Video Track Priority Changed in Resolve 17

In Resolve 17.x, rendered video clips placed on the top track of a project no longer play with priority over the clips in the tracks below it (which may contain FX, cropping and grading work), as they used to. What now happens is that Resolve 17.x is processing all the tracks in the timeline without giving priority to the clean and rendered track in the topmost position, as it did in all earlier versions.

To get around this I am having to disable all video tracks except the top track when I'm working like this. It is certainly an unusual change of behaviour in the evolution of DaVinci Resolve. Is this a feature, or is it a bug? Big headache for long-form feature film making.

You're the first I've seen report this, but I'm having difficulty imagining what the cause might be.

Any chance you could create a smaller project and share that as an Archive (.dra) so we can examine directly?

[Rohit Gupta]

Are you rendering in a format which has a valid alpha channel?

Are you rendering in a format which has a valid alpha channel?

No; the topmost file is plain 8bit H.264 1080p 25fps - definitely super easily playable with minimal processing overhead. No alpha. No treatments. This used to work and is common to pretty well every other NLE I've used.

You're the first I've seen report this, but I'm having difficulty imagining what the cause might be.

Any chance you could create a smaller project and share that as an Archive (.dra) so we can examine directly?

Not sure if that would help, yet. Because I've found out a bit more:

If the topmost clean H.264 clip (let's call it a proxy clip) is of a different resolution compared with the timeline, then the stuttering playback issue manifests. If it is of the same resolution then there is no issue.

I work mostly in DCI 4K 4096x1716 timeline resolution at 25fps. Ordinarily, any H.264 proxy file at pretty well any resolution and with minimal compression will play perfectly in DR when placed in the topmost position over the graded and treated clips in the timeline; it used to take priority. Now when I render a section of timeline at the same DCI 4K 4096x1716 resolution as the timeline then it plays perfectly when placed in the topmost position and it does take priority, as expected. A smaller 2K 2048x858 H.264 clip (note: same aspect ratio and framerate) will not play smoothly when placed in this situation. Timeline is set to scale full frame with crop, so the proxy at smaller resolution upscales to full screen. This didn't used to be an issue in previous versions of DR. So it appears to be a scaling-related phenomenon. Bear in mind that these are only my initial assesments.

Naturally, all the proxy clips are flattened 8bit video with no treatments and they all upscale and play perfectly when placed on the bottom-most position on the timeline (track V1).

Ther best way to test this is to make a 4K project, introduce some footage and apply treatments by adding some film grain and maybe some denoising to the footage - just to severely bump up the required processing power until it won't play smoothly directly from the timeline. Then render out the timeline as a proxy H.264 clip at best quality at (1) timeline resolution and (2) and a lower resolution, say 2K. Introduce the proxy clips over the original treated timeline clip and see what happens.

OK, that is a very different description of the issue than the first post.

Does the 'proxy' play without issue in the Source Viewer?

OK, that is a very different description of the issue than the first post.
Does the 'proxy' play without issue in the Source Viewer?

No; not different; certainly more detailed.
Yes; the "proxy" plays perfectly in the source viewer.

Alright.

What happens if you place that lower res proxy onto V1 of a new 4K timeline, no other clips.

Alright.
What happens if you place that lower res proxy onto V1 of a new 4K timeline, no other clips.

Then the clips play perfectly, as expected.

OK.

And when that clip is in either the original or the new timeline, it fills the frame?

OK.
And when that clip is in either the original or the new timeline, it fills the frame?

Yes it does.

OK, I ran a test in Studio 17.1 for Windows and I'm seeing the same behavior.

I'm thinking it may be related to the Input Scaling. Despite those settings being sufficient for the clip to fill the frame, Resolve isn't quite recognizing that and is processing the clips underneath, even though they don't actually appear in the frame because of the clip on top.

That's the second oddity I've seen with Input Scaling in 17.1.

Tried a UHD mp4 on V1 and HD mp4 on V2. Individually they play back in real time, but even with no effects applied, if I enable both V1 and V2 (filling the frame), I only get 19fps in a 23.98 timeline.

Reverse those Tom. UHD on V2. What happens?

Reverse those Tom. UHD on V2. What happens?

Well I didn't save the project so I'm having to recreate this timeline, using the same set of sources but inevitably different clips.

Somehow I'm now getting real time playback no matter which resolution clip is on top (yes I've checked the settings), so I added a Box Blur to the UHD clip.

HD clip on its own is real time. UHD on its own is 15fps.
If I have both enabled, then no matter which on is on top I get 12.5fps.

Now I switched it so the HD clip is the one with the Box Blur.

HD clip on its own is 14.5fps. BUT HD clip on top of UHD clip is still 14.5fps, and UHD clip on top is real time! So I don't know what's going on there. Anyway I'll leave it to BMD to fix from here.

If I have both enabled, then no matter which on is on top I get 12.5fps.

That's what I saw as well. Resolve is rendering the effects, even though the clip with those effects isn't visible in the frame at all.

Weird.

For the sake of completeness, I can report that Resolve version 17.1.1 does not show this issue. Rendered clips placed in timelines play as they should; Resolve is no longer trying to process the FX of any clips which are on lower tracks in the timeline.

Nice!