Exposure stacking for video light painting

I'm looking to create an exposure stacked video for a light painting project. Ie. I want to blend all moving (and static) subjects evenly.

Below is a rough example assuming I have 5 frames that make up an input frameset (IF) and output frameset (OF). The OF should look like:

OF 1 = IF 1
OF 2 = IF 1 + IF 2
OF 3 = IF 1 + IF 2 + IF 3
OF 4 = IF 1 + IF 2 + IF 3 + IF 4
OF 5 = IF 1 + IF 2 + IF 3 + IF 4 + IF 5

I understand that the exposure will increase each frame; it would be nice to counteract this during the rendering process, but my main goal is to show the current frame with cumulative motion from previous frames. Exposure compensation is a secondary goal. Is this possible within Resolve / Fusion?

Thanks,
Dustin

The first way that comes to mind is to use the Trails tool to combine the images. Set the Alpha Gain to 0 in order to get a simple Add (Not the first Gain control; the Alpha Gain slider down below the Apply Mode controls). To counteract the exposure gain, use a BrightnessContrast tool and set this expression on Gain: 1/(time+1)

Gain is simply a multiply, or scale, on the luminance, so this formula will progressively darken the result based on the number of frames that have been rendered. I have added +1 to the time under the assumption that you're starting at frame 0, Fusion's default. If your global range starts at 1, then 1/time is what you should use.

I am not sure how small the number can go before you start running into

Trails is unlike other Fusion tools—its output is cumulative instead of being recalculated each frame, so when you use it, you have to start at the beginning and just let it render. If you need to make a change, you have to return to the beginning of the timeline and click the Restart button to clear its previous calculations.

Nice! I was able to get Trails and BrightnessContrast tool, and this is exactly what I was looking for

I ran into a small issue with the Fusion Trails Render Cache, but all is good after resetting the Trails tool and then also deleting the Render Cache immediately before exporting.

Thanks so much.

Oh, that's good advice! I hadn't considered how Trails would interact with Resolve's cache.

Here's the first test I did with this technique - https://my.panomoments.com/u/MomentCapt ... ign-trails

Thanks again for your help.