> I must admit I'm kind of newbie in image editing... I was trying to 
> resize a photo to 5x7 cm, using the scale image dialog, using 
> centimeters...  however the resulting image is smaller, and also the 
> grid, when set to cm, (using 100% zoom) uses something that's smaller 
> than a centimeter...  what am I doing wrong?

Ciao, Lorenzo
Here is a colleague speaking (DISI, Genova), so I feel obliged to reply,
even if only to review things that have already been said - although
spread across several replies.

You have two different issues here, either of which is not really
as much an issue, but rather a point to be clarified.

They are (A) output size and (B) image resolution.

Output size (such as viewing on screen or printing) depends on the number
of pixels. Usually setting 100% on screen you have one-to-one
correspondence between image pixels and LCD pixels. So the displayed image
depends on the pixel pitch (= size) of your screen, or the pixel pitch
of your printer, when printing.

To calibrate the screen size, I place an A4 sheet on a Word (really
an Openoffice Writer) blank document, and play with the zoom factor
until the short side of the blank doc is reasonably identical in size
to the real sheet, which is 21cm. Then I use that zoom factor across
all programs where I need it.

And that's one story.

A somewhat different story is when you resize an image. Or rather - when
software does that for you.

Resizing an image means resampling it for output, so that it has fewer
or (by interpolation) more pixels.

Resizing on the screen is explicit: you set the zoom level, the software
does the math. Resizing for print, instead, is at least in part implicit.

There is an image property which is called resolution (may be different
in vertical and horizontal). Resolution is expressed as pixels per unit
length (e.g., pixels per inch -ppi- or pixels per cm). This is used
to decide the final printing size of your image.

Width  = Horz.Resolution x NumpixelsX
Height = Vert.Resolution x NumpixelsY

When printing, the image will then be RESAMPLED (this may not be clear
a-priori) to fit this size with the output device resolution.
Of course you can usually change that by setting an output zoom level,
or even by directly setting the desired size (both available in Gimp).
This setting is much more important than what you see on-screen.

To sum up, when working for the web, resolution is useless, because
you don't think in terms of physical size. Instead, when printing,
it is important to set the image resolution, which is NOT the number
of pixels as commonly (mis)interpreted, but the conversion factor
used to translate pixel counts into physical sizes.

In Gimp, resolution is set either when creating an image (under
"Advanced", not visible initially, defaults to 72 ppi) or afterward
by navigating to "Image -> Print size...".

--Stefano Rovetta

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