Simulate accumulation of long term exposure

kentavv wrote on 8/9/2016, 10:07 AM
I would like to create a long term exposure using video. With each frame being equivalent to the long term exposure of the all the frames before it.

Imagine a dark room with a moving light. Each frame shows the complete path up to the point taken by the light. The final frame shows the entire path of the light.

Perhaps said another way, I want to take a video, and replace each frame with a blend of all previous frames. Control over the blending function, similar to Photoshop's layer blending would be fantastic.

I hope the question makes sense.

Thank you, Kent


john_dennis wrote on 8/9/2016, 10:59 AM
Depending on how "long term" your final piece might be, you could render to a PNG image sequence and actually use Photoshop's layer blending.

Have you considered stacking multiple copies of your video on many tracks, shifting by one to n frames and changing the opacity of the tracks to achieve the blend you're looking for? Haven't tried it but multiple tracks in Vegas Pro are analogous to Photoshop layers.
kentavv wrote on 8/16/2016, 1:08 PM
Thank you for the suggestion. I ended up using ffmpeg to convert frames to images, then a Python program to build the accumulation images, then ffmpeg to compose the images. It's a bit convoluted, but only time consuming the first time... hopefully :)
Chienworks wrote on 8/16/2016, 4:33 PM
For real accumulation of exposure you'd want to use "additive" rather than opacity. Just keep in mind that it would be easy to overexpose, depending on how bright the frames are.

Note also that any fast movement may show up as discrete steps rather than a smooth blur, since you're combining discrete image frames rather than having had the shutter open continuously.