AG_Boston wrote:
This is commonly done in the "remote sensing" world. These are people who analyze satellite images for various reasons. This is called "change detection" in the world of remote sensing. It's done to look at things like large crops to determine if various diseases, and pests are impacting crops. It's also done to see if crops might need more, or less water. There are also other reasons for doing change detection, such as analyzing cities to study urban growth year over year. I tried doing this using Gimp a few years ago, and got it to work, but it was a haul, and not worth the effort. There are analytical issues that come into play, such as "nearest neighbor". Unless you absolutely want to do this, I'd recommend not exploring it unless you really want to go down the rabbit hole on this. A program that is used to analyze satellite images is called ENVI. There are some YouTube videos on this topic, but WOW they are boring. L3 Harris has some info on their site, and it's also pretty dry.
https://www.l3harrisgeospatial.com/Lear … ystems-GIS
https://www.l3harrisgeospatial.com/docs … lysis.html
You can probably find a way to load a photo into ENVI, but I've never tried it.

There was a opensource version of a tool that worked like ENVI, but I can't remember what it is called. Look around for remote sensing analysis software and you might find something.

If it's mostly for star specs in the sky I assume it's not quite right for me, but I'll check out a YouTube video anyway.

The Other Place wrote:
You could probably just use the "subtract" or the "difference" layer blending mode to make a mask.  Then, you would just add the masked layers in the desired order.

I don't currently have Photoshop, but do you know of a good YouTube video that explains addition, subtraction, and multiplication functions in layers? I am completely unfamiliar with them.

Mad Hatter Imagery wrote:
I don't currently have Photoshop, but do you know of a good YouTube video that explains addition, subtraction, and multiplication functions in layers? I am completely unfamiliar with them.

Paint Shop Pro has "difference" and "exclusion" blend modes.  Try those.

Also, you might consider switching to GIMP.  It's free and open source, and it works on Windows, Mac and Linux.  It's powerful with a lot of blending modes and other features, plus there are zillions of GIMP tutorials online.

In regards to a video that explains layer blending modes in Paint Shop Pro, here's one on YouTube.

The Other Place wrote:

Paint Shop Pro has "difference" and "exclusion" blend modes.  Try those.

Also, you might consider switching to GIMP.  It's free and open source, and it works on Windows, Mac and Linux.  It's powerful with a lot of blending modes and other features, plus there are zillions of GIMP tutorials online.

In regards to a video that explains layer blending modes in Paint Shop Pro, here's one on YouTube.

Thanks. I already have Gimp somewhere. It's not the most intuitive. I was going to check out PS prices again.

Mad Hatter Imagery wrote:
I already have Gimp somewhere. It's not the most intuitive. .

GIMP is just as "intuitive" as anything else -- you're just conditioned to what you already use.

The Other Place wrote:

GIMP is just as "intuitive" as anything else -- you're just conditioned to what you already use.

I was always told photoshop was well organized while gimp is kind of chaotic. It always seemed messy to me and I think it crashed a lot.

Mad Hatter Imagery wrote:
I was always told photoshop was well organized while gimp is kind of chaotic.

Don't believe everything that you're told.  The person who told you that likely was conditioned to Photoshop.

Mad Hatter Imagery wrote:
It always seemed messy to me

Messy?  In what way, exactly?  Again, you are conditioned to something else, so you would likely find it confusing.

Mad Hatter Imagery wrote:
... and I think it crashed a lot.

You "think" it crashed?  I know it doesn't crash -- it's always been rock solid for me.

Mad Hatter Imagery wrote:
I don't currently have Photoshop, but do you know of a good YouTube video that explains addition, subtraction, and multiplication functions in layers? I am completely unfamiliar with them.

What editing software do you have - AND USE?
You have certainly posted lots of technical questions. YouTube videos will be specific to particular software.

Mark Salo wrote:
What editing software do you have - AND USE?

He says he uses Paint Shop Pro.

Mad Hatter Imagery wrote:
I was always told photoshop was well organized while gimp is kind of chaotic. It always seemed messy to me and I think it crashed a lot.

I've always found Gimp to be more intuitive and Photoshop to be confusing, and it also crashes more. But I've also used Gimp more than photoshop. It's really just about getting familiar with each program.