Monday, February 21, 2011

Premiere Pro: Red, yellow, and green render bars and what they mean

Each Premiere Pro class that teach find me answering the question about the "render bars" and what they mean. Here's a good answer direct from Adobe: "If you’ve worked with Adobe Premiere Pro even a little bit, you’ve noticed that colored bars—red, yellow, and green—appear at the bottom of the time ruler at the top of the Timeline panel, above clips in a sequence. These colored bars are often referred to as render bars. But what do they mean, and what does this mean to your work?

First, we need to understand what it means to render a preview.

In the context of computer graphics, rendering is the creation of an image from a set of inputs. For Premiere Pro, this essentially refers to the creation of the frames in a sequence from the decoded source media for the clips, any transformations or interpretations done to fit the source media into a sequence, and the effects applied to the clips.

For clips based on simple source media that match the sequence settings and have only simple effects applied, Premiere Pro can render the frames that make up the sequence in real time. In this case, each frame is rendered for display just before the CTI (current time indicator) reaches it. Premiere Pro caches these results so that it doesn’t unnecessarily redo work when you revisit a frame.

For more complex sets of effects and more difficult source media, Premiere Pro can’t always render the frames of the sequence as fast as needed to play them back in real time. To play these frames in real time, they need to be processed and saved ahead of time, so that they can be read back and played instead of being recalculated on the fly. The creation of these frames to be saved for rapid playback is what is meant by rendering a preview.

By the way, it’s common but confusing and misleading jargon to refer to rendering of previews as rendering all by itself. Rendering for display, rendering for final output, rendering for previews—these are all valid uses of the word rendering. Don’t fall into the trap of using this general term to refer only to the specific case of rendering for the purpose of creating preview files for real-time playback.

Note: Rendering of previews is only for preview purposes. Preview files will not be used for final output unless you have Use Previews option checked on output—which you should not use except in the case of rough previews. Using preview files for final output will in almost all cases cause a decrease in quality. It can speed things up in some cases, so it may be useful for creating a rough preview in less time.

With that preparatory definition out of the way, what do the colored bars mean?

Green: This segment of the sequence has a rendered preview file associated with it. Playback will play using the rendered preview file. Playback at full quality is certain to be in real time.
Yellow: This segment of the sequence does not have a rendered preview file associated with it. Playback will play by rendering each frame just before the CTI reaches it. Playback at full quality will probably be in real time (but it might not be).
Red: This segment of the sequence does not have a rendered preview file associated with it. Playback will play by rendering each frame just before the CTI reaches it. Playback at full quality will probably not be in real time (but it might be).
None: This segment of the sequence does not have a rendered preview file associated with it, but the codec of the source media is simple enough that it can essentially be treated as its own preview file. Playback will play directly from the original source media file. Playback at full quality is certain to be in real time. This only occurs for a few codecs (including DV and DVCPRO).

Note the uses of the word probably above. The colors aren’t a promise. They’re a guess based on some rather simple criteria. If you have a fast computer, than a lot of things marked with red may play back in real time; if you have a slow computer, then some things marked with yellow may need to be rendered to preview files before the segment can be played in real time ..."

Click here to continue reading - and find out what causes a segment to get render bars of a certain color as well as how the Mercury Playback Engine changes things.

Enjoy.

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