Why doesn't Multi-frame Rendering work with PNG sequences and only uses one core?

[DanijelC]

I am using the latest version of After Effects (25.1) on a computer with an AMD Ryzen Threadripper PRO 7975WX 32-Core processor, 256 GB RAM, and an RTX 4090. Despite having the 'Enable Multi-frame Rendering' option enabled, it is using only 1 core to render PNG image sequences. Is this normal? Are there any other settings that would allow me to better utilize all of these cores?

[Warren Heaton]

PNG image sequences as Source Footage or as the export format should not disable Multi-frame Rendering.

Do you happen to have a non-MRF effect applied?  That will prevent MFR when enabled.  You'll see an exclamation point "!" next to the Effect in the Effect Controls panel.  

Pictured: Effect not optimized for Multi-frame Rendering indicator

Pictured: Effect not optimized for Multi-frame Rendering dialog box

[DanijelC]

Hi,

No, I haven't noticed that any effect has been labeled as problematic. I have now tried with a very simple composition, a series of PNG images with alpha and a static image in the background. Without any effects. Again, MFR doesn't work when exporting PNG images.

Danijel

[sskaz]

Exporting to PNG is much slower than most other image formats. Whatever compression algorithm they’re using is very slow. Here’s an article benchmarking how much slower it compared to other formats (ProRes, JPG, TIFF, and EXR).

I don’t know why it would disable MFR, but it certainly doesn’t help. Maybe one CPU core gets used for exporting frames to disk and holds up the process in some way?

If your workflow requires PNG, I would render to something else and then use another tool to convert those finished frames to PNG. Media Encoder could work. If lossless is required, use TIFFs, otherwise give EXRs a try.

[DanijelC]

Thank you for the suggestion. I was also considering this option. For comparison, I tested exporting images from a video in PNG and TIFF formats. For exporting 240 frames with a resolution of 1920x2400 px, it took 380 seconds for the PNG format and only 14 seconds for the TIFF format. From the task manager, it was clear that in both cases, only 1 processor out of 64 was used.

[Warren Heaton]

@DanijelC 

Are you seeing the Multi-frame Rendering indicator on the Current Render Status bar?

Pictured: Ae 25.x Current Render status bar indicating Multi-frame Rendering

I'm guesssing that Multi-frame Rendering is enabled in the preferences (Memory & Performance > Performance > Enable Multi-Frame Rendering) with the % CPU reserved for other applciatons at the default 10% and that it is happening, but that you are not seeing activity across multiple cores while rendering.

Pictured: macOS Activity Monitor CPU Usage window while rendering a PNG Image Sequence in After Effects

If you'd like to provide a link to the project file (just the project with no linked media) either here or a direct forum message, I can try to reproduce the issue on a Threadripper based system at work.

Edit:  It looks like Field Kuang joined this thread.  You're in good hands to get this resolved.

[DanijelC]

First of all, thank you very much for your help. The green bar did appear during rendering, but in the info box under concurrent frames rendering, it showed 0.

Aftereffects and Task manager during PNG export

I opened a new project and imported a 240-frame long video, without effects. With MFR enabled and 10% reserved for other applications (even when I assigned 0% to other applications, there was no difference).

Exporting the PNG sequence of 240 frames with a resolution of 1920 x 2400 px took 380 seconds. For comparison, I exported the same sequence in TIFF format, which only took 14 seconds. In both cases, I monitored the process in the task manager. In both cases, only 1 processor was used.

Link to AE file:

https://www.dropbox.com/scl/fi/71lspwlgm2qponfwrmeuk/PNG_Export_MFR_Test.aep?rlkey=hwob7asnczjlxvwuz... 

[Field Kuang]

MFR should support PNG sequence without any problem. Please see the attached screenshot of PNG sequence MFR with 9 threads, taking all cores of the CPU.

A few questions:

1. It appears that MFR threads for PNG sequence do take about 10-15secs to spin up from one to multiple for me. As for the other formats, MFR threads spin up almost instantly. Does your render stay at single thread at all time for a long time?

2. Could you try importing a long video file to AE and render it out as PNG sequence without adding any modification to it? If MFR works for this simple comp but not your comp, it would be really helpful if you could share your project with us. We could investigate the root cause of the problem that prevents MFR from kicking in.

[DanijelC]

Thank you for your help. I tried with a 240-frame long video. Throughout the 380 seconds of PNG export, only one thread was active. I also tried with the TIFF format. Here too, only one thread was used, but it exported 240 frames in just 14 seconds.

[Field Kuang]

I did some research and testing and I think this is what's happening.

Rendering in After Effects consists of two main stages: Compositing and Encoding. Compositing involves calculating effects, transformations, and generating the individual frames. The Encoding stage is to compresses those frames into your selected format.

For formats that support asynchronous encoding or have extremely fast serial encoding, MFR is very noticeable. However, PNG only supports serial encoding and is also relatively slow at encoding. This means MFR applies only during the Compositing stage, but not during Encoding.

I've observed that the complexity of a Comp influences how much MFR benefits the rendering process for PNG. With more complex comps, MFR is more likely to be visible because Compositing takes longer, giving the encoder time to keep up. For simpler comps, Compositing finishes quickly, resulting in a queue of frames waiting for the slower PNG encoder to encode them one by one. This bottleneck effectively limits MFR and makes it behave like SFR.

The screenshot I shared earlier was from rendering a very complex 4K comp with 3D, lighting, and special effects. In this case, the Compositing stage was slow enough for the PNG encoder to keep pace. However, when I rendered a simpler comp with one solid, I experienced behavior similar to SFR, as you described.