File I/O -> Experimental HEVC decoding: Has severe problem with a clip

John-Callahan wrote on 7/6/2025, 6:15 AM

I have been using the option in preferences: File I/O ->Enable experimental HEVC decoding. I noticed recently whilst rendering a series of clips (11 clips = 8min 26sec) that at about 46% into the render it slowed down substantially. Using a series of tests I narrowed down the problem to one of the clips. The clip was only 9sec and 13frames (238frames), but the render time increased by a factor of about x120. I tried rendering with and without that option as well as using AMD 9070 acceleration, Radeon Graphics acceleration and not acceleration (CPU) and found they all suffered the same. Below are the statistics for rendering that clip, with and without the use of that setting:

GoPro12 4K (HEVC) clip - Gx010011.mp4 (9s+13frame = 238frames)

Without using experimental HEVC decoding: Renders in 1.82s = 130.5 fps
Using experimental HEVC decoding: Renders in 3min 42s = 1.07 fps

The rendered video results

Without using experimental HEVC decoding: Video is 33.2MB and looks the same as source
Using experimental HEVC decoding: Video is 19.4MB and flickers

Attached are the source video clips and the two rendered clips.

Comments

EricLNZ wrote on 7/6/2025, 6:29 AM

@John-Callahan Please upload your original problem clip to a cloud service such as Google Drive so others can download and try it on their systems.

John-Callahan wrote on 7/6/2025, 6:37 AM

I uploaded the simple Vegas project file (.veg), the source MP4 and the two rendered videos as mentioned in my original post:

https://1drv.ms/f/c/23fbf66dfe042b97/Ep_IUAKYA9pKlLgwVU748WsBDqQZszPakpxWl6srXWU5Hw?e=dki0wM

Dexcon wrote on 7/6/2025, 6:51 AM

Experimental HEVC decoding was useful for me with GoPro 11 HEVC video in Vegas Pro 21 because it reduced the amount of stuttering on the timeline (not completely though). From various posts on the forum, the Vegas team have been developing VP's video engine and HEVC performance is so much better in Vegas Pro 22 to the extent that I no longer need to activate Experimental HEVC decoding in File I/O.

Yes, your issue is rendering, but if you're getting better performance without the experimental option activated, why not keep it de-activated for rendering - maybe 'experimental' is no longer a good 'back-up' option in Vegas Pro 22 given the improvements made in VP 22's video engine.

Gid wrote on 7/6/2025, 7:07 AM

@John-Callahan I noticed a very big improvement with GoPro HEVC after VP21 build (300) - Experimental unticked/off, I too now don't have it turned on, so If you're getting better performance with it off leave it off.

I believe the 'experimental' was before the VP team made the improvements in VP21 (300) & is poss now obsolete, they've left it I guess because of the numerous amount of file types & it may help someone.

John-Callahan wrote on 7/6/2025, 7:08 AM

@Dexcon - I was trying it as I have many timelines with HEVC video. Some timelines use a combination of AVC and HEVC.

As far as the issue being rendering, that is not the case. I can see the affect in the preview window. It manifests as a delay in getting the frame number. Without the option selected when I click anywhere on the timeline the preview shows that frame and the frame number changes. However, with that option enabled, when I clip at a point in the bad clip the preview does not update immediately and the frame number still shows the previous number, but with an additional '....'. For instances, the current preview is showing frame 500 and I click to another part of the timeline and it shows the preview and new frame number immediately. However, if I am on frame 500 and click in the problem clips the preview does not update yet and the frame number will show 500..., but eventually will update to show the actual frame.

Unless there is a significant increase in performance using this 'experimental' option, I will leave it off. I only spent the time with this issue to post it for the Vegas (MAGIX) developers to use in any debugging they need to do.

Dexcon wrote on 7/6/2025, 8:59 AM

Thank you for uploading a sample of your GoPro HEVC video.

On both my desktop and laptop (both VP22 b250) with 'Experimental HEVC decoding' off, the video played smoothly (Best/Auto) and the clock/timer updated instantly. On the other hand - and on both computers with 'Experimental HEVC decoding' enabled - timeline playback was absolute rubbish - massive stuttering/stalling with mostly only a black image in the preview screen, and with very sluggish clock/timer updating. This was far worse than I've experienced before with HEVC video in any version of Vegas Pro.

With similar GoPro UHD 4.2.0 25 fps video from my GoPro 11, a MediaInfo reports shows the bitrate as being circa 121 Mbps, but MediaInfo shows your video as having a bit rate of just 7,822 kbps - that's about 7.64 Mbps, close to 16 times smaller.

BTW, my projects have a mix of UHD MP4 AVC and HEVC - and before VP22 I usually needed to activate Legacy AVC Decoding in File I/O to get better timeline performance with AVC, but that's not needed any more in Vegas Pro 22.

Please note that Experimental HEVC Decoding was introduced in Vegas Pro 17 and, as to my understanding, was introduced to deal with HEVC media not then fully supported by Vegas Pro. I suspect that the Vegas team is focused in tweaking the video engine to fully cope with HEVC rather than supporting a years old 'back-stop' for HEVC.

RogerS wrote on 7/6/2025, 9:20 AM

The experimental decoder is the old one and unless you need to use it for a particular file type I'd avoid it. If a file isn't working well with the current decoder let's work to figure out and report the problem so it can be improved.

Howard-Vigorita wrote on 7/6/2025, 1:42 PM

What @RogerS said. I would only use Experimental Hevc on footage from a camera that doesn't work with the vp22 default decoder. Which is impractical in multicam combined with footage that works better with the default. I would just transcode the errant footage with either Shutter Encoder or ffmpeg.

I shoot 4k exclusively in hevc and only buy cameras whose footage works with Vegas. So I've never had to do that with my own footage. Right now I'm shooting multicam with 3 hevc 4k cameras: Zcam E2, Canon xf605, and Canon C70. Or if traveling light, with a Samsung S23 on a BeastGrip.