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[Q] Understanding the 72 DPI problem

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[Q] Understanding the 72 DPI problem Daniel Carrera 27 Mar 00:35
  [Q] Understanding the 72 DPI problem John Culleton 27 Mar 14:51
   [Q] Understanding the 72 DPI problem O'Smith 27 Mar 15:28
[Q] Understanding the 72 DPI problem Kevin Myers 27 Mar 16:37
  [Q] Understanding the 72 DPI problem O'Smith 27 Mar 20:27
  [Q] Understanding the 72 DPI problem Daniel Carrera 28 Mar 04:16
[Q] Understanding the 72 DPI problem Kevin Myers 28 Mar 04:46
Daniel Carrera
2003-03-27 00:35:00 UTC (over 17 years ago)

[Q] Understanding the 72 DPI problem

Hello,

I've been using Gimp for a while. I keep hearing that Gimp has a 72dpi limitation and Photoshop doesn't, but I really don't understand how that is. I am interested in trying out some printed postes, and I want to understand this issue beforehand.

This is how I understand the situation, perhaps someone can explain to me where I'm wrong:

1) Gimp understands images as a matrix of pixels. There is no concept of physical units like 'inches'.

2) If I have an image that is 600x600 pixels, and I print it out so it comes out at 1x1 inches, I have printed a 600dpi image.

3) The jpeg, gif and png formats are matrices of pixels. They don't have a notion of 'inches'.

4) The eps format does have a notion of index.

Given facts (1)-(4), I can produce a 10x15in poster at 600dpi this way: - Start with a jpeg/png that is 6000x9000 pixels. - Do some gimping.
- Export to eps and set width and height to 10x15in. - Print the eps file.

I don't see where the 72dpi limitation is. It seems that Gimp has no knowlege of dpi except when it exports to eps.

Where exactly is Gimp's limitation? In which way does Photoshop /not/ have this limitation?

Thanks for the help,

John Culleton
2003-03-27 14:51:17 UTC (over 17 years ago)

[Q] Understanding the 72 DPI problem

On Wednesday 26 March 2003 06:35 pm, Daniel Carrera wrote:

Hello,

I've been using Gimp for a while. I keep hearing that Gimp has a 72dpi limitation and Photoshop doesn't, but I really don't understand how that is. I am interested in trying out some printed postes, and I want to understand this issue beforehand.

Gimp defaults to 72 pixels. But when you create a new item, or import a PostScritp file etc. you have the option of setting the dpi.

This is how I understand the situation, perhaps someone can explain to me where I'm wrong:

1) Gimp understands images as a matrix of pixels. There is no concept of physical units like 'inches'.

On most screens you have an op[tion of inches or pixels.

2) If I have an image that is 600x600 pixels, and I print it out so it comes out at 1x1 inches, I have printed a 600dpi image.

Correct AFAIK. I fyou brought this image into Gimp it ould automatically scale it at e.g., 25% so I wouldn't overflow the screen. But it would still be a 600 dpi image internally.

3) The jpeg, gif and png formats are matrices of pixels. They don't have a notion of 'inches'.

Gimp does. I resize images to a specific size all the time. Remember that Gimp is not handling your image in an external format. It is handling it in its own format.

4) The eps format does have a notion of index.

Given facts (1)-(4), I can produce a 10x15in poster at 600dpi this way: - Start with a jpeg/png that is 6000x9000 pixels. - Do some gimping.
- Export to eps and set width and height to 10x15in. - Print the eps file.

You may find that a file that is as you describe is huge and unwieldy. But I think it could be done.

I don't see where the 72dpi limitation is. It seems that Gimp has no knowlege of dpi except when it exports to eps.

No, it also has knowledge of dpi when you create a new image. And it appears to maintain the dpi when it imports an image.

Where exactly is Gimp's limitation?

Gimp is more oriented to online images than to printed images. But the only serious limitation it has AFAIK is that it does not work in CMYK internally. So for color printing you have a couple of choices: 1. Produce an RPG file and convert it to CMYK using an external program. 2. Use Gimp to create CMYK separations.

DISCLAIMER. I am a (perpetual) newbie and some of the above may be incorrect.

John Culleotn

O'Smith
2003-03-27 15:28:26 UTC (over 17 years ago)

[Q] Understanding the 72 DPI problem

On Wednesday 26 March 2003 06:35 pm, Daniel Carrera wrote: > Hello,
>
> I've been using Gimp for a while. I keep hearing that Gimp has a > 72dpi limitation and Photoshop doesn't, but I really don't > understand how that is. I am interested in trying out some printed > postes, and I want to understand this issue beforehand. (big snip)
==========================

Daniel, Maybe this site will help you understand the differences between Photoshop and Gimp. It should also explain why you should use Gimp in most cases as well. :o)

http://manual.gimp.org/manual/GUM/Migrate.html

It's a good site to pass along to someone thinking about the move to Gimp or Gimp & Linux, when they are trying to get away from all the expensive software on a Windows or Mac machine.

I don't know the site, but I know there is info about the Film Gimp that many of the Hollywood companies are using now in Linux to produce all the films graphics. I am not sure when the two Gimps will begin to migrate, but it's pretty certain all the attributes Film Gimp has will end up in Gimp as well.

Patrick

Kevin Myers
2003-03-27 16:37:27 UTC (over 17 years ago)

[Q] Understanding the 72 DPI problem

There are some VERY misleading statements on the web page referenced below regarding the specific question originally posted by the potential GIMP user in this thread. That web page does say, incorrectly, that the GIMP only has one resolution, and that is 72 dpi. That may have been true at some point in time, but that is most definitely NOT true in the version that I am using (1.2.4).

First of all, that statement ONLY applies anyway when your monitor resolution is 72 dpi. My monitor resolution is 100 dpi (.25mm dot pitch, 1600x1200 on a 21" screen), which the GIMP supports directly, regardless of my next point.

Secondly, there is a preferences item under Interface/Image Windows named Use "Dot for Dot" by default. With this option disabled (my normal usage), images at different resolutions than screen resolution will still be displayed at their normal size on the screen by default.

Finally, you can specify both resolution and size in the standard File->New dialogue, as well as in the preferences options for new files.

I'd also like to bring up one very important GIMP advantage that I didn't see mentioned at all in the Photoshop vs. GIMP comparison. The GIMP can in many cases handle LARGER images than Photoshop. Photoshop (ALL versions) is limited to 32K pixels in any single dimension, whereas the GIMP is only limited by available memory (potentially including virtual memory). I routinely process images with over .5G total pixels, and over 300K pixels on one axis using the GIMP. Photoshop cannot handle these images at all. In fact, I have found very few other applications that can.

It would sure be nice if someone could make note of these facts on the subject web page.

s/KAM

----- Original Message ----- From: "O'Smith"
To:
Sent: Thursday, March 27, 2003 8:28 AM Subject: Re: [Gimp-user] [Q] Understanding the 72 DPI problem

On Wednesday 26 March 2003 06:35 pm, Daniel Carrera wrote: > Hello,
>
> I've been using Gimp for a while. I keep hearing that Gimp has a > 72dpi limitation and Photoshop doesn't, but I really don't > understand how that is. I am interested in trying out some printed > postes, and I want to understand this issue beforehand. (big snip)
==========================

Daniel, Maybe this site will help you understand the differences between Photoshop and Gimp. It should also explain why you should use Gimp in most cases as well. :o)

http://manual.gimp.org/manual/GUM/Migrate.html

It's a good site to pass along to someone thinking about the move to Gimp or Gimp & Linux, when they are trying to get away from all the expensive software on a Windows or Mac machine.

I don't know the site, but I know there is info about the Film Gimp that many of the Hollywood companies are using now in Linux to produce all the films graphics. I am not sure when the two Gimps will begin to migrate, but it's pretty certain all the attributes Film Gimp has will end up in Gimp as well.

Patrick

-- --- KMail v1.5 --- SuSE Linux Pro v8.1 --- Registered Linux User #225206
On any other day, that might seem strange...

O'Smith
2003-03-27 20:27:35 UTC (over 17 years ago)

[Q] Understanding the 72 DPI problem

On Thursday 27 March 2003 10:37 am, Kevin Myers wrote:

There are some VERY misleading statements on the web page referenced below regarding the specific question originally posted by the potential GIMP user in this thread. That web page does say, incorrectly, that the GIMP only has one resolution, and that is 72 dpi. That may have been true at some point in time, but that is most definitely NOT true in the version that I am using (1.2.4).

First of all, that statement ONLY applies anyway when your monitor resolution is 72 dpi. My monitor resolution is 100 dpi (.25mm dot pitch, 1600x1200 on a 21" screen), which the GIMP supports directly, regardless of my next point.

Secondly, there is a preferences item under Interface/Image Windows named Use "Dot for Dot" by default. With this option disabled (my normal usage), images at different resolutions than screen resolution will still be displayed at their normal size on the screen by default.

Finally, you can specify both resolution and size in the standard File->New dialogue, as well as in the preferences options for new files.

I'd also like to bring up one very important GIMP advantage that I didn't see mentioned at all in the Photoshop vs. GIMP comparison. The GIMP can in many cases handle LARGER images than Photoshop. Photoshop (ALL versions) is limited to 32K pixels in any single dimension, whereas the GIMP is only limited by available memory (potentially including virtual memory). I routinely process images with over .5G total pixels, and over 300K pixels on one axis using the GIMP. Photoshop cannot handle these images at all. In fact, I have found very few other applications that can.

It would sure be nice if someone could make note of these facts on the subject web page.

s/KAM

====================

Great idea Kevin! I don't know myself how old the site is and the data contained therein, but it sounds like it could stand some updating. I am sure everyone here appreciates you clearing up some older points about Gimp that have been resolved in the newer versions. Gimp is a good program and gets better with each new version. I don't know who updates the mentioned site or who you would need to contact to make note of the differences you point out, but I am sure it would be worthwhile to do so.
http://manual.gimp.org/manual/GUM/Migrate.html

I think someone on this list is putting together a revised FAQ as well that will be somewhat more updated. I seem to remember someone putting a call for questions and anyone that could answer those questions, so maybe that person or persons could update us on the progress of that.

Thanks again for bringing all of us up to date on other ways Gimp proves to be a better choice than Photoshop!

Patrick

Daniel Carrera
2003-03-28 04:16:15 UTC (over 17 years ago)

[Q] Understanding the 72 DPI problem

Thanks Kevin, I do feel much more informed now. I still have a few questions, which I hope you could answer.

On Thu, Mar 27, 2003 at 09:37:27AM -0600, Kevin Myers wrote:

There are some VERY misleading statements on the web page referenced below regarding the specific question originally posted by the potential GIMP user in this thread.

I looked at the site. The content of the manual is great, but it deals with Gimp 1.2.2. So I guess this is just outdated informatin.

BTW, I am more than a "potential" user, I am a fully-converted GIMP lover :). However, I've only ever used GIMP for the web, and I want to understand the issues involved in producing something for the printed press.

Finally, you can specify both resolution and size in the standard File->New dialogue, as well as in the preferences options for new files.

I just noticed that, I can't believe I didn't see it before.

My first question: If Gimp sees images as a matrix of pixels, what does it /mean/ for an image for have 100ppi? Are you changing the ppi by just zooming in and out?
The concept of dpi/ppi only seems to come in when you print an image, so I'm confused.

I'd also like to bring up one very important GIMP advantage that I didn't see mentioned at all in the Photoshop vs. GIMP comparison. The GIMP can in many cases handle LARGER images than Photoshop.

This is very interestig.
Elsewhere on the site give, it says that Photoshop is faster for large images and GIMP is faster for smaller images. Is that still true?

Photoshop (ALL versions) is limited to 32K pixels in any single dimension, whereas the GIMP is only limited by available memory (potentially including virtual memory). I routinely process images with over .5G total pixels, and over 300K pixels on one axis using the GIMP. Photoshop cannot handle these images at all. In fact, I have found very few other applications that can.

That sounds huge. Where do you use those images? posters? How large should an image be so that it can be printed as a poster and still look good? (say, a 22x34in poster).

How much memory do you need to handle an image that large? Can a regular PC (e.g. Athlon XP 1800+) manipulate those images and be fast enough to be useful?

Thanks for your help.

Kevin Myers
2003-03-28 04:46:43 UTC (over 17 years ago)

[Q] Understanding the 72 DPI problem

See below. Hope this helps.

s/KAM

----- Original Message ----- From: "Daniel Carrera"
To: "gimp users"
Sent: Thursday, March 27, 2003 9:16 PM Subject: Re: [Gimp-user] [Q] Understanding the 72 DPI problem

Thanks Kevin, I do feel much more informed now. I still have a few questions, which I hope you could answer.

I'm not yet a complete GIMP expert by any means either. There are MANY features that I still know nothing about at all. Glad to help out where I can, and pay back a little of the help that other GIMP users have provided to me.

On Thu, Mar 27, 2003 at 09:37:27AM -0600, Kevin Myers wrote:

There are some VERY misleading statements on the web page referenced

below

regarding the specific question originally posted by the potential GIMP

user

in this thread.

I looked at the site. The content of the manual is great, but it deals with Gimp 1.2.2. So I guess this is just outdated informatin.

BTW, I am more than a "potential" user, I am a fully-converted GIMP lover :). However, I've only ever used GIMP for the web, and I want to understand the issues involved in producing something for the printed press.

Finally, you can specify both resolution and size in the standard

File->New

dialogue, as well as in the preferences options for new files.

I just noticed that, I can't believe I didn't see it before.

My first question: If Gimp sees images as a matrix of pixels, what does it /mean/ for an image for have 100ppi? Are you changing the ppi by just zooming in and out?

No, you're not changing the dpi of the actual image by simply zooming in and out.

The concept of dpi/ppi only seems to come in when you print an image, so I'm confused.

You're right that printing is one important place where dpi comes into consideration. The other place is when you DON'T have the Dot for Dot option selected, and want your image displayed on the screen at actual (i.e. printed) size. The GIMP allows you to provide screen size and resolution information when it is unable to determine it automatically, so that it can scale images appropriately for display.

I'd also like to bring up one very important GIMP advantage that I

didn't

see mentioned at all in the Photoshop vs. GIMP comparison. The GIMP can

in

many cases handle LARGER images than Photoshop.

This is very interestig.
Elsewhere on the site give, it says that Photoshop is faster for large images and GIMP is faster for smaller images. Is that still true?

Sorry, I can't answer that one. Maybe some other folks can chip in. All I know is that for most of what I normally use it for, the GIMP is fast enough for my needs, even with my huge images. However, there are a few of the filters that I use which are certainly slower than I would like (notably unsharp mask). But scaling, resolution, and color depth changes, and many other filters are reasonably quick. FYI, I primarily work with grayscale and line art images (although the GIMP presently converts those to gray scale too, which is a bit of a pain for me, since I then have to use some other application to convert them back).

Photoshop (ALL versions) is limited to 32K pixels in any single dimension, whereas the GIMP is only limited by available memory (potentially including virtual memory). I routinely process images with over .5G total pixels, and over 300K pixels on one axis using the GIMP. Photoshop cannot handle these images at all. In fact, I have found very few other applications that can.

That sounds huge. Where do you use those images? posters?

Raster images of well logs for oil and gas wells. These are essentially strip chart recordings of various geophysical properties up and down a borehole. Even when the depth axis is scaled down significantly (e.g. 1 inch = 100 ft), these logs can still be tens or even hundreds of feet in length. In many respects as far as the GIMP as concerned, you could think of them as very long, narrow posters. I also deal with other very large technical documents, such as maps of subsurface formation topography, and geologic cross-sections. These documents are typically palletized color, 300 dpi, and roughly 3 to 5 ft high by 4 to 8 ft long.

How large should an image be so that it can be printed as a poster and still look good? (say, a 22x34in poster).

Not sure what exactly you're asking here. But for true posters, with large text that would be viewed mainly from a distance, I would think that 100 dpi or even 72 dpi would probably be adequate. On the other hand, I deal primarily with images that are going to be viewed for up close detail, even though the images are physically very large, unlike typical poster applications. So, my file size and resolution requirements are much larger than normal.

How much memory do you need to handle an image that large? Can a regular PC (e.g. Athlon XP 1800+) manipulate those images and be fast enough to be useful?

I am using a 2+GHz P4 machine with 1.5GB of RAM. I have also used a 1GHz Duron with 640MB, but that can really slow to a crawl on my largest images that force swapping virtual memory to disk in order to handle the image size. Otherwise, the speed really isn't all that bad on the Duron either. For reasonably normal size posters, I would guess that your Athlon XP based system should be just fine as long as you have a decent amount of RAM installed. The amount that you really need depends on the exact size, resolution, and color depth of your images. If you can live with 256 palettized colors, then a 3 ft by 4 ft poster at 300 dpi (probably far more than adequate resolution) would require 3*12*300*4*12*300 = 155MB, plus some additional for the program itself and the OS. I'd probably recommend at least 256MB of RAM as a decent starting point for large, high resolution poster work.

Thanks for your help.
--
Daniel Carrera
Graduate Teaching Assistant. Math Dept. University of Maryland. (301) 405-5137