Re: [Gimp-user] At 100%, my selection hasn't the size it should have

[Steve Kinney <admin pilobilus net>]

On 01/04/2018 12:04 PM, qsdqsd wrote:
> > First and most important. Gimp is a raster editor and works in pixels.
> > It can show cms as units for your convenience, but it still works in
> > pixels.
>
> When I use the scale tool, I can see the dimension in pixels of my selection. I
> can also switch the unit and see measurement in mm. When printing this image,
> will I get the mm annouced in Image --> scale image ? Which are the same seen
> with the scale tool by the way. Because if I get what I have at the screen, it
> will be much smaller.
> Indeed, taking off the dot for dot, changes quite a lot the visual dimension.
> And it is much closer but still different...
> The screen resolution seems to be good though... I don't understand the
> difference between the image properties and the measurement on the screen...

When editing images for print, step one is to decide what the size in
inches or cm the printed image will be, and the resolution needed in
pixels per inch or cm.  Multiply the physical size of the finished
printed image by the desired resolution, to find the height and width in
/pixels/ of the image file you need to create.  From that point on,
forget that "inches" or "centimeters" exist.

Typically, high resolution color printing is done at 300 DPI (or more),
while lower resolution black and white printing is typically done at 150
DPI.  These are general rules only, but they do give good results in
most applications.

Example:  Editing a photo that will be 4 x 6 inches on the printed page,
you might want 300 dpi resolution if it's a color image that will be
printed on glossy paper.  4 x 300 = 1200, 6 x 300 = 1800, so your
finished image will be 1200 x 1800 pixels in size.  Once you have
calculated the height and width in pixels, you can forget that inches
exist: 1200 x 1800 is the size of the finished image as seen by your
image editor.

Example 2:  Editing a photo that will be 4 x 6 inches on the printed
page, you might want 150 dpi resolution if it's going to be printed in
black & white on regular paper.  4 x 150 = 600, 6 x 150 = 900, so your
finished image will be 600 x 900 in size.  As before, once you have
calculated the height and width in pixels you can forget that inches
exist:  600 x 900 is the size of the finished image as seen by your
image editor.

Editing images for screen display (photos for display on web pages,
etc.), physical measurement units do not matter at all, just the height
and width in pixels.

:o)