie is the source or fill or background for any effect on any other layer it will be rendered.
...and because you have only 8 gigs of RAM in your machine, it's tough to keep memory free to hold previous RAM Previews... especially if you're trying to use multiprocessing, which you shouldn't.
Thanks for the responses and apologies for the slow reply (I spent some time following-up on the clues from the kind responses and did a more focused reading of AE's Help page based on the clues).
To answer Mr Szalam's questions:
I'm using AE CS6 version 126.96.36.199
Installed RAM: 7.9 GB
RAM Reserved for other applications: 2 GB
RAM Available for AE: 5.9 GB
[Not selected] Reduce cache size when system is low on memory
Previous Multiprocessing settings:
Installed CPUs: 4
CPUs reserved for other applications: 1
RAM allocation per background CPU: 1.5 GB
Actual CPUs that will be used: 0
Current Multiprocessor settings: [Not selected] (due to advice given in the responses)
Copy that on the RAM part. Time for a little hardware upgrade :-)
For Mr/Mdm bogiesan's advice, thanks. Now I know more of what to look out for in simplifying future projects. For this current case, the pre-dendered movie-layer is the only layer-segment as I intended it to be a completed segment in the hope of avoiding repeated renders ... alas, it didn't turn out the way I hoped ... oh well :-)
For Mr Dave La Ronde's advice, copy that. I've disabled multiprocessing. Indeed, I'm using sub-par hardware for the intended purposes.
To sum up:
From AE's Help-pages:
1. "If you have a nested composition ... you can save time during each rendering operation by pre-rendering the nested composition ..."
Takeaway: Re-rendering will occur under all situations, be it a plain movie-layer, effects-applied movie-layer, etc. Pre-rendering was designed to save time on nested compositions only (i.e. re-rendering nested compositions take longer than re-rendering the pre-rendered version of the nested composition).
2. "The rendering of a movie is the frame-by-frame rendering of each of the frames that make up the movie.".
Takeaway: Even plain-movies get re-rendered. Since pre-rendered movies are the same as imported movies, everything gets re-rendered without exception.
Although Premiere Pro CS6 seems to handle movie footages with transition-effects more smoothly (e.g. during scrub-through), but I guess AE CS6 has much more to keep track of if it needs to tell the difference between a plain-movie and an effects-applied one ... perhaps a more optimized approach is indeed to render everything without exception. Perhaps a dedicated no-render-layer-slot can help AE to decide what not to render (i.e. anything dragged into such a no-render-layer will never be rendered) ... but that's just from a novice hobbyist's limited-knowledge perspective (I'm not a professional effects artist, which may be obvious from the sub-par hardware I use ... I bought the Teachers and Students Edition of CS6, which was kindly offered by Adobe at a steep discount ... and I use it to satisfy my curiosity of how those stuff-of-dreams sort of movie-effects are created ... for now, I'm only able to play with basic video montages :-)
Thanks for all the responses. Case closed as AE seems to be designed to render everything without exception.
You are misunderstanding how Premiere Pro handles transitions and playback. Transitions, if the codec of the original footage supports it, can be rendered in real time, if the source codec does not, they are rendered when you do a preview so that the program window, the place you view your edited video, is acting as a media player for existing video footage. Through the playback engine PPro is able to playback multiple streams of video at the same time using the Mercury playback engine.
After Effects is a compositing program that layers pixels and performs math on every pixel. If you have cache enabled then rendered frames are not re-rendered when you re-do the ram preview, but if you make any change in the timeline, everything from that point forward must be rendered again because there is no way to easily tell what pixels are going to change in the layers after you make a change without performing the calculations.
AE is a great tool for producing shots and short sequences but it was never intended to be used as an editing program to put together movies. It is just not the right tool for that. Integration of Compositing and Editing will improve as both PPro and AE are more tightly integrated, but this is the way it is with all software used to create a movie. Compositing is a separate operation from Editing and they are handled differently.
Thank you, Mr Gerard, for explaining the difference between PPro and AE.
Perhaps just a thought ... would the following scenario be sound?
>> Although AE was designed for pixel-level calculations, perhaps it can somehow determine that a layer-segment has not been changed? such as a freshly-imported movie or a newly-pre-rendered movie (which is meant to be a completed segment of a project). For pure (no changes) footage-segments, perhaps AE can invoke the media-player engine instead of attempting to re-calculate every pixel? AE can always revert to pixel-level calculations when it reaches any changed/effects-applied segment.
I agree that AE is a great tool. But unless all short sequences can be completed in single sessions, AE's current design means that everything need to be re-calculated and re-cached the next time we open the same partially-completed project. If any segment can simply be played (media player), it seems an overkill to re-calculate and re-cache every pixel, which consumes resources that can otherwise be used to serve those other segments that have indeed been changed (and/or have any effects applied).
Just my five cents' worth from an amateur hobbyist's view.