This thread supersedes the previous one concerning my experiments.
After various tests I decided to publish the "official" filter.

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Plug-in Title : "PointillismEffects"
Gimp Menu : "<Image>/Diego/Artistic"
Release : 1.0 (initial)
Author : Diego Nassetti (dinasset@gimpchat.com)
Licence : GNU General Public License

System Environment used in development
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Platform : Windows 10 64-bits
Graphic Apps : Gimp 2.8.16
Language : python as included in Gimp

Extrn Filters : (Gimp) LumaInvert (script-fu-invert-YUV.scm by "Stratadrake")
(Gimp) Gimpressionist (additional Presets defined)
(G'MIC) none

Summary
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Filter to implement the creation on a NEW IMAGE (dimension is a user option)
starting from an RGB* input image/drawable; the core function
works on a duplicated layer replacing each pixel with a (quasi)random colored pixel
having more or less the same average luminosity of the original.
Filter has been inspired by Gimp-chat thread "interesting but not what i'd expected"
by trandoductin.


Details
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Flow implemented to get the final result:

1. calculate the new image width and height scale for the output (user choice for dimensions)
2. save and set-up context values
3. create the new image by appropriately scaling the input image dimensions
4. create the BG layer for the new image by copying the input layer, scaled
5. add alpha to the layer is not present
6. add a preliminary preprocessing to normalize (people) or partially equalize (others) the colours
7. create a new layer (dup of the BG) for random pixels
8. transform this layer using an ancillary function (using tiles to be fast)
source values are exaluated multiplying values by 1+luma factors
a threshold is established to identify the prevailing colour
(colours different of less than the threshold are considered equal)
each pixel is replaced (partially) by a random colour chosen among the "surrounding" colours (a triplet)
e.g. a source pixel with a prevailing cyan colour is replaced by a cyan, green or blue nuance
9. create a copy and apply a GIMPressionist effect (preset is a user choice)
10 .the created painted layer is submitted to a partial retinex effect
11. raise BG to the top (grain-merge)
12. again (softlight)
13. dup but invert YUV
14. visible layer on top, excluding the bottom BG
15. apply a curve to the values, after desaturation
15. flatten the image is requested

Acknowledgments
---------------
Previous experimental release has been tested by Issabella, Marek, Dalia, Pat, Animicule
outcomes/comments helped in making the improvements implemented in current release


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other info:

for those who have tested my experimental previous release
these are the main differences:
a) new parameter "Strength" allows the user to choose between:
low : suitable particularly for portraits
high : for the generality of the images
(including portraits for those who like the effect)
b) new preset selection method (search for a file) allows the user
to choose between:
user defined presets (like those attached or others)
Win users find them usually under (user)/gimp-2.x/Gimpressionist/Presets
std GIMPressionist presets
Win users find them usually under C:/Program Files/GIMP 2/share/gimp/2.0/gimpressionist/Presets
click over the parameter and browse either folder
c) new parameter "Paint layer mode" allows the user to choose the combination
of the GIMPressionist painting layer and the Pointillistic layer among:
normal : standard mix
softlight : increases the colour
color : diminishes the contrast
saturation : increases the lightness
darken only : increases the structure

suggestions for further personalize the outcome:

the outcomes when using the "low" strength show usually very "moderate" effects
hence likely they do not need special treatments
the outcomes when using the "high" strength may be easily modified by operating
on the opacity of the top layer
if the user likes to modify modes and opacity of the lower layers it is suggested
to set off the visibility of the 2 top layers then recreate the "now visible" layer
after the modifications
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attachments:
initial release of the filter

initial set of GIMPressionist presets

removed

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removed

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removed

removed

image by Meemaw

soft and strong effects




and (why not?) 50-50

I like this new version!!!

Do you have to have 2 layers for this script to work? I ask because I get errors when I run it.

which error do you get mackenzieh?
I always run it with a single layer (RGB)

if not against your privacy, can you post the image you want to test?

there are info messages displayed concerning the pixel nuances processed and the gimpressionist preset used
but these are just INFO messages

these is an example of the info messages you get



they tell you what I said on my previous post

thanks, can it be another toy to play with?

Those errors in the image that you posted is what I see when I run the script.

Here is the photo I use


I also get these errors:


My initial question wasn't answered. Does this script need 2 layers to work?

OK, they aren't errors but info messages

your image, "half and half"



(adjusted ratio W/H before applying the filter twice, then mixing)

I still get errors. Check my edited post. Post number 17.