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?


Lorenzo Bettini, PhD in Computer Science, DI, Univ. Torino
HOME: http://www.lorenzobettini.it MUSIC: http://www.purplesucker.com
BLOGS: http://tronprog.blogspot.com  http://longlivemusic.blogspot.com

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