Here's a simple way I use to enhance an image, that gives nice results which can be done on any image in Gimp. It's not just for photo enhancement, it's also good for subtle overall image sharpening as well.
I suggest applying this enhancement as the last step when working with any image. Above all making sure that you're doing all your working on a copy of the original image.

Before After

Very Nice Wallace

Thanks CT-Gimper101.
It's something I often use to subtly sharpen an image and thought other might find is useful.

Thank you Wallace. That is very useful and the improvement is noticeable.

Thank you Wallace... nice job

This is great, thanks Wallace! I've been using a similar process, but your method is quicker and I hadn't tried inverting the colors. More ideas to play with, hooray!

Sometimes, for a stronger effect, I'll set the final layer to Grain Merge instead of Overlay.

If the image is noisy and I don't want to sharpen the noise, I'll take the final gray layer and give it a 1 or 2 or 3-pixel gaussian blur. Just enough blur to soften the grainy bits.

This page has some ideas about how much to blur: http://clownfishcafe.blogspot.com/2013/ ... ening.html

This is, far and away, my favorite way to sharpen a photo. I only wish this technique had a name. I've been calling it do-it-yourself highpass sharpening, but I gather this is a little different? What should we call it? Because we really should teach it to everybody.

It's very similar to a high-pass filter. I haven't really gaven thought to naming it. I guess it could be called a sort of DIY High-Pass.
The blur amount has a lot to do with the image size, but the link you provided does shed some light on the blurring process.
It would be easy to experiment with this to achieve the results one might be after, without relying on filters to hopefully do the job for you.
Some Gimp users may also not know how to apply a high-pass filter, nore have a script/filter installed that does this. It's a simple method that can be used on any image, without the need of a filter.

Thanks Graechan.
That's easy for you to say, I'm not a scripter/coder like you.
I know you have only good intentions, but afterall the point being is not having to rely on a script.

Works well Wallace... here is my before and after.
Before

After


Thank you Wallace

Wallace, Cool technique on the homestead.

This feels eerily similar to Filters>Enhance>Unsharp mask (which under the hood is also subtracting a blurred version of the the image from the original to bring out the high-frequency elements...).

looking good Molly!


Yeah thanks Odin.
I couldn't figure out what image to use for the demo, so I used my old childhood home.


Could be, I wouldn't doubt it. Only no use of filters.
I wish I could remember where I learned this technique from.

Yes it does work:

Before After

But this is what Unsharp Mask can do:

It's quite understandable what Unsharp Mask can do.
I did mention that it was A "subtle overall image sharpening", emphasis on the word "subtle".
However... there is no use of a script when using this so called "DIY High-Pass" technique as described.

Thank you Wallace. Very nice indeed
It showed a marked improvement on this photo but I still have to get a spot out (unsharp mask) didn't help
Before

After

I had to try this method. Here is my try. Photo just cnverted to jpg.

Sunrise 2015-01-07

Original:

After Wallace-method:


Thank you Wallace for yor method!

No problem Himppu.
I really can't claim this technique as my own. I picked it up somewhere, but for the life of me, I can't remember where.