The most simple way to use GIMP in combination with the PhotoFlow plug-in under Linux is to download and run the AppImage package that is hosted on the pixls.us web site. This screencast explains the details of how to run the GIMP AppImage and briefly introduces the tools that are provided with it.
One of the possibilities offered by the PhotoFlow GIMP plug-in is to apply non-destructive edits in Lab colorspace to any GIMP layer. The Lab colorspace (or better CIELab) is an alternative representation of the pixel data that, unlike RGB, separates the luminance information from the color information. It is therefore possible for example to adjust the luminance of the image without changing the color components, or to adjust the color saturation in a way that is more natural and pleasing than in RGB.
Let's see how this works in practice.
The starting point is an existing image opened in GIMP. To run the PhotoFlow plug-in on this image, go to the "Filters" menu and select "Filters -> PhotoFlow...":
If needed (if the selected layer was already created with the PhotoFlow plug-in), select "Create new" in the little dialog that pops-up when the plug-in is started:
At this point, any tool inserted above the "colorspace conversion" layer will be working in Lab colorspace. For example, the "curves" tool gives the possibility to separately adjust the curve of the "L", "a"and "b" channels. Here I have applied an S-shaped curve to the "L" channel, aimed at increasing the contrast and slightly reducing the brightness of mid-tones:
Adding contrast in the "L" channel has often the tendency of simultaneously reduce the overall saturation of the image, like in this example. To compensate this effect, I added a "basic adjustments" layer above the curves, and increased the saturation a bit. When working in Lab colorspace, the saturation is adjusted by multiplying the "a" and "b" channels by a constant proportional to the value of the slider: the constant is > 1.0 if the saturation adjustment is positive, and < 1.0 if the adjustment is negative:
Most of the tools in PhotoFlow are able to process pixel data in Lab colorspace. Moreover, several tools can restrict their action to a specific channel. For example, here I have applied a final sharpening step that only processes the "L" channel and does not modify the color information:
When sending the image data back to GIMP, the PhotoFlow plug-in also stores the editing configurations as meta-data attached to the newly created layer. This allows to re-open the plug-in and further tweak the edit if needed. To do that, one has to select the layer created by the plug-in, start again the plug-in from "Filters -> PhotoFlow..." and then select "Edit current" in the pup-up dialog:
This will restore the plug-in in exactly the same state as it was when the layer was modified the last time:
Any further modifications will be saved again in the GIMP layer together with the updated image, and will be available for further tweaking...