Re: [Gimp-user] Scaling up photo

[Robert Krawitz <rlk alum mit edu>]

On Thu, 21 Mar 2019 00:04:08 -0400, Liam R E Quin wrote:
> On Wed, 2019-03-20 at 17:57 -0400, Rick Strong wrote:
> > Does anyone know of really good software for scaling up an 8x12 inch,
> > 300 pxi photo to, say, 16x20 or 20x24 inches or larger? Will GIMP do
> > that? 
>
> At 10 to 12 feet, 144dpi will be fine. Fine Art magazines and books
> rarely go over 150 lines per inch in a dot screen, so although 300dpi
> is recommended, 150 is fine, as long as you don't have text at small
> sizes.

So first off, I agree and if anything would go further and say that
there's rarely going to be a problem at even 75-100 dpi.

One is often viewing a ~20" print from rather closer than 10-12';
that's not an atypical viewing distance from a much larger TV.  But I
don't disagree with the conclusion for a photograph, where there's not
normally extremely high spatial frequency data at high contrast, even
if the image is reasonably sharpened.  Even highly detailed scenes,
such as trees, fields of grass, and such usually in my experience
don't have a lot of pixel-to-pixel detail, and if they do, it's only
going to be right at the focal plane.

Text and line art (including high quality photographic reproductions
of same) are another matter, but that doesn't sound like the use case
here.

Just to give you an idea, [1] has long been one of my own favorites,
and I've printed it at 16x24 many times.  It has quite a bit of fine
detail throughout.  It was taken in 35mm film with an inexpensive lens
(Tamron 28-200, in the late 1990's when that was an early superzoom),
and might charitably have 2000 lines/inch resolution or 6 megapixels,
which would work out to 125 dpi on the final print.

[1] https://rlk.smugmug.com/Other/Landscapes/i-qGZ7Jx9/A

> So, 8x12inch at 300dpi is 16x24inch at 150dpi.
>
> Now, newspapers use 75 or 72dpi screens generally. So try looking at a
> newspaper photograph from ten feet away and see if you're OK with the
> quality.

That's a bad comparison; there are other reasons why newspaper photos
are poor quality (cheap paper and ink, bad channel-to-channel
registration, and so on).

Which also brings up the issue of print technology -- the printer's
screening, whether a fixed grid dot screen like offset printing or the
much higher frequency but less regular dithering of an inkjet or
laser, will usually help hide pixelation in a continuous tone image,
and if not, good quality scaling as Liam suggests will hide the
pixelation.  Pixelation is a lot harder on the eye than a small bit of
softness.

> If so, your image is good for 32x48inches unchanged (or just changing
> the image’s Print Size).
>
> If you're sending this out to a print shop, ask what resolution they
> need. If they insist on 300dpi, you can use gimp to scale it up; the
> results will not generally be as good with Fractal, though. You might
> also be able to use the liquid rescale / resynthesizr plugin to get
> better quality, because you're asking gimp to invent detail.
>
> Liam


-- 
Robert Krawitz                                     <rlk alum mit edu>

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