Hi

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?

thanks in advance Lorenzo

On 11/03/2010 09:28 AM, Tőkés Ábel wrote:

11/2/2010 11:13 PM keltezéssel, Lorenzo Bettini írta:

Hi

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?

thanks in advance Lorenzo

Hi,

I think a possible reason can be: The resolution of your monitor is better than the resolution set in Scale Image window. I tried and experienced the same thing as you, and I got a 5x7 cm square with X and Y resolution 89 pixels/in on my monitor.

The reason of this is that at 100% zoom level, gimp indicates each pixel of the picture on one pixel of the monitor, independently from resolution. Resolution and image size in pixels determine together the image size in cm. The meaning of image size of 5x7 cm for eg. 72 pixels/in when your monitor has a different resolution is that if you print your image with the set resolution, you will get the an 5x7 cm image. (72 pixel/in printed is quite poor resolution.)

Hi Abel

does this mean that even if I don't see it correctly on the screen, the image will be printed with the correct dimensions 5x7 cm?

thanks Lorenzo

On 11/04/2010 09:32 AM, Stefano Rovetta wrote:

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...".

Ciao --Stefano Rovetta

Ciao Stefano :)

summarizing: what should I set to be sure that when printing I have exactly the desired size in cm?

thanks cheers
Lorenzo