[digiKam-users] More on changing image resolution

[Daniel Bauer]

Am 24.08.19 um 22:35 schrieb Northern Colorado Event Photograph:
> Yes, I understand that ". . . a digital image doesn't have an intrinsic 
> size in inches, only a size in pixels."
> 
> And I also understand that "Resolution plays a role when printing . . . ."
> 
> But that is my point.
> 
> Suppose you are processing (for example) a low res JPG image out of a 
> smart phone,  And you have a request for a print (4 x 6 or 5 x 7).
> 
> To provide a satisfactory print, it may make sense to increase the 
> resolution of this image.  Yes, I understand that this is not usually a 
> good idea and can potentially result in a number of issues with the 
> final product.

If you increase the resolution of an image (that is pixels per inch or 
cm) then logically the image size (in inch or cm) gets smaller /when 
printed/.

So if you have an image with 1000 pixels width and you set the 
resolution to 100 pixels per inch, your image prints 10 inches wide.
If you set the resolution to 200 pixels/inch it prints 5 inches wide; 
500 pixels/inch = 2 inches, 10pixels/inch = 100 inches).

With all those changes in resolution actually nothing is changed in the 
image. All the pixels stay the same. The only thing that changes, is 
"how many pixels the printer takes to fill one inch". But you achieve 
exactly the same when you simply put the print size in the print dialog 
(the printer or its driver will simply divide the pixels that are 
available in the image by the width or height you entered).

So in the above example with your 1000px wide image (lets say its saved 
with resolution of 70 px/inch) and you enter 10inches wide in the 
printer dialog, it will be printed with a resolution of 100 pixels/inch. 
If you enter 5 inches width, it prints with a resolution of 200 px/inch. 
It doesn't have any influence what resolution is saved in the image.

Also if you go to a print-lab with your file you just say: print it at 
this or that size. You don't have to care about the resolution.

The same happens when you paste the image in a print publishing software 
(for example adobe's pdf crating software). Here indeed you give a 
resolution and print size for the document, because it is aimed for 
printing in a defined size. When you paste the image file it will be 
pasted in the "cm-size" according to your document-resolution setting 
and the available pixels in your image file. (you can then drag it 
smaller and the software will actually change the image size, cut away 
from the original pixels. You can drag it larger and the software will 
fill the missing pixels with "invented" ones or lower the resolution, 
depending on your settings).

The only thing you have to care is that you have enough pixels in your 
image to achieve a good print quality in the desired size.

Now, if you have that 1000px wide image and want it to print 10 inches 
wide with a resolution of 200 px/inch, there are not enough pixels in 
your image. You'd need 2000 pixels width. Theoretically you can enlarge 
your image (in Gimp for example). This will add new pixels. Those new 
pixels are calculated and are not from the original image. In most cases 
it will give a bit a blurred result.

Most printers can soften the otherwise visible pixels when there is not 
enough resolution, and that mostly gives better results than adding 
pixels in Gimp or photoshop. I just printed a 2.5m (98.42") poster from 
a file with 5110 pixels which means a resolution of 52 pixels/inch only 
and there are absolutely no pixels visible. Of course it's not a 100% 
sharp when looked at from close, but from 1m distance the result is perfect.

> Nevertheless, it is worth a try, if for no other reason that to make the 
> requestor (customer/client) happy.

The client will be happy if there are enough pixels to reach the quality 
in print he wants. You don't have to care about the resolution 
information that is saved in the file.

> Now 300 ppi doesn't make sense in this case, but 120 or 150 might.
> 
> Unfortunately, if I correctly understand the messages in this thread, 
> there is no way in digiKam to make this change independent of changing 
> the dimensions (resizing) of the image.

Digikam can only change the pixel-size of an image.

The only moment when you really want to change the resolution is when 
you must deliver an image in a given print size in a given resolution 
(e.g. A4 in 250 dpi). Then you must use the Gimp.

I had such cases and still simply delivered the image in 72dpi but with 
the exact amount of pixels that would fill A4 at 250 dpi. The client 
didn't even notice it, because an image file of 2067 pixels with and 
72dpi will print exactly 21cm wide when the printer is set to 250dpi, or 
will print exactly at 250dpi when the printer is set to 21cm width.

So, in general: forget about changing resolution in images unless you 
deliver to a print shop that doesn't know how his printer works  :-)



-- 
Daniel Bauer photographer Basel Málaga
https://www.patreon.com/danielbauer
https://www.daniel-bauer.com