[Gimp-user] New User Need Help Enlarging/Enhancing Image

[gstalnaker <forums gimpusers com>]

> Here is one that was taken from my camera and edited in Lightroom.  I
> have no idea how large this would be able to be printed.  Do you?  Is
> there anywhere online I can submit several pictures to see how large
> they can be made without losing quality?  I really want to use this
> picture somehow!  Thanks again.

nickib - rich404 gave you the most detailed and correct answer (though rather a
lot of info in a compact way). Let's see if I can help. The issue you're having
is the difference between what a computer/phone screen shows you and what a
print of the same image will show you and the image file that a camera creates.

My monitor right now is displaying 2560x1440 pixel (dots-per-inch) resolution.
At 26" wide, my monitor shows an inch of image using 2560 pixels/26 inches = 98
pixels (or 98 DPI). Nice. High-density displays have more DPI.

Printers, on the other hand, have MUCH MUCH more DPI than most consumer
displays. My $60 HP Color InkJet printer prints at 1440 DPI. Printers can get
much better than my cheap HP.

My Nikon N7000 camera creates images at even larger DPI: Image Size - Large;
Size (pixels): 4928x3264 (1.5:1); Print Size / In: 16.4x10.9 (a resolution of
4928/16 = 300 DPI). My LG V30 phone image resolution is 4656x2328 (2:1). The
difference? FAR FAR superior lens/sensor on the Nikon than the LG. The teeny
sensor on the LG phone cannot compete and almost always significant image detail
is lost in its images than what the Nikon can capture with its much larger sense
(and superior lens; check out https://www.dpreview.com/forums/thread/4155644 for
a discussion of the physical size of the image sensor regardless of its
"megapixel" rating). Benefit of the LG? I almost always have it with me.

NOTE what the Nikon manuals says is the approximate size for an image printed at
Large: 16"x10" -- that's MUCH smaller than you're talking about. Keep this in
mind when contemplating how big you can go before undesirable image artifacts
become *visible* (Partha explicitly mentions this).

To make an image print well use an application like Lightroom/GIMP to modify the
image file so that it will PRINT at printer DPI (don't rely on image programs
like Microsoft's image viewer that do these things "behind the scenes" where you
cannot control them). The larger the resolution of the image you start with, the
better the PRINT will be because the image software you use to get the image
file to PRINT resolution will have less computational work to do. Do NOT rely on
what you see on a monitor (remember they have MUCH lower DPI than printers do).

GIMP lets you do this in the SCALE option. With an image open from the Image
menu select Scale Image. Modify the X resolution and Y resolution so that they
are 150 DPI or larger (personally I'd use no less than 300x300). Note that doing
this will usually cause the Width/Height values to greatly increase. This is
okay. BUT ... doing this also makes the image size when saved to a harddrive
also much larger.

GIMP uses sophisticated computations to take an inch of image at one DPI and
make an inch of image at another DPI. But it is not magic. A poor source image
may not scale well. If you're going from an image that prints well at 16"x10"
(my Nikon or my LG phone) to 40"x20" you'll most likely see issues, if you see
them, in long diagonals across an image (e.g., the roof line of your image).

Another issue as other posters have said, is that prints are not usually the
same dimensions as modern displays. Prints are typically done at 4"x6", 5"x8",
8"x10", 14"x16", 16"x20", etc. Many modern displays, however, are more like
widescreen TVs, etc. This means that you probably, as one poster said, want to
CROP your original so that it fits a standard print size. Or not ... if your
frameshop can print non-standard sizes and frame them. They can tell you this.

If you do need to crop, GIMP provides a CROP tool. With an image open, SHIFT-C
(or left-click on image and select Tools - Transform - Crop). Make sure you can
see the CROP tools properties. There is an option for Size and a dropdown to
select the scale to use: select INCHES. Now click on the image and drag left.
The boxes next to Size: will display values. You can now manually modify them
(e.g., to 8" x 10") and the crop rectangle will adjust its size accordingly. Now
you can put your cursor in the middle of the crop rectangle and drag it around
to show the part of the image you want to keep. Pressing the RETURN key will
perform the crop.

I will be honest, I'm not hopeful you can get a good printed image at the size
you want from a cell phone camera and expecially from the image you posted on
this thead (it already has pixelation that would make it unacceptable to me,
personally). I know I cannot with my LG phone. But, GIMP can do some pretty
magical things if you want to work at it long enough.

-- 
gstalnaker (via www.gimpusers.com/forums)