Auto Mask makes playback stutter / lag

Although they have similarities, the internal implementation of Auto Mask differs from Magnetic Mask. Auto Mask is incredibly fast at defining the mask, but incurs more runtime overhead. If you select that clip and do CTRL+R to render it to cache, it should then play smoothly. But rendering the clip may take some time.

By contrast, Magnetic Mask's "Analyze" feature can take time, but when it finishes, it tends to play more smoothly without needing to render to cache with CTRL+R.

This difference is fully expected. But if you have a case where Auto Mask has sluggish playback even after rendering to cache (IOW the render dots vanish above that clip), then please report more details.

If so, is it Auto Mask itself causing it, or is it some combination of effects plus Auto Mask? There are some known cases (not limited to Auto Mask) where certain combinations of effects may not get the expected performance benefit of rendering to cache. You can help us see if that case applies by selecting that clip, Edit > Remove Attributes, then remove all attributes except for one using Auto Mask.

Since Auto Mask is often applied as an option on other color effects such as Color Wheels, as a troubleshooting procedure, you may have to temporarily remove or disable each of those except the one using Auto Mask. IOW it may not appear as a stand-alone effect. But the overall area to examine is if the clip contains other effects besides the Auto Mask-enabled effect. If so, selectively remove those as an investigative step to see what combination of effects cause this. This would also apply to computationally-intensive timeline operations such as retiming with Machine Learning.

Auto Mask has a lot to do in real time. However you should not experience this after rendering the clip it's applied to as FCP is basically just playing back a regular single-stream video file it has tucked away instead of the actual clip itself. I just tried it and could not replicate the lag. It took a long time to render though on my M1 Ultra Mac Studio with 64GB of RAM. Are you sure it finished rendering? How does it look when you export that section?

There’s really nothing to “fix” here. If it plays OK after you render the section, it will be fine in your exported file.

I often run into sections of a time line with a lot of effects that won’t play smoothly in real time. However, the final exported file always plays smoothly and looks fine. When I first started using Final Cut Pro, I was concerned as you are, but I don’t worry about it any more.

That is just not true, only if you render the clip first you will have a stutterless playback. MacStudio M4. A search will tell you the same. Except for rendering there is no fix, just like Old_Video_Guy says.

levskomatic wrote:

That is just not true, only if you render the clip first you will have a stutterless playback. MacStudio M4. A search will tell you the same. Except for rendering there is no fix, just like Old_Video_Guy says.

Correct…there is nothing to “fix” here.

My point is that it is not necessary to have stutterless playback from the timeline in order to have a properly rendered, smooth playing final file for your client. All the “rendering” needed will be done when the file is exported.