Maybe here i reinvented the wheel but i invented a fast and simple procedure to adapt the color of the render to the bg, that doesn't require any extra plugin or script

For me works well on most of the images (but i will also explain how to solve the few times that is not sufficient )

But please check if you like the result :
here 2 Before/After examples

1 BEFORE



1 AFTER



2 BEFORE



AFTER



Now i will explain BUT the best explanation may be look and play with the XCF that i included as attachment ...just toggle in/out layers visibility should explain better then my english

For image 2 i used i simplified the procedure so are just 3 simple steps

1) duplicate the BG and move ON TOP
Obviously the render will disappear ... but don't worry
2 ) Apply a generous amount of gaussian blur on the dup
3) Now change the layer mode to Color and lower the opacity to taste (50% may be a good starting point )

This already give often a good result (But don't forget to blur ! or you will risk to get instead then nice reflex sharp colours blotches and/or spray of sharp coloured noise ,that is the most ugly noise
(BW noise may even be used for artistic effects but nobody like sharp coloured noise )

Anyway for the second i uses the complete process :
quoting myself

" 1 Do as usual the BG in bg, the render(s) on top
2 Duplicate the BG twice, BLUR a lot the 1st duplicate
3 Set the blurred BG copy (just above the render ) in COLOR mode lower opacity ( 50% may be a good starting point )
4 The second BG copy, above the first go in SOFT LIGHT (or overlay) mode, again lower opacity
5) now lower a bit (no too much !! may be 70 or 80%, ) the opacity of the render(s)
6) to fine tune copy again the render(s) put on the very top in NORMAL mode and lower opacity to taste

yes are 6 steps but very simple

Again remember to BLUR a lot (on the 3200x2400 original i used 18 of gaussian blur ) the "BG copy layer" in Color mode
(if not instead then sort of nice coloured reflex you will get ugly sharp colour blotches )

The second dup in Overlay mode is to intensify the color ; the Color mode should transfer the color component of the layer on top on that below but preserving the original grayscale

But the technical definition of "colour " doesn't correspond fully to that of human eyes ,so the transferred colour may look more washed , or more "acid" (because our eyes/brain separate the colors from the lightness/shadow only if there is a clear shadow or point of light:
To clear we see vermilion and brown as 2 different colors, but a brown may have identical hue and saturation of vermilion and differ only because darker

So to get something close to the "same colours (as seen by our brain) often the Color mode is not sufficent, add another copy but this time in soft light(or overlay) mode will often fix
(again lower the opacity to taste, too much opacity may raise too much contrast ..and this layer doesn't need to be blurred )

Before conclude a last but important tip :
i used this because as i said work well very often

But the usual procedure would be instead then use dup of the bg ,add a transparent layer , set it in color mode and brush the desired colors on it .

I prefer use a copy of BG because much quicker and, only in case of problem, correct the copy , simply erasing the undesidered color zone (instead then erase you may use layer mask that allow easier correction ) or replacing that part with the desired color

Humm..the xcf are too big for GC even resized

but no problem you may download from my DA sta.sh

http://www.deviantart.com/download/4376 ... d5pzppy.7z

2 last words

if you prefer you may just add a trasparent layer put in color mode and paint there the color cast you wish to add, you will see the result in real time

anyway i found quicker use BG dups, that often work well without extra work,
and correct it only if needed

That's a really keen way of doing things. Usually in my compositions I end up laying a final layer over the top, fill it in a color that complements the whole of the image and then set the mode to overlay. This works well on wallpapers etc, when I didn't actually use any renders and just went with filters from scratch. I find it brings the colors together nicer in the end. I'm glad I'm not the only one that thought of it and your way of doing it is way more sophisticated than mine.

thank you for making this technique into a tutorial, PhotoComix !

btw, never guessed you are italian

i use often a effect layer in overlay mode (or soft light that for gimp are identical ) , but i seldom use solid color fill ,usually i dup or a variant of the image to edit

Several of my Gmic filters (i.e. Graphic Novel and Graphic Boost) are derived by the same idea:
a dup, filter it , and merge back with the original , in the filters i allows much more merging mode but overlay remain the default (or hard light but just because the option to reverse layer position: hard light is identical to overlay applied on swapped layer)..but here i am going OT

Back to the tut , the base of the color to add come the layer in Color mode ,and again here the tip is too blur well the dup without fear for the sharpness because that come from the BG layer

The layer in overlay mode is to enhance this and also too boost a bit saturation and contrast



Thank Esper , and yes i am Italian