Workaround: render animation issues

[ryichando]

Who this affects: Anyone rendering long simulated animations, especially on GPUs with limited VRAM (the report that prompted this was an RTX 4070 Laptop, 8 GB, Blender 5.0, Windows 11).

The symptom

When rendering a long animation with Blender's built-in Render Animation (Ctrl+F12), frames render at normal speed for a while (e.g. ~5-10 s each), then after roughly 100-120 frames the per-frame time suddenly jumps 70-100x (a 10 s frame becomes several minutes). Dedicated GPU memory climbs steadily in the frames leading up to the slowdown until it plateaus at the VRAM ceiling, after which everything crawls.

Closing and reopening Blender, then resuming from where you left off, temporarily resets it, until it happens again a few dozen frames later. The slowdown reproduces even when rendering to PNG rather than to a video container, so it is not specific to the output format.

Why it happens

Blender's Ctrl+F12 renders the whole frame range as one continuous job that holds onto the simulated mesh's GPU data for the entire run. Over a long sequence the VRAM footprint keeps growing until it saturates, and once the GPU starts spilling, render time collapses.

This is most likely caused by my own misuse of the PC2 cache rather than a Blender problem, but I couldn't track it down.

The workaround

Use the add-on's own Render Animation button in the Debug menu instead of Ctrl+F12.

It renders the animation one frame at a time as separate, self-contained renders, with a brief pause between each. That pause lets Blender release the previous frame's GPU memory before the next frame starts, so VRAM stays flat and per-frame time stays constant for the whole sequence. Confirmed in #44 to keep render times steady where the built-in path slowed to a crawl.

A couple of things to know:

• This button outputs image frames (e.g. PNG), not a video file. If your output is set to a video format, use an image format instead.
• To turn the frames into a video afterward, use any external tool, e.g. ffmpeg:

  ffmpeg -framerate 30 -i frame_%04d.png -c:v libx264 -pix_fmt yuv420p out.mp4
  

  Blender's built-in Video Sequence Editor can also compile the image sequence directly.

We're still looking into the underlying cause (it may be related to how the PC2 cache is used), but for now the custom Render Animation button is the reliable way to render long sequences without the slowdown.