PHASE 1: SHADOW RECOVERY

duplicate the source layer, call it "COPY", set mode = ADDITION, set opacity at 60% (filter default, you may use the amount you like)
duplicate the source layer, call it "SHADOW", set mode = OVERLAY, set opacity at 60% (filter default, you may use the amount you like)
desaturate and invert the "SHADOW" layer
to both layers add a mask (WHITE)
copy the "SHADOW" layer and merge it into both masks
remove the "COPY" layer
(as you already noticed, it seems useless to create then delete it, maybe a previous release did some "merge", I don't know)

CHECK THE RESULT OF PHASE 1: you should see your image with the shadows a bit clearer and more detailed

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PHASE 2: SMART SHARPEN

duplicate the source layer, put it on top, call it "SHARPEN"
now decompose the source layer into a new image using COLOURS/COMPONENTS/DECOMPOSE model=HSV (the filter uses model=Value but I do not see it in interactive mode)
the created image shows 3 layers, you need only the layer named "value"
copy this "value" layer and paste it into the "SHARPEN" layer, set the mode to "VALUE"
create a duplicate of the source image (either you duplicate it at the beginning or perform copy source layer and paste as a new image)
name the (only) layer of this image "MASK"
apply the edge filter (SOBEL with 6.0 as amount)
apply levels stretch
convert the Mask Image to Grayscale
apply a gaussian blur (6.0 as both amounts, RLE)
(back on the main image) add a mask (WHITE) to the "SHARPEN" layer
copy the "MASK" layer from the Mask Image and merge it into the mask of the "SHARPEN" layer
at this point you may delete the Mask Image, no need anymore
apply Unsharp Mask to the "SHARPEN" layer with a (default) amount of 0.5
set "SHARPEN" layer opacity to 80%  

CHECK THE RESULT OF PHASE 2: you should see your image a bit more sharpened

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PHASE 3: LOCAL CONTRAST

create 2 new layers on top "from visible", call them "TEMP1", "TEMP2"
apply Unsharp Mask on "TEMP1" with a radius=60.0 and an amount of 0.4 (filter default, you may modify it at your choice)
set "TEMP2" mode to GRAIN_EXTRACT
create a new layer on top "from visible" and set mode to GRAIN_MERGE
remove "TEMP1" and "TEMP2"

CHECK THE RESULT OF PHASE 3: you should see your image a bit more locally contrasted

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PHASE 4: FINAL ENHANCEMENTS

create 3 new layers on top "from visible", call them "OVERLAY", "OVERLAY2" and "SCREEN"
desaturate all 3 layers
set modes respectively to OVERLAY, OVERLAY, SCREEN
set opacity to 50% (filter defaults)
add a mask to the "SCREEN" layer (ADD_COPY_MASK) and invert it

if you like (filter default is "no") play with COLOLRIZE on those 3 layers

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I hope it works.





