Yeah, it seems that they make some easy thing unnecassary complex.
First, whenever AE is displaying visual content, it needs to render this content. So, actual, we are always rendering, no matter if final render, preview render, playback, or just a single frame. "Render" means to display things.
AE needs to render before it can playback. That's how the software works in its very basics. AE renders each frame directly into the memory (RAM), and in addition there is a cache on the harddrive to speed up renderings a bit.
To indicate which frame is rendered and available, and which frame needs to be rendered to come available, there is a small green line under the render area in the composition. Every frame which is covered by this line will be play back in realtime. It can happen that the playback starts in realtime and suddenly goes very slow. That's when you pass the point of avaiable frames. AE often cannot render frames in realtime.
Sometime, playback is slow even everything is coverd by the green line so you need to render again or purge the cache. And as I already wrote, if you don't have much going on, rendering will happen in realtime.
AEs preview mechanics has changed a few years ago and is now confusing - even to me.
You can now press spacebar and rendering will start, but AE is displaying the frames and playing the audio in addition to this. If a project is light weighted and rendering is fast, this behaviour get mixed up with playback. If the project get heavier and rendering need its time, people wonder by suddenly their "playback" get's slow.
Actual you need to press space and have to wait until the end of your render area - that's the "render" pass - and only after this AE will playback the rendered area in realtime - that's the playback pass.
This is of course kind of stupid...
So, what do I do?
I press space and let AE render a few seconds, less than 10 mostly. Since I was working on this part of my animation already, the rendering is pretty fast - a lot of the frames are already available. I wait until this certain part has passed, and press space to stop rendering, or just click on a previous time mark, or scrub the CTI. In my settings, the CTI (current time indicator) doesn't move to the new timemark where I press space again, but keeps on where I started the rendering, so I can easily preview the same part again. Usual I have already seen something which I like to fix, so I do the fix and repeat the process. After some passes, I can press spacebar and the rendering is fast enough due to the cached frames so I see this certain part in realtime.
I only do this for short timespans - if I want to see a longer sequence like a whole animation of 30 seconds for final review before output - I mark in and out point of the render area and press 0 on numblock.
So, Num 0 was how you do the preview rendering in the older days of AE. By doing so, AE starts rendering, but doesn't preview the frame and doesn't playback the audio at the same time. All you see is the frame where you started rendering, you see the green bar filling and if you have the info panel activated, you see a message like "rendering frame 10 from 890".
Once the render area is filled with the green bar or RAM is full, the playback starts automatically and now you can be sure that this playback will be realtime and it won't suddently drop.
If you press comma on numblock, AE only renders and plays audio. So this is great if you search for a certain cue in the audio track.
You'll find all those behaviours in the preview panel, so it's worth so spend an afternoon getting in touch which them. I dislike all the options there, that's what make things complicated. What turns out for me the best is using spacebar for short preview where it's okay if it's not realtime, and Num 0 for longer parts where it's critical that it is and stays realtime. So, you can tell it a rough preview and a fine preview, if you will.
Since we are on topic, changing preview resolution, fps and skipping frames will become more and more essential, as you dive deeper into AEs possibilitys.
You easily end up rendering 10 seconds for one frame and waiting 30 minutes just to preview a few seconds if you work in full resolution, full fps all the time. I usual preview everything in half resolution and as soons as rendering time ascends, I drop resolution down to on quarter. When you work on timings, you don't need a crystal clear image. If you work on the visuals, you don't need realtime playback. A longer rendering time per frame is okay then, because you will only render this frame for now.
I hope I could bring some light to this topic - don't hesitate to ask if something is still unclear.
*Martin
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