blurring boundaries

[APDamien]

I have an image that I want to extend to the right. Most of the right border is a vaguely orange-red color, lighter near the top of the image, darker near the bottom.

I want to make that image larger by extending that border to the right. If all of that border followed that pattern, I would just copy it piecemeal and paste it next to the existing border. But there are a few places where bits of furniture etc. touch the right border, and extending those in a straight line would look weird.

So far, I have copied pieces of that right border using a rectangular selection tool at a 400% zoom (with the grid spacing set to 1, this makes each pixel visible to the naked eye) and repeatedly pasted them to the right until I reach the right edge of the (expanded) image.

But where other things touch the right border of the original image, there are breaks in that nice smooth flow from light orange to dark orange, making a hard visual boundary where they meet.

I want to "erase" that boundary, making the transition gradual across the entire right area.

I thought of applying a few hundered iterations of a "relaxation" algorithm: color(x,y) = (color(x+1, y+1) + color(x+1,y-1) + color(x-1,y-1) + color(x-1,y+1))/4 , but I thought it would be better to learn how to do this in GIMP, if I can.

Suggestions?

See the attached image for what I've managed to do so far.

Btw, the messy area in the top center is the result of my trying to erase boundaries with the smudge tool. It didn't work as well as I'd hoped.

[rich2005]

It would be better if you could give the sample before your editing.

Various ways, spring to mind.

It might be possible with a mask and a gradient fitted under the image as this:

   

..or there is a plugin 'liquid-rescale' which might do the job, difficult to say.

[tmanni]

Such a simple background (2 colors, smooth transition, no structure, small fine details) can be easily recreate by hand using frequency separation method.
Idea is to have one layer for the background colors and one layer for the background texture.
An example:
   

The "background colors" layer is created using the gradient tool (but you can also use a big soft brush)

To create the "original details" layer:
1) duplicate the original one
2) set the blending mode of the new layer to "Grain extract"
3) apply a Filters > Blur > Gaussian blur... with small sizes
4) merge down

The "new details" layer is filled with the Clone tool, taking content from the "original details" layer.

The original image is put on top with a layer mask to reveal the arm and have a smooth transition between the original background and the new one.

[APDamien]

(07-20-2020, 08:42 AM)tmanni Wrote: Such a simple background (2 colors, smooth transition, no structure, small fine details) can be easily recreate by hand using frequency separation method.
Idea is to have one layer for the background colors and one layer for the background texture.

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Thanks for the detailed explanation. The previous two suggestions assumed a lot more expertise in GIMP than I have. Until recently I've done all my work in a combination of PowerPoint and Windows Paint.  (Power point for adding text and positioning the parts of my image, Paint for trimming the edges, sometimes rotating the image and/or fixing minor glitches.)

I expect to continue using PP for most of the work, but maybe I can use GIMP in things like extending the background of the original image.