Re: [Gimp-user] Sizes of an image - display v printing

[Ofnuts <ofnuts gmx com>]

On 12/03/2020 19:44, chuckie wrote:
> I am little confused ...
>
> I have an image that is 828 pixels wide.
>
> If I open it in Paint, and HyperSnap or GIMP and view it at 100% and use a
> ruler, the width of the image is about 23cm.
>
> If I drag the image into a Microsoft Outlook email (embed it) or go to the GIMP
> Print Size window, they both report it to be 3.5cm wide!
>
> So, the GIMP editor shows it at a nice size of 23cm visually, but to print, or
> embed it in Microsoft Outlook email it is only 3.5cm width.
>
> Why is it then that the display of it in the editor is 23cm? What changes need
> to be applied so that the image will embed into Microsoft Outlook email at 23cm?
> Certainly, I was expectign it to embed at the scale I see it in the GIMP viewer
> and not the anticipate scale for printing.

When you look at your picture at 100% in Gimp, Paint and other
applications, it is displayed "dot for dot", one pixel of the picture is
one pixel on your screen, and therefore it is displayed using your
screen definition  (so given the size you report we can assume that the
resolution of your display is 828/(23/2.54)=91 pixels/inch).

On the other hand your  picture has an embedded "print definition" which
is used for printing, in your case 828/(3.5/2.54)=600 pixels/inch. This
sets a theoretical size for the picture, size at which the picture is
printed (as long as everything in the print stack abides to it).

Print definition (in Pixels/inch), print size (in inches) and size (in
pixels) are related by the formula:

print size = print definition * pixels

Note that this means that you can only pick two of these values, the
third is determined by the first two.

In Gimp you can un-tick "View>Dot for dot", in which case the picture
will be shown at the print size, if your screen resolution is properly
set  (see Edit>Preferences>Interface>Display).

When you create the image, you should set the size in pixels big enough
so that you can print it at the required size with a print definition of
at least 200 pixels/inch (photo) preferably 300 or more if there is
text/logos or other computer-generated graphics. The advanced options in
"File>New..." let you enter a print size and print definition, and
compute the required size in pixels.