Understanding Rendering--with Trim Paths

You need to have a good grasp of CODECs and File Formats in order become better; have more confidence with the decisions you make.

CODECs are like different dishes you eat. Some dishes are meant for breakfast and others for lunch, dinner etc. Each dish is better suited for a specific meal.

Each CODEC was developed to provide a solution to a specific acquisition, production and delivery problem.

For production (or post productionin this case), you will want to use a high quality CODEC - the higher, the better but with an appreciation that file sizes may get prohibitively high. So, pick a CODEC with lossless compression. Most folks use DNxHD, Cineform, Apple ProRes; to name a few. You will want a CODEC that supports Alpha Channel if you're working with CG. Stay away from H264 as a production format. H264 was developed as a delivery format. Consumer and some prosumer cameras capture to H264. If time permits and you have a lot of footage, convert the footage into a mezzanine format/CODEC - one of the production CODECs mentioned earlier.

On pre-rendering, this is highly advisable as this helps to lower the final render times. Pre-render at the end of the day or during breaks that are sufficiently long for you to get the pre-render done. Again, use a high quality CODEC for your pre-renders. Pre-rendering can significantly speed up your final renders. As a general rule, do not allow your final render to take longer than 8-10 hours - about the duration of an overnight sleep. This is to allow you to get in the next day and start working instead of sitting on your butt; waiting for the render to complete.

There may be slight quality degradation when working with a pre-render that has been compressed. However, there is a very high chance that the drop in quality will not be noticeable.

Lastly, Media Encoder off course supports Alpha Channel output. You just have to ensure you select a CODEC that supports Alpha Channel and then seelct options that will indeed render the Alpha Channel since all CODECs that support Alpha Channel can also output without an Alpha Channel; the obverse isn't true.

Why render precomps at all? If you need them pre-rendered to improve speed when working on the final comp, then render them as proxies (File>Create Proxy), and in your final render turn off proxies in your quality settings.

If you do need to use intermediate renders for whatever reason, the best bet is one of the codecs designed for the purpose, e.g. Apple Prores 4444 or Avid DNxHR. They both support alpha, and are significantly smaller than uncompressed. Both are lossy, but minimally so, and they also support higher colour depth (e.g. 10 bit colour) which is critical if you're doing further processing or grading on them.

h.264 is a great codec for web delivery, but a terrible intermediate codec: it is highly compressed with lossy compression, it usually uses inter frame prediction and is usually only 8-bit, no alpha. You can do lossless, all I-frame, 10-bit h.264 (though I'm not sure if any of the Adobe encoders do it), but at that point the file sizes will rival that of proRes or DNxHD.

But mostly, don't render precomps.