I took my first attempt at fireworks photos last night. I did okay with them, but I couldn't balance the light so that the fireworks came out looking good along with the foreground. Either the foreground was exposed well and the fireworks were blown out, or the foreground was all black and the fireworks came out nice. So, I went home a little disappointed.
While going through my photos I had an idea -combine 3 of the photos: the foreground, and some individual firework shots, and then I'd get something closer to what I had hoped for.
So, I opened a shot of the foreground that I had taken for test purposes in order to get a starting point for my exposure settings:
And two separate fireworks photos that I had taken:
And I opened them in Gimp in different layers (with the background being the first or "background" layer).
I then set the Layer Mode to "Lighten Only" (you do this in the "Layers" window which you can open by clicking on the "Windows" menu > "Dockable Dialogs" > "Layers"). This is the key. "Lighten Only" means that only the parts of the firework layers that are brighter than the background layer will show up. This means the black sky is automatically ignored and just the fireworks show up in the image. No cutting or selecting required -neat huh?
After that it was just a matter of resizing and positioning the fireworks where I wanted them in the image and erasing the parts of the trails that overlapped the tree line so it'd look like they were coming from behind the trees.
When that all looked good, I flattened the image and then created a duplicate layer that I flipped and positioned so I'd get the reflection in the water of the fireworks. I erased the tree line in the new layer since I only wanted the fireworks to be reflected, then applied a "Motion Blur" to the layer so the reflection of the fireworks would be blurred just like the lights on the water.
After that I was done. Here's the result: