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Why do 720p and 1080p videos need to ram preview? Why not just play?

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Ryan McAdamWhy do 720p and 1080p videos need to ram preview? Why not just play?
by on Feb 4, 2016 at 6:51:55 pm

I understand why an animation with many effects, or a video with Post Processing needs to ram preview in order to see a real time preview. But, I don't understand why a video that comes in as it is needs to be ram previewed in order to playback smoothly.

If I have 1080P video with an audio track, why do I need to ram preview in order to see it playback in real time? There are no fx added and there is no Post Processing why does it not just work?

My biggest problem with this is that it essentially makes pre-rendering useless. A pre-render is supposed to speed up the process but if it also has to be ram rendered it yields little benefit.

I understand ram previewing is part of after effects and it's not supposed to be a real time editor like premier pro. However, I feel that a video of any kind with no post processing, effects, or key frame animation added should ram preview almost instantly. Am I doing something wrong? Is there any way to fix this? I've tried the usual (lower resolution, render every other frame during preview).

Any help or explanation is welcome. Thank you so much.


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Michael SzalapskiRe: Why do 720p and 1080p videos need to ram preview? Why not just play?
by on Feb 4, 2016 at 9:51:30 pm

To quote Rick Gerard:

Sony Vegas, Premiere Pro, Final Cut, Avid are all NLE's (Non Linear Editors) and they are specifically designed to playback a video stream. With any of them, if you stack enough layers or effects on the video they will have to render a new video stream based on pixel based calculations for every pixel in the stack. This rendering, especially for HD sources or for complex plug-ins, will take quite a bit of time.

After Effects, Flame, Fusion, Shake -- are all pixel based image processing applications that act very much like Photoshop. They calculate the values of every pixel in every frame, come up with a new pixel, and then play those pixels back as a video stream. More importantly, AE and all the other pixel based compositing apps, always work internally with completely uncompressed pixel data. NLE's rely on codecs and in some cases, hardware, to playback the video. It's an entirely different way of working with moving images.

In After Effects you enable the preview by loading a bunch of frames into RAM then the video stream is played back. You start the process with the 0 key on the numeric keypad and not with the Space Bar as you do in nearly every NLE ever created. The length of the preview depends entirely on how much free ram you have available and it takes some time to generate these new pixels. The more layers, the more effects, the more calculations that need to be performed the longer it will take to process the RAM preview. There's currently no way around this rendering time. A modern NLE will handle an amazing number of video streams simultaneously, but as soon as you exceed the capability of the system you're stuck with a render. Most NLE's, given the same number of calculations, actually take a little longer than After Effects to do the same kind of effects. Open GL, and other GPU acceleration helps many NLE's achieve higher performance but it has yet to be implemented into a pixel based compositing app. The sad truth of the matter is that if you want to do compositing in any of the available compositing apps, you have to wait for renders. They are getting better. Memory management and efficiency is improving. GPU accelerated effects are being added, but for now, that's about as good as it gets.
I hope this helps. As long as you use After Effects to create shots and don't try to make it do the work of a NLE you should be fine. Movies come from

NLE's, amazing shots come from AE.


Granted, AE's previewing system has changed since he wrote this (you can use spacebar now, hooray!), but the underlying principle is still the same; AE works on completely uncompressed pixels, so it's recreating the image for every frame and not simply playing back a video file.

Does that help?

- The Great Szalam
(The 'Great' stands for 'Not So Great, in fact, Extremely Humble')

No trees were harmed in the creation of this message, but several thousand electrons were mildly inconvenienced.


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Ryan McAdamRe: Why do 720p and 1080p videos need to ram preview? Why not just play?
by on Feb 8, 2016 at 9:55:26 am

That does help. Thank you for the detailed explanation.


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Michael SzalapskiRe: Why do 720p and 1080p videos need to ram preview? Why not just play?
by on Feb 8, 2016 at 2:20:11 pm

You're welcome. And don't forget to thank Rick Gerard if you see him running around somewhere. :)

- The Great Szalam
(The 'Great' stands for 'Not So Great, in fact, Extremely Humble')

No trees were harmed in the creation of this message, but several thousand electrons were mildly inconvenienced.


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