Geoffrey wrote:

So here's a question which will demonstrate my ignorance. I've got some digital photos I took that, when opened with GIMP are identified as being 75dpi x72dpi. I need images that are 300 dpi, so is it possible to convert the 75dpi image to 300 dpi?? I selected to scale the image and happened to notice that I can make this change at this point. Is this doing what I'm expecting?

That is a question that only a mind reader can answer. (Sorry :-))

Here's the thing: a resolution in dpi is not a property of the image per se, it's a property of the way the image is displayed, on the screen or on paper. When you see a resolution of 72x72 dpi for an image, what those numbers represent is somebody's judgement that the image will look good when displayed with 72 pixels per inch; and usually this means "look good when displayed on a monitor", because nothing looks very good when it is printed at 72 dots per inch.

Now suppose you have a 300 x 300 pixel image, with a nominal resolution of 72 dpi, and suppose you want to convert it to 300 dpi for printing. There are two approaches you could take. (Actually more, but let's keep it simple for the moment.)

(1) You caould say, okay, 300 pixels is about 4 inches at 72 dpi. I want my print to have the same size. So, I will scale the image to 1200 x 1200 pixels, and set the resolution to 300 dpi.

(2) You could say, okay, I don't want to make the image look blurry by scaling up the number of pixels, so I will simply set the resolution to 300 dpi without changing the pixel dimensions, thereby getting a print about 1 inch across.

Both of these are legitimate choices, and so are many others. The best way to do it depends on your image and your printer. What makes it complicated is the fact that printer dots have much poorer color resolution than monitor dots. On most modern systems, a monitor dot encodes 24 bits of color information. On a typical high-quality printer, a single dot encodes about 6 bits of color information. Thus, simply changing the resolution so that a single monitor dot becomes a single printer dot is usually not the best thing to do.

Best,
-- Bill

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On Mon, Sep 13, 2004 at 02:22:24PM -0400, Geoffrey wrote:

So here's a question which will demonstrate my ignorance. I've got some digital photos I took that, when opened with GIMP are identified as being 75dpi x72dpi. I need images that are 300 dpi, so is it possible to convert the 75dpi image to 300 dpi?? I selected to scale the image and happened to notice that I can make this change at this point. Is this doing what I'm expecting?

open up one of the images in gimp and tell how many pixels each image has in its width and height.

carol

Quoting Geoffrey :

The actual image is going to a professional shop to be placed in an ad. Thanks for your feedback.

A lot of printers (companies, not cheap piece of hardware on your desk) read the
DPI information out of the file to know what size to print the file at, so it is an important, if not an integral part of an "image".

I work a lot with machine vision, and as part of our project, we make sure that
the DPI information is properly filled in the images we acquire, so we can easily convert between physical and pixel coordinates in our processing. (a trivial and silly example, as there are other ways of doing it)

Jeff

On Monday 13 September 2004 02:22 pm, Geoffrey wrote:

So here's a question which will demonstrate my ignorance. I've got some digital photos I took that, when opened with GIMP are identified as being 75dpi x72dpi. I need images that are 300 dpi, so is it possible to convert the 75dpi image to 300 dpi?? I selected to scale the image and happened to notice that I can make this change at this point. Is this doing what I'm expecting?

=========

Geoffrey,

In the last Linux Journal magazine, October 2004, Issue 126, there is a wonderful article on Gimp and using it for professional photography! One of the best articles I've read and more than helpful and encouraging about the abilities of Gimp. The article itself is not online, but the resources are listed there. The author, RW Hawkins, has his site there as well and would be very helpful should you have detailed questions.

That's not to say people on this list are not just as qualified to answer you, because they are! I just thought this might be of some interest to you and others, since you brought up digital photos and I know there is a lot of interest about that and the Gimp's ability to work with them effectively.

Here is your resources site: http://www.linuxjournal.com/article.php?sid=7704

Hope that helps and if you don't have a copy of the mag, go get one, it's a nice article for info and on the Gimp!

Patrick

On Tuesday, September 14, 2004 4:22 AM [GMT+1=CET], Geoffrey wrote:

So here's a question which will demonstrate my ignorance. I've got some digital photos I took that, when opened with GIMP are identified as being 75dpi x72dpi. I need images that are 300 dpi, so is it possible to convert the 75dpi image to 300 dpi?? I selected to scale the image and happened to notice that I can make this change at this point. Is this doing what I'm expecting?

This series may be useful:
http://www.scantips.com/basics01.html

Bob Long

On Mon, Sep 13, 2004 at 01:39:09PM -0700, Carol Spears wrote:

On Mon, Sep 13, 2004 at 02:22:24PM -0400, Geoffrey wrote:

So here's a question which will demonstrate my ignorance. I've got some digital photos I took that, when opened with GIMP are identified as being 75dpi x72dpi. I need images that are 300 dpi, so is it possible to convert the 75dpi image to 300 dpi?? I selected to scale the image and happened to notice that I can make this change at this point. Is this doing what I'm expecting?

open up one of the images in gimp and tell how many pixels each image has in its width and height.

okay, i am replying to my own posting, and there is some rule against this, i am fairly certain.

i would really like to know how many pixels you are working with. please?

carol

While we're on the topic of scaling/resampling maybe someone could clarify something for me. I read somewhere that when scaling an image for display on a monitor that it was best to change the dpi by the same ratio to avoid interpolation. Elsewhere I have gotten the impression that dpi was totally and always irrelevant to images on the screen.

My limited understanding is that when scaling an image from, e.g. 1600 pixels wide to 400 pixels wide, that it is best to change dpi in the same ratio, say, from 72 dpi to 288 dpi. And further that it is even more important when upsampling an image. The desired result being the avoidance of any blurring from the software not being able to map the scaled image pixel for pixel.

Is this accurate? Does any of it actually show up on the screen or would it only apply to printed images?

Thanks,

Barton

Hi,

Barton Bosch writes:

While we're on the topic of scaling/resampling maybe someone could clarify something for me. I read somewhere that when scaling an image for display on a monitor that it was best to change the dpi by the same ratio to avoid interpolation. Elsewhere I have gotten the impression that dpi was totally and always irrelevant to images on the screen.

My limited understanding is that when scaling an image from, e.g. 1600 pixels wide to 400 pixels wide, that it is best to change dpi in the same ratio, say, from 72 dpi to 288 dpi. And further that it is even more important when upsampling an image. The desired result being the avoidance of any blurring from the software not being able to map the scaled image pixel for pixel.

Unless you disable "Dot-for-Dot" in the GIMP's image window (it's in the View menu), you will always see one pixel of the image taking up one pixel on screen (of course only in 1:1 zoom ratio). The dpi setting is irrelevant for the image display. It becomes of importance if you want to use the rulers or the measure tool with real-world units. Then GIMP needs to know how large a pixel will end up on the final print or projection or whatever your final media is.

Changing the dpi setting alone won't alter your pixel data. It's just some meta information. Even if you disable dot-for-dot view in GIMP, the dpi information is only used in the display routines. Your pixel data is not changed.

Sven

Hi,

Geoffrey writes:

Thanks for the insites Sven. I had seen the dot-for-dot option, but didn't really know what it was for. So, if I understand you correctly, by opening a file and setting the dpi from 75 to 300 and then saving it really hasn't changed anything?

Nothing but the resolution information. The actual image data remains unchanged.

Sven

Sven Neumann wrote:

Hi,

Barton Bosch writes:

While we're on the topic of scaling/resampling maybe someone could clarify something for me. I read somewhere that when scaling an image for display on a monitor that it was best to change the dpi by the same ratio to avoid interpolation. Elsewhere I have gotten the impression that dpi was totally and always irrelevant to images on the screen.

My limited understanding is that when scaling an image from, e.g. 1600 pixels wide to 400 pixels wide, that it is best to change dpi in the same ratio, say, from 72 dpi to 288 dpi. And further that it is even more important when upsampling an image. The desired result being the avoidance of any blurring from the software not being able to map the scaled image pixel for pixel.

Unless you disable "Dot-for-Dot" in the GIMP's image window (it's in the View menu), you will always see one pixel of the image taking up one pixel on screen (of course only in 1:1 zoom ratio). The dpi setting is irrelevant for the image display. It becomes of importance if you want to use the rulers or the measure tool with real-world units. Then GIMP needs to know how large a pixel will end up on the final print or projection or whatever your final media is.

Changing the dpi setting alone won't alter your pixel data. It's just some meta information. Even if you disable dot-for-dot view in GIMP, the dpi information is only used in the display routines. Your pixel data is not changed.

Okay, thanks for the definitive answer. The few references to changing the dpi when scaling must have been in regards to printed images.