I've been experimenting with this for a while and am trying to find the best method.

I have a camera feeding images to a PD patch. A new image comes in say every 15 mins and I want to sum this image individually or layer/stack it on top of the previous image. So ideally; visually the background of the image stays the same, it is only movement or people within the image that is added each time. The best method I have used so far has probably been [pix_diff]

So image 1 loads into [pix_diff] left inlet. Image 2 loads into right inlet. I then take a snapshot of this, save as new image and load the new image back into the left hand side of [pix_diff]... another image gets loaded into right inlet etc. It's not perfect, the image inverts every second time around because of finding absolute difference between the photos.

Other methods I have tried:
[pix_mix] - the new info in each image eventually washes out over time as new images get added.
[pix_compare] - only lighter OR darker pixes are replaced. so not all new info is added to new image, looks messy and lacks continuity
[pix_composite] - similar to pix_mix - old info becomes transparent or washed out as new info is added
[pix_add] & [pix_subtract] causes whiteout & blackout

If I could figure out a method that finds all new info in the image and adds this. I'm sure it can be done, just my basic knowledge of gem and pd doesn't allow it yet!

[alpha 1] in your gem path will allow alpha blending of images.

Looking at the [alpha 1] help example it appears to use the same technique for blending as [pix_mix]. This would work for 2 pictures but I don't think this blending technique works for multiple pictures because it averages between pixes and info eventually gets washed out and lost. i.e. if you have 6 pictures of a lawn; in 1 of the pictures a person stands on the grass, other 5 are just grass. Using this blending on the 6 pics - the result will be 5/6 grass, 1/6 (pretty transparent) person. I need the person to remain visible and not become transparent as more pics are added, if this makes sense. Which is why the [pix_diff] seems to work best for me so far. It gets the absolute difference between pixes rather than blending.

this is a pretty fun challange. if i get it right you want to add just the pixels that changed from the previous picture (background stays the same, foreground layers — let's say a person — overlay and so on), correct?

Exactly Yeah it's been good fun so far, I'll attach what I've been working on to give an idea..

http://www.pdpatchrepo.info/hurleur/Sum_Images.pd

If you need a fast, but more complicated version, you should check out:

help/browser/Gem/examples/10.glsl/multitexture.pd

It is included in pdextended i guess

Theres a puredata version of openCV that might be more feature-ful in detecting motion and such.