On Tue, 07 Apr 2009 12:06:43 +0100, norman wrote:

In a book I am reading, under the heading of scanners, it says that the scanner resolution should be 600 pixels per inch. When I look at scanner specifications resolution is quoted as dpi. Please, are these the same?

Yes, dpi = dots per inch = pixels per inch.

* norman [04-07-09 08:35]:

Could you please also clarify another thing. In the specification for a scanner it has Optical Resolution 4800 dpi X 9600 dpi. From this how should I calculate pixels per inch?

Are you testing us here?

You have given the dots per inch, dpi, resolution of the scanner and were just told that dots/pixels per inch measurements were the same. What is it that you want to know?

On Tue, Apr 07, 2009 at 02:21:26PM +0100, norman wrote:

Are you testing us here?

You have given the dots per inch, dpi, resolution of the scanner and were just told that dots/pixels per inch measurements were the same. What is it that you want to know?

There is no question of testing. As dpi = pixels per inch then I can see resolution is 4800 ppi by 9600 ppi. Firstly, why the two numbers, I would have thought resolution needed only one number and secondly, if this sort of resolution is readily available why does the author of the book take pains to imply that 600 ppi is something important?

Two numbers are because the resolution is higher in one direction than the other. 4800 ppi used to be marketing speak for a lower ppi with some math tricks to make it look higher (like scan the image multiple times at a low dpi and create a higher resolution surface). I don't know if they still pull that kind of thing.

I have no idea what the author wants to imply, but few printers are capable of outputting more than 600ppi, and you are dropping below human visibility there.

As for your original question. if you have 4800x9600 ppi, and you scan an inch square of material, you will end up with 4800x9600 pixels. Thus the question if you are having a laugh, as your question seemed trivial.

Jeff

Hi Gimp users and developers,
I noticed that using two displays is cause of error in the tool position. The tool position does not match with the pointer position. What is even more tedious is that if I go back to the single display configuration, Gimp continues to fail. I have to reinstall Gimp. Maybe you know a workaround, at least to avoid the tool re-installation. Thanks,
Giovanni

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I'm 36 with a post grad degree & I can't figure out how to get an image to specific size
Cheers
M

-----Original Message-----
From: gimp-user-bounces@lists.XCF.Berkeley.EDU [mailto:gimp-user-bounces@lists.XCF.Berkeley.EDU] On Behalf Of norman Sent: Wednesday, 8 April 2009 12:08 AM To: gimp-user@lists.XCF.Berkeley.EDU Subject: Re: [Gimp-user] pixels to dpi

< snip >

As for your original question.
if you have 4800x9600 ppi, and you scan an inch square of material, you will end up with 4800x9600 pixels. Thus the question if you are having a laugh, as your question seemed trivial.

I am really sorry you see my questions as trivial, they are not meant to be. Much of my difficulty is one of understanding the terminology used. For example and, assuming I understand correctly, if I scan a photograph which is, say, 5 inches square and then display that scan on my monitor, it will measure 24,000 pixels X 48,000 pixels. To test this on my rather cheap Canon LIDE20 I scanned a picture which is 5 inches square saved the file, opened the file in GIMP, cropped so that only the picture was there and GIMP said it was 729 pixels X 729 pixels.

Please explain and, just in case you think I am some youngster trying to get his homework done, I was 81 years old last birthday.

Norman

On Tue, 2009-04-07 at 15:37 +0100, norman wrote:

I scan a photograph
which is, say, 5 inches square and then display that scan on my monitor, it will measure 24,000 pixels X 48,000 pixels. To test this on my rather cheap Canon LIDE20 I scanned a picture which is 5 inches square saved the file, opened the file in GIMP, cropped so that only the picture was there and GIMP said it was 729 pixels X 729 pixels.

729/5 = ~145 ppi. Assuming you're reading the size of the image correctly in GIMP, it appears your scan wasn't at that much higher resolution. Note that scanners convert reflected light (analog signals) into pixels (digital signals) and can do this by varying the range of sampling of the light. Sometimes the higher resolution they advertise is actually a function of their software and not of their hardware. Their hardware may not be able to sample at those higher rates. In that case, and if you aren't using their software, you probably won't get the higher ppi resolution.

If you are using their software to scan (I haven't read this whole thread but in this case it would mean you're using Windows) then try opening the image in another program and see if it will tell you the pixel size of the image. If you get two programs telling you that the image is 729x729 pixels, then your scanner/scanning software isn't doing what it says its doing.

Please explain and, just in case you think I am some youngster trying to get his homework done, I was 81 years old last birthday.

I hope I'm still learning new things when I'm 81 (I'm on the high side of the 40's). :-)

On Wed, 2009-04-08 at 00:18 +0930, Michaela Baulderstone wrote:

I'm 36 with a post grad degree & I can't figure out how to get an image to specific size

Image->Scale, unless I misunderstand your need here.

* norman [04-07-09 10:40]:

I am really sorry you see my questions as trivial, they are not meant to be. Much of my difficulty is one of understanding the terminology used.

yes, there are several aspects to consider.

For example and, assuming I understand correctly, if I scan a photograph which is, say, 5 inches square and then display that scan on my monitor, it will measure 24,000 pixels X 48,000 pixels.

that is correct

To test this on my rather cheap Canon LIDE20 I scanned a picture which is 5 inches square saved the file, opened the file in GIMP, cropped so that only the picture was there and GIMP said it was 729 pixels X 729 pixels.

cropped? that changes the size and the size gimp reports :^)

Please explain and, just in case you think I am some youngster trying to get his homework done, I was 81 years old last birthday.

Well, from a 68 year old youngster, you have a falicy in your comparison. Your monitor displays as xx ppi, usually between 72 and 100, so the image appears very large as you scanned to 24000 ppi. If it is being displayed at 100 ppi, that makes the full image 240 inches wide :^)

Norman,

I hope I have not misconstrued your question / wording.

You used the word "scan" in conjunction with your camera. Your camera is not a scanner. It has a very different dpi/ppi than a scanner.

Scanners use A x B dots/pixels maximum per square inch. However, what you get from your scanner (up to that maximum) is what you tell it to give you, for example 600 x 600 dots/pixels per square inch.

On scanners, most humans have little use for their so-called maximum resolution. However, one practical application for using higher resolution settings is if you want to scan a small object, such as a postage stamp, and blow it up to a wall-sized poster. Scanners scan at 100% of actual object size, so a 1x1-inch object at 600 x 600 is going to show up in your image software as 600 x 600 pixels/dots. The resolution of your computer monitor and the zoom/magnification (for viewing purposes only) setting in your image program will affect how large it appears on your computer screen. However, if you PRINT that image on a 600 dpi/lpi/ppi PRINTER, then it will come out 1x1-inch on the paper (unless you make it larger, i.e. more pixels/dots, in your image program).

(I find it nearly impossible to explain to folks the difference between viewing and printing in regard to resolution. It takes a while to "get it".)

So, to scan that postage stamp that you want to turn into a wall poster, you might scan it at a very high 2400 x 2400 resolution. Then, in your image program, change the size from 1x1-inch up to 20x20 inches AT THE VERY SAME TIME AS YOU _reduce_ the resolution to 300x300 dots/pixels per inch. My understanding is that you have to do it at the same time for best results. You thus are SPREADING the 2400 x 2400 pixels over an area of 6000 x 6000 pixels (20 x 300). The result won't be crisp and clear when printed, but then it is a wall poster meant to be viewed from some distance. Good luck finding a printer that can print it (a 36,000,000 pixel/dot, 20x20 inch image). ;-)

Cameras use a different C x D maximum dots/pixels per square inch than scanners. You might have to find the info buried in the manual. Scanners and cameras are two completely different animals in several ways. But more to the point, it is my understanding that they use a maximum total image/data size for the picture. However, again, what you actually get is what you tell it to give you, up to that maximum. You probably got 729 x 729 (i.e. 531,441 dots/pixels or "half megabyte") because you told it to use a particular size/quality. I don't have a lot of experience with cameras seem to use words to describe the image "size" or "quality", or "speed", such as "good", "better", "best". If "good" is 729 x 729 and you use the setting of "good", then whatever is in the picture is 729 x 729. If that is not enough detail, then the picture must be taken zoomed in so that whatever it is you want in the picture is taking up more of the image area.

I suggest if you have further questions about this, you find a mailing list or forum about cameras, scanners, and computer graphics, etc. The Gimp list is meant to be most about GIMP.

BTW, You got a LOT more action on this than I get when I ask a GIMP question. Nobody seems to want to answer my GIMP questions. :-(

Jay

On 04/07/2009 10:48 AM, Michaela Baulderstone wrote:

I'm 36 with a post grad degree & I can't figure out how to get an image to specific size
Cheers
M

-----Original Message-----
From: gimp-user-bounces@lists.XCF.Berkeley.EDU [mailto:gimp-user-bounces@lists.XCF.Berkeley.EDU] On Behalf Of norman Sent: Wednesday, 8 April 2009 12:08 AM To: gimp-user@lists.XCF.Berkeley.EDU Subject: Re: [Gimp-user] pixels to dpi

< snip >

As for your original question.
if you have 4800x9600 ppi, and you scan an inch square of material, you will end up with 4800x9600 pixels. Thus the question if you are having a laugh, as your question seemed trivial.

I am really sorry you see my questions as trivial, they are not meant to be. Much of my difficulty is one of understanding the terminology used. For example and, assuming I understand correctly, if I scan a photograph which is, say, 5 inches square and then display that scan on my monitor, it will measure 24,000 pixels X 48,000 pixels. To test this on my rather cheap Canon LIDE20 I scanned a picture which is 5 inches square saved the file, opened the file in GIMP, cropped so that only the picture was there and GIMP said it was 729 pixels X 729 pixels.

Please explain and, just in case you think I am some youngster trying to get his homework done, I was 81 years old last birthday.

Norman

On Tue, Apr 07, 2009 at 03:37:57PM +0100, norman wrote:

As for your original question.
if you have 4800x9600 ppi, and you scan an inch square of material, you will end up with 4800x9600 pixels. Thus the question if you are having a laugh, as your question seemed trivial.

I am really sorry you see my questions as trivial, they are not meant to be. Much of my difficulty is one of understanding the terminology used.

Not a problem at all. Professionals get this kind of stuff wrong all the time.

Yes, if you were to scan a 5 inch square image at 4800x9600 ppi, then you should get 24,000 x 48,000 pixels. So, something else is going on. Your hardware is only capable of 600dpi (I just looked it up on Canon's website), it can double sample in one direction to get close to 600x1200, then interpolate to get up to the ranges of 2400. So, it really doesn't make sense to scan any higher than your base of 600dpi. But that doesn't address the other problem you mentioned. 729 pixels / 5 inches is 149dpi, which is off by a factor of 4 from what the hardware is supposedly capable of and 16 from what you think you scanned at. I would make sure that the GIMP isn't loading some embedded thumbnail instead of the real image. If you are in windows, you should be able to right click on the image and go to properties. The size that Windows thinks the image is is usually stored in the "Summary" tab. If you are on a Mac, I would suggest using Preview, then there should be a view image info menu button somewhere (not near a mac at the moment). If you are on Linux, then you should be able to type "file imagename.ext" and get the file size.

Please explain and, just in case you think I am some youngster trying to get his homework done, I was 81 years old last birthday.

I didn't think that for one second. I assumed that unit conversion wasn't understood rather than that you were seeing something contrary to what the math tells you. I apologize.

Jeff

On Tue, 7 Apr 2009 09:36:41 -0400, Jeffrey Brent McBeth wrote:

if you have 4800x9600 ppi, and you scan an inch square of material, you will end up with 4800x9600 pixels. Thus the question if you are having a laugh, as your question seemed trivial.

Actually, you'll most likely end up with 9600x9600 pixel image in the program, with the image simply stretched in the dimension that was scanned with lower resolution.

Also, most consumer scanners are limited to 1200DPI (some even just 600DPI) optically, and anything more is often simply interpolated by the scanner driver.

Just to muddy the waters further, when you get into dpi with printers, it opens up a whole 'nother can of worms...at least as I understand from researching the subject on the net.

With regard to printing, dpi and ppi are two seperate things. A printer might have a certain dpi rating, but that is NOT (again, if I understand correctly) the same as ppi. Why, because there may be more than one dot per pixel. You can have a single pixel that may be rendered by the printer with a grid of very small dots...and that pixel will still be at 300 ppi on the page, but that 300 may be at a much higher dpi, depending on the resolution of the printer. Capische? In other words you'd have 1200 dpi, rendering 300 ppi, or somesuch.

For printing purposes, what I came across was that anything up to say 5x7 inches, you want to print at 300 pixels per inch. As the print size gets larger, this value can go down. For instance, an 11 x 14 print could probably go at something like 250 pixels per inch, or even 200. A much larger print, like a 16 x 20, maybe 150 pixels per inch.

Why? Because smaller images are usually viewed much closer than larger ones. So smaller images need more resolution. But as you get farther away from the image, less detail is needed for an "acceptable" image. It is also my understanding that in some respects digital imaging is more forgiving than film, since it tends to have a smoother perceived "graininess" than film. One of my photography teachers didn't like us blowing 35mm up larger than 5x7, since he felt that image quality tended to suffer.

I used to have a medium format camera (6X7), and had some 5x7's printed off of it. I was blown away by how incredibly sharp they were. However, nowadays, some of your high end digital cameras have just as good of perceived resolution. It is my understanding that 6 megapixels is the bare minimum needed to equal the resolution of 35mm.

However, there is also the actual physical size of the digital image sensor to consider. Smaller sensor with lots of pixels will be "noisier" (i.e. granier) than a physically larger sensor with fewer megapixels. The larger the sensor, the better, as it makes for smoother noise. If you get a chance to see anything shot with a Canon 5D, you will see what I am talking about. The 5D has a full 35mm size sensor and the output from this camera is phenomenal. I am no expert, but it wouldn't surprise me to find that this camera has quality as good as older film based medium format cameras, if not better.

As for scanners, they have a certain "native" or "optical" resolution, at which they are best used. The software that comes with them can "interpolate" and create higher resolution images, but it isn't done by actually scanning something. As is mentioned earlier in this thread, it uses some fancy math to fill in the extra pixels not actually scanned. The results can be blurry. You get the same effect with scaling an image progressively in Gimp. You take an image, scale it up by 10%, take the result and scale THAT up by 10%, and so on. The result will be fairly smooth, i.e., not pixelated, but will be blurry, since there is no actual information added to what you started with, instead the computer "guesses" at what would have been there at a higher resolution.

--- On Tue, 4/7/09, norman wrote:

From: norman
Subject: Re: [Gimp-user] pixels to dpi To: gimp-user@lists.XCF.Berkeley.EDU Date: Tuesday, April 7, 2009, 12:16 PM < great big snip >

This topic has certainly brought lots of reaction and my thanks to all.
My intention is to buy a new scanner which I want to use for scanning
photographs prior to carrying out restoration work. I use Ubuntu 8.10,
GIMP and XSane.

The book I have says that I should look for a scanner that captures at
least 10 bits of data and has an optical resolution of 600 pixels per
inch. I don't have lots of money and this interest in restoration is
just as a hobby and not a business. So, could some kind person guide me
in the right direction, please.

Norman

norman wrote:

In a book I am reading, under the heading of scanners, it says that the scanner resolution should be 600 pixels per inch. When I look at scanner specifications resolution is quoted as dpi. Please, are these the same?

Useful information here regarding terminology: http://www.scantips.com/basics01.html

On Wed, Apr 8, 2009 at 12:44 AM, Jay Smith wrote:

Norman,

So, to scan that postage stamp that you want to turn into a wall poster, you might scan it at a very high 2400 x 2400 resolution.  Then, in your image program, change the size from 1x1-inch up to 20x20 inches AT THE VERY  SAME TIME AS YOU _reduce_ the resolution to 300x300 dots/pixels

Just to clarify: This is unnecessary. If you want to blow up something, just reduce the resolution. Halving the resolution is all that would be required to turn a 600x600 (1 inch square) 600dpi image into a 600x600 (2 inch square) 300dpi image. The inches measurement should automatically update itself, since it's dependent on image resolution.

David

David,

Thanks for that clarification.

I made the mistake of thinking of how I do it in Photoshop which has a single "resize image" dialog where things are not so neatly divided as they are in Gimp. In Gimp there is a "Print Size" dialog. My bad.

Jay

On 04/07/2009 07:01 PM, David Gowers wrote:

On Wed, Apr 8, 2009 at 12:44 AM, Jay Smith wrote:

Norman,

So, to scan that postage stamp that you want to turn into a wall poster, you might scan it at a very high 2400 x 2400 resolution. Then, in your image program, change the size from 1x1-inch up to 20x20 inches AT THE VERY SAME TIME AS YOU _reduce_ the resolution to 300x300 dots/pixels

Just to clarify: This is unnecessary. If you want to blow up something, just reduce the resolution. Halving the resolution is all that would be required to turn a 600x600 (1 inch square) 600dpi image into a 600x600 (2 inch square) 300dpi image. The inches measurement should automatically update itself, since it's dependent on image resolution.

David

On Wed, Apr 8, 2009 at 12:25 PM, Jay Smith wrote:

David,

Thanks for that clarification.

I made the mistake of thinking of how I do it in Photoshop which has a single "resize image" dialog where things are not so neatly divided as they are in Gimp.  In Gimp there is a "Print Size" dialog.  My bad.

Hmm, maybe this needs clarification too:

The 'print size' dialog is one way to change image resolution in GIMP (which I discovered thanks to your mentioning it!). The other way, which I have always used myself, to change DPI is by using the Scale Image dialog.

(I think what you were suggesting could be done using the Scale Image dialog.)

I understand why Photoshop's resize image dialog might require two actions (it looks like allowing 'size in inches' and 'dpi' to disagree allows Photoshop to mediate actual printed DPI to best compromise between the two.)
If Photoshop really requires you to do both of those steps rather than just changing resolution, I wonder whether there is some other dialog better for your purposes that does only require the resolution change. I certainly hope other software like PSP manages to auto-update inch sizes, though :)

I actually prefer the Print Size dialog now, cause it's simpler for the purpose of tweaking dpi:)
(of course I never make prints -- I just use this to alter the way the image is displayed :)