I filed this in the back of my mind until I worked out how to do it. This is one way and there must be more.

1) start with a series of frames (imported from a video)
2) save as an xcf with the correct timing.
3) make the series reverse using 'reverse/mirror layers'
4) save reversed series as another xcf.
5) use 'Wills Animation Merge' set to addition to merge the two xcf streams together.
It doesn't matter which one is first or second.
Multiplication and hard light also works but for this addition was the best. Out of 36 layers I had to delete layers 19 and 36.
Step thru with Animation Playback to check any layers that are frozen. This is caused by the maths in the animation merge script.
I used 3 scripts, Wills Animation Merge, reverse/mirror layers and also transoductin's Run Code on Visible Layers to take the alpha off all layers.

This works quite well.
The flames look nice, the reverse is more visible higher in the air with the clouds hugging each other.
I've used similar approach for steam discharge, tobacco smoke and the like.

I think the easiest way to do this, is to use a video editor. Take a short clip into the timeline and split it in the middle. Then swap the order of the clips so the 1st is now the 2nd. Put a blend transition between the clips. You could do the same in Gimp I suppose.

Example:

Hi Nidhogg - I appreciate your fine eye and feedback. I notice you posted an image of the US pres on another thread. Doesn't bother me but have you seen the GC site policy?
Controversial political, religious and ethnic discussions are not appropriate topics on Gimp Chat. Please refrain from posting images of political figures, past or present. Gimp Chat enjoys a multi-national membership and we want to keep it free from controversies coming from those kinds of issues.

To me the GC site policy looks just like the hippocratic oath for doctors being implemented on Easter Island just before they wiped themselves out.

Hi racer-x - Thats a nice warm fire and it really shows the difference between the Northern and Southern Hemispheres right now. The reflections on the guitar are excellent. I would be disappointed it you didn't do graphics for a living.

My gif is only a background for a blatant political statement about how the earth's biggest coal miner is currently burning itself to death.
The trouble with bushfire footage is its always shot be people who have had the sh1t scared out of them and its jumping all over the place.

No, I don't do Graphics for a living. It's always been just a hobby of mine. I've always worked hard with my hands for a living and am now forced into early retirement.

For close-up shots of fire, it's best to start with a short clip around 4 seconds because the wood is burning away. I grabbed this video on-line. https://pixabay.com/videos/search/flame/

I think I'm safe. It wasn't tasteless or controversial and falls in the comics category.
If anyone is offended by a gently stippled celebrity pic, (used extra soft brush) I can replace it with a stippled image of Rocky Balboa.

I wouldn't sweat it Nidhogg, I think political figures are fair game. Especially corrupt and dangerous leaders like the current President* of the US. The only people that get worked up are political junkies anyway. I'm a registered Independent and hate politics, the only things I'm interested in is policies.

I just discovered that ShotCut supports Track Masks, cool....

That's pretty cool ani
I haven't used ShotCut, but I installed AVLinux on another HDD and been testing KDEnlive for simple video editing.

The burning skull looks awesome racer

Rocky Balboa is interchangeable with potus

I made this one with Kdenlive but it looks like nobody sets up a tripod close to an out of control fire
I used 2 'Dissolve' transitions but there is still a jerk in it.

@ Tas_mania, It's best to use footage shot on a tripod or it looks pretty bad.

@ Nidhogg, Out of curiousity I downloaded AVlinux and ran it on a live SD card. It might be a good way for newbies to get started in multimedia editing as it includes a lot of tools. However most of those tools are older.

I run most of my editing tools in AppImage versions. That way you get the latest and they are a lot easier to work with and already have all needed dependencies. AppImages work great with whatever Linux desktop you like. Best of both worlds in my opinion.

Made the clip with Kdenlive, edited with Gimp, made gif with an online app.

Nice one Tas_mania. Can you imagine having to go to work everyday like that? I wonder how long those people can hang on to the side of those train cars before they fall away?

Nice seamless ani Tas_mania.
Does it go to opposite direction than the coal train you did recently? These must be the miners going to work
Racer-x, yeah some of the tools in AVLinux are probably outdated.
It would have been quite a chore to install low latency kernel, all the apps and configure them to work fluently.
When I tried the same video apps on Ubuntu, they were useless because of the lag and didn't include as many effects.
But I'm using it for audio primarily, recording some musical keyboard play using VSTi plug-ins & Audacity.

Never tried AppImages, but I'll give it a shot next time.

I've always liked trains. These overcrowded trains are free because the conductor would have to move along the roof to check tickets. I think these trains are very dangerous because sometimes somebody on the roof stands up and gets hit by a bridge or tunnel. Luckily human beings are easily replaced.....



racer-x gave me the idea of a burning skull avatar that burned-out.

I think I made a mistake on the train gif. I use Gimp to delete every second frame, then I renumbered the layers , then deleted every second frame again.
First I deleted even numbers then odd numbers. Because they were renumbered I actually deleted frames in series.

If you delete half of the frames, you are effectively reducing the frame-rate in half. To compensate you should speed it up by reducing the frame delay to around 60 ms. I do all that in ffmpeg. It's exactly what I did with the flaming skull avatar.