Tuesday, October 21, 2008

Oversampling

With all these new high resolution video sources out there, it great to know you can pan into a 2K, 3K or 4K frame and still deliver sharp results at your 720p/1080i output.  However, this oversampling advantage can be easily lost with incorrect use or setup within your NLE. We often see user setups that run the larger source media on a timeline that matches the target resolution, e.g. 4K source in a 1080i/p timeline.   This seems to be an obvious configuration, but I'm going to point out why this is the wrong way to setup your project, particularly if you want any oversampling advantage when re-cropping a larger frame.  Using Premiere Pro as an example, dragging 4K media into a 1080/2K timeline defaults to center cropping the image. This is an annoying default as you likely don't want any cropping for most clips, other than a few scenes that might need a little re-framing, so the 50% center crop default is a nuisance.  The are two solutions to address this center crop:
1) Go through all your clips on the timeline, adding a motion filter to re-frame back to the entire image (particular trickly to do fast with mix resolution sources.)  
2) Use the neat feature, "scale to frame size", which can be set up as the default for these types of projects.

If you pick 2), as it is certainly the easiest, you have lost all your oversampling advantages.  Really!  This scale happens BEFORE any of your timeline based filters see the data.  So 4K images on a 1080 timeline becomes only a 1080 source when you zoom in; push in more than 20% and the image will get soft.  There is nothing wrong with 4K on a 1080p timeline in this mode, as long as you remember to turn that feature off when doing any re-cropping, it is just too easy to forget.

If you go with solution 1), you still have your oversampling benefits for re-framing.  But now you have to be careful as the NLE is doing the resize not the importer, so you have to make sure all your scaling filters are deeper than 8-bit, otherwise you will have lost precision.  Adding a motion filter would seem best, as the Premiere motion filter does support 32-bit processing -- within Prospect HD/4K you can turn on the feature that displays the depth of the filter stack (handy to confirm you are maintaining quality.)  This additional scaling step can make things slower during your edit.

The better solution is to do your edit at your largest source resolution, i.e. editing 4K at 4K and 2K at 2K.  Set the "scale to default size" on so any lower resolution media (e.g. over-cranked sequences) are scaled to fit (no issues with the upscaling.) There are no limitations to doing this, particularly as Prospect 4K already dynamically decodes larger source data to half or quarter resolution as needed for editing speed.  There are many advantages, such as freeing the range of output options for frame size and frame aspect, to be performed at maximum quality.  4K within a default scale to frame 1080 project, will have a little poorer 720p exports, as scales would be 4K to 1080 then 720, rather than 4K oversampling and scaling directly to 720.   I've received some feedback to suggest some feel that zooming into 4K frame in a 4K timeline would result in softer images than the same source in a 2K timeline; they are missing the fact that oversampling happens upon export to whatever the target resolution may be, not how it previews at 100% pixel view on the timeline.  When pushing into a 4K frame, within a 4K project, you can check your sharpness at 1080/2K by setting the program window view to 50%, or look at the 1080p feed over HDSDI, both will allow you to see if you are pushing in too far and preview the quality of the final output.

While instinct tells us for most projects a 1080p export will be plenty of resolution, that doesn't automatically extend to being the ideal timeline format, particularly for oversampled re-framing.

7 comments:

Anonymous said...

very good to know....
so digital zooms should be done on a 4k timeline for say 1080p output...
any issues with cineform express workflow and this scaling....or is it all automatic when relinked

David said...

Yes, digital zooms should be done at the source resolution (4K or whatever) no matter you export resolution (unless you are only posting on youtube.com.) Express files work great with this workflow and they were planned to do so. All Express files are 1K, no matter the source resolution 2K, 3k or 4K. So you should edit you express files within a 1K preset for the best speed, then once locked, import your edit into a 4K sequence and relink to the full resolution clips. Here is where the scale to frame size is particularly useful, now all your master clip fits 4K, even if they were over-cranked 2K, and any re-crops you do at 1K match perfectly at 4K (but know with oversampling.) Now all you need to do is render to your target. Easy.

Rosferatu said...

Thanks for this David.
I know you're not a Final Cut Pro guru but we are using ProRes now to edit (we love Express but Apple kinda hobble that for us) and then are XML'ing that into After Effects for finishing.
Our XML links us back to the 4k Cineform Footage in After Effects nicely.
Do your comments then apply? Is After Effects making the same "mess" that Premiere, FCP etc would?
Thanks again for this post. Its got us all here fascinated in our little studio,
-Ross Webb

David said...

Ross, your setup is fine, compositing tools don't have the same limitation as NLEs.

Anonymous said...

FCP does not suffer from this.
This is adobe "feature"

-Kaspar

David said...

Kaspar,

Don't get too confident about FCP as it can do far worse things. We seen when you mix sequence and source resolutions, it can unexpectantly fall back into 8-bit modes rather than than using 32-bit float. FCP is a finicky product.

Kaspar said...

hi

I mean all of the have their own quirks, if you ever happen to develop for avid you will find them there. FCPs problem is bit depth, but mixed res is much better implemented than any other product in the price range.

-Kaspar