When I work on a photo using the GIMP, I typically follow a technique that focuses either on the Grain Merge filter or the Value filter.
Before I decide the values for any filter, I create two layers, copied from the base picture:
And later I might create some masks or shade layers, and I’ll explain those later.
My base image:
First: the desaturated layer (BW) I often use a luminance style desaturation, since that provides more intersting contrast. I might use the Brightness-Contrast filter on that layer afterwards, in order to boost that layers effect. When it comes to using that layer, recently I use it as a Value layer at full opacity. I used to use it as a Grain Merge layer at about 50% opacity, but I tend to find that effect too bold.
Next, I use a color enhance step on the CE layer. This I tend never to use at full opacity, that would be absurd. I generally apply a color enhanced layer at about 20-40% opacity as visually appropriate. This helps me get to the saturation level I pre-visualized for the photo.
Recently, I’ve been using a channel-mixer layer. In this case, i clone the original layer as I would normally, but I choose to Channel Mixer, and often I pull down the blue and green channels to boost the golden or red hues of the scene. You don’t have to do this much, even 10% for each channel can be plenty. And when I do this, I might apply it as a Color or as a Soft Light layer in the mix.
Some times I apply some sharpening, but most of the time I tend not to, as Unsharp Mask can be a very heavy hammer to wield on a photo.
The combination of a desaturated Value layer enhances constrast, the channel mixer and or the color enhancement layer provides the color boost I typically want. Some sharpening is optional. Please try the technique and let me know what kind of results you find!
Here’s another shot at channel mixing, layer mask on desaturated layer for value channel.
Here is a 1920×1080 wallpaper.
Here is a 3200×1080 panorama.
A 16:9 wallpaper and a panorama.