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In a world of real-time editing and effects software, it's easier every day for new artists to do a great deal of work without really needing to render out their work in order to sufficiently preview it. Especially for After Effects artists who are coming from Premiere or Final Cut Pro, the idea behind the RAM Preview might not make complete sense. 

In today's article, two parts – the Backgrounder is for new AE artists, and the Tips section should be useful to everyone.

rampreview-1.pngYou've no doubt noticed the little green "tape" above your timeline (pardon, "Time Bar") in After Effects.  In fact, if you're coming from other motion graphics or editing software, it's probably familiar: in most circumstances, it refers to portions of your project that are rendered.

The same is true in After Effects.  But in After Effects, you'll be seeing a whole lot more of the little green bar.  While editing software generally streams video straight from the disk to the screen, motion graphics software always represents your project as more of a "recipe card": the computer must manipulate the original video into an intermediate format.  This requires a substantial amount of extra processing power – quite often, it requires so much extra time that the computer can't maintain a real-time framerate during preview playback.  That's where RAM Previews come in.
When you render a RAM Preview, you're performing the intermediate calculations – "baking the cake," as it were – just as if you were outputting a final image.  Once a frame is rendered, it plays back at real time, just like source clips (and outputted clips) do.  A RAM Preview, then, is very similar to rendering a clip to disk: the difference is that a RAM Preview saves time by rendering to a temporary buffer in your computer's memory instead of writing to disk.

To render a RAM Preview, press the 0 key on your numeric keypad or click the RAM Preview button in the Preview pane.
rampreview-2.pngAE will render each frame in your Work Area as quickly as it can, then play through the entire Work Area in realtime.  RAM Previews are limited by the amount of RAM in your computer – when your RAM fills up, AE will quit rendering additional frames, and play back as much as it could fit in the buffer.

RAM Preview Tips
  • To free up RAM by erasing your RAM Previews, use Edit -> Purge -> Image Caches.rampreview-3.png
  • You can configure two different sets of RAM Preview options: one uses the default RAM Preview shortcut (0 on numeric keypad or click RAM Preview button), while the other uses Shift plus the shortcut (so Shift+Num0 or Shift-click RAM Preview button).  For example, you could set up a "quick and dirty" RAM Preview with lower resolution and framerate, then a "high quality" RAM Preview that renders more slowly but plays back at full quality.
  • You can make your RAM Previews render faster in a few ways. 
    • First, consider dialing down the frame rate for playback.  You still get a sense of the timing, but you only have to render a fraction of the frames.  Counterintuitively, you use the "Skip" setting to render at a lower frame rate; adjusting the "Frame Rate" setting renders all frames but plays the comp back in slow- or fast-motion.
    • Also, consider using a lower resolution for the preview.  If you can settle for half resolution and half framerate, the rendering process is doing 1/8 the work of a full-rez, full rate output.rampreview-4.png
    • Finally, you might save time by setting your work area – personally, I'm too lazy for that; I just click to stop the RAM Preview render when it has finished rendering the region I'm interested in.

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Revealing Affiliated Clips in Final Cut Pro 7 was the previous entry in this blog.

Preparing Slideshow Assets for DVD Studio Pro is the next entry in this blog.

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