I got the idea for these two filters a long time ago and even sketched out some prototypes a way back. But now that a convolution filter has made its way into the next release, I figure it's time to push these two out into the world as well.
They are pretty simple - and really aren't "filters" so much as they are "store and load" commands you can use in a layer's filter chain. The first is "Snapshot" and the second is "Snapshot Reblend". The first filter remembers the image as it's passed through it, and the second lets you composite that image back into the filter chain.
What's this useful for? Have you ever thought about how drop shadows are made? The most common way is to apply a gaussian blur + grayscale filter + the original image composited back on top. What if you wanted to make a custom shadow which keeps its color and instead of a gaussian blur, it uses a zoom blur?
Here's our deer again, this time with a transparent background. You can see that we're storing a copy of the image, then blurring it, and then drawing on top of that blur, which gives us a drop shadow to the image.
The Snapshot Reblend filter has a couple of options, including the ability to change the blending mode. If we change it to something like "Destination Out", we're essentially erasing where our original pixels were and are left with just the shadow, which is another useful effect. You see it above - but this time with a motion blur.
You could also use this technique with the "Fuzz Stroke" filter to get an outline of your opaque areas.
These effects have always been possible, but you had to use copies of your image on multiple layers and then things might get out of sync and it could be a mess to update. With the Snapshot and Snapshot Reblend filters coming in Acorn 7.3, we just made those annoying to update effects so much easier to do.
If you'd like to play with these filters right now, you can grab them from Acorn's latest builds page.