Re: [Gimp-user] Best way to upscale an image

[Liam R E Quin <liam holoweb net>]

On Sat, 2018-11-10 at 11:02 +0100, Helmut Jarausch via gimp-user-list
wrote:
> for a photobook I need to upscale an image (showing a landscape) by
> a  factor of 2.

Usually fine art prints in books are done on a 144 or 150 line screen,
and normally you want twice the lines-per-inch resolution, which means
you need to aim for 300 image pixels per printed inch.

> What are good (best?) methods to do so with Gimp, GMIC or other
> tools (I'd prefer open source on a Linux machine)

I sometimes scale up larger, e.g. 6x, then do a gaussian blur (radius 4
or 5 in this case), make sure the levels span the full range from black
to white in every channel, then scale back down (e.g. with nohalo if
you are patient) 3x. Then check curves and/or levels again after
scaling, and finally use unsharp mask (since the earlier and more
subtle "sharpen" filter seems to be dead), usually with "amount"
greater than "radius" (this is just a rule of thumb, and it depends
very much on your image - you need to avoid introducing a halo).

As you mention it's a landscape, use 16bpp or 32bpp floating point 
linear (image->precision) for best results

> I've read about neural network algorithms. Are they usable for
> larger (7MP) images without enormous computer power?
Yes, but they have the best results with "anime" style images as far as
i can tell. As others have mentioned, the liquid rescale plugin may
work, but it might also give bizarre results.

i think of 7MP as a fairly small image, but, ths system has 32G of RAM.
Make sure your GIMP tile cache is large, and that not too many other
programs are running. i'm currently editing an image 18,000 x 26,000
pixels for example. As cameras sport increasingly high resolution,
people are finally getting used to working with print-sized images :)

Liam (slave ankh on IRC)


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