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 Post subject: Help explaining changing DPI in IMAGE>SCALE IMAGE [SOLVED]
PostPosted: Thu Jul 14, 2016 2:24 pm  (#1) 
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I'm going to make my ebook covers into paperback covers on Createspace (Amazon's company). So the big buzz is you have to have a 300 DPI image to make for an effective paperback cover. Well, as far as I can tell what's important is having enough pixels to make a paperback cover work. So if my ebook cover is 1800X2700 I have enough to make a 300 DPI cover of 6" by 9". Simple.

Question #1. What is the difference between a 72DPI 1800X2700 ebook cover and a 300DPI ebook cover? In my mind--nothing. They both have 4,860,000 pixels. Am I wrong?

Question #2. If I have an image in GIMP and I go to the Scale Image feature and it says 72 DPI and I change it to 300DPI, what does that do? It can't add pixels.

Thanks.

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Last edited by iggy on Sun Jul 17, 2016 2:44 pm, edited 1 time in total.

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 Post subject: Re: Help explaining changing DPI in IMAGE>SCALE IMAGE
PostPosted: Thu Jul 14, 2016 2:48 pm  (#2) 
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iggy wrote:
GIMP Version: 2.8.14
Operating System: Linux
OS Version: XUBUNTU
GIMP Experience: New User



I'm going to make my ebook covers into paperback covers on Createspace (Amazon's company). So the big buzz is you have to have a 300 DPI image to make for an effective paperback cover. Well, as far as I can tell what's important is having enough pixels to make a paperback cover work. So if my ebook cover is 1800X2700 I have enough to make a 300 DPI cover of 6" by 9". Simple.

Question #1. What is the difference between a 72DPI 1800X2700 ebook cover and a 300DPI ebook cover? In my mind--nothing. They both have 4,860,000 pixels. Am I wrong?

Question #2. If I have an image in GIMP and I go to the Scale Image feature and it says 72 DPI and I change it to 300DPI, what does that do? It can't add pixels.

Thanks.

DPI settings are mainly for printing.
The DPI comes into play when you attempt to print the image.
The difference is obvious 72 DPI vs 300 DPI when printing.

It’s important to begin with a high-quality image,
which means the highest resolution and image dimensions you can get.
When it comes to source images,
bigger is better,
because you can go down in size,
but not up,
without losing quality.

DPI: Dot’s per inch. The number of dots in a printed inch.
The more dot’s the higher the quality of the print (more sharpness and detail).

PPI: Pixels per inch.
Most commonly used to describe the pixel density of a screen
(computer monitor, smart phone, etc…)
but can also refer to the pixel density of a digital image.

Resolution: Resolution is the measure of pixels in the display,
usually expressed in measurements of width x height.
For example a monitor that is 1920 x 1080 is 1920 pixels across and 1080 pixels down.

Higher resolution means more detail.
Higher DPI means higher resolution.
Resolution is not “size”,
but it’s often confused with it because higher resolution images are often bigger,
but that doesn’t necessarily have to be the case.

Print: 300dpi is standard,
sometimes 150 is acceptable but never lower,
you may go higher for some situations.

Web/Digital: DPI doesn’t equate to digital it’s a print measure.
It was commonly believed for a long time that 72dpi was ideal for web.
If you hear that it’s simply not the way things work.
When talking digital, we’re concerned with the actual resolution.
How that image prints is another matter.

If you are creating images to use for print and the images are “too small” the odds are the resolution wasn’t high enough.
The image might look huge on your computer but still print out quite small.
To add to the confusion,
your screen resolution will also determine how big the picture appears to you when viewing it on your computer.

A monitor set to 1024 x 768 will show an 1024 pixel wide by 800 pixel tall image as a full screen image.
On a monitor that is 1920x 1080 the image will only take part of the screen.
Long story short,
the image will look much smaller on that screen,
even though the image is the same size because the screen has higher resolution.

Here are some examples to show you the difference,
no matter what your monitor resolution,
it’s all relative!

The first example below has a lot of detail.
The second example is at 72dpi but scaled up to the same size so you can see the difference in detail.
The actual image would be about 1/4 the size when you go from 300dpi to 72dpi,
but at the same height and width is where you can actually see the difference.
------300dpi example------------------------72dpi example
Image----Image

Hopefully this has helped you get a little clearer on the differences between DPI and PPI,
resolution and why if you have someone do something for you in print,
there will be different requirements than for the web.
It’s also why that digital camera with higher megapixels takes better pictures than one with lower,
(lenses and other factors being equal),
because it gives you more resolution to capture more detail.

Another important note about monitors,
even though 72dpi is standard for the web,
monitors have slightly different resolutions depending how you have the monitor set and how big the monitor is.
For example,
a 19″ monitor set to 1024×768 will show 70ppi
(pixels per inch, monitors use pixels which are square not round,
but pixels and dots for the sake of this conversation are otherwise analogous).
By comparison,
a 19″ monitor set to 1280×1024 will have a resolution of 87ppi which means you fit more on the screen and get more detail,
but everything looks smaller.


Side bar:
image files with higher resolution,
(more dpi),
will also have a bigger file size because they contain more data.
Start with the biggest images you can,
but when putting images on the web they should be set to 72dpi,
it’ll save a ton of bandwidth and they’ll load faster.
Yes,
they’ll be smaller than the original,
but should in most cases be plenty big,
because of monitor resolution (ppi) sizes.


One last thing,
don’t confuse “image size” with “file size.”
Image size refers to the dimensions of the image,
while file size is how much space the image takes up on a hard drive (kilobytes or megabytes).

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 Post subject: Re: Help explaining changing DPI in IMAGE>SCALE IMAGE
PostPosted: Thu Jul 14, 2016 6:35 pm  (#3) 
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Joined: Nov 16, 2011
Posts: 5130
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Wallace made some excellent points. The main thing to remember is 'start' your design with a 300ppi canvas.
Here's one example of why; importing the same 3048x1536 image to a 72ppi and a 300ppi canvas, both 11x8.5", landscape. If the top image had been scaled down to fit the 72ppi canvas before the canvas was changed to 300ppi, it would appear much smaller than the image currently on the bottom 300ppi canvas.

Image
to scale

_________________
Image
Gimp 2.8.18, Linux, median user
Gimp Chat Tutorials Index
Spirit Bear (Kermode)


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 Post subject: Re: Help explaining changing DPI in IMAGE>SCALE IMAGE
PostPosted: Thu Jul 14, 2016 7:51 pm  (#4) 
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GimpChat Member
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Joined: Sep 21, 2015
Posts: 174
Location: Itasca, Illinois
Wallace wrote:
iggy wrote:
GIMP Version: 2.8.14
Operating System: Linux
OS Version: XUBUNTU
GIMP Experience: New User



I'm going to make my ebook covers into paperback covers on Createspace (Amazon's company). So the big buzz is you have to have a 300 DPI image to make for an effective paperback cover. Well, as far as I can tell what's important is having enough pixels to make a paperback cover work. So if my ebook cover is 1800X2700 I have enough to make a 300 DPI cover of 6" by 9". Simple.

Question #1. What is the difference between a 72DPI 1800X2700 ebook cover and a 300DPI ebook cover? In my mind--nothing. They both have 4,860,000 pixels. Am I wrong?

Question #2. If I have an image in GIMP and I go to the Scale Image feature and it says 72 DPI and I change it to 300DPI, what does that do? It can't add pixels.

Thanks.

DPI settings are mainly for printing.
The DPI comes into play when you attempt to print the image.
The difference is obvious 72 DPI vs 300 DPI when printing.

It’s important to begin with a high-quality image,
which means the highest resolution and image dimensions you can get.
When it comes to source images,
bigger is better,
because you can go down in size,
but not up,
without losing quality.

DPI: Dot’s per inch. The number of dots in a printed inch.
The more dot’s the higher the quality of the print (more sharpness and detail).

PPI: Pixels per inch.
Most commonly used to describe the pixel density of a screen
(computer monitor, smart phone, etc…)
but can also refer to the pixel density of a digital image.

Resolution: Resolution is the measure of pixels in the display,
usually expressed in measurements of width x height.
For example a monitor that is 1920 x 1080 is 1920 pixels across and 1080 pixels down.

Higher resolution means more detail.
Higher DPI means higher resolution.
Resolution is not “size”,
but it’s often confused with it because higher resolution images are often bigger,
but that doesn’t necessarily have to be the case.

Print: 300dpi is standard,
sometimes 150 is acceptable but never lower,
you may go higher for some situations.

Web/Digital: DPI doesn’t equate to digital it’s a print measure.
It was commonly believed for a long time that 72dpi was ideal for web.
If you hear that it’s simply not the way things work.
When talking digital, we’re concerned with the actual resolution.
How that image prints is another matter.

If you are creating images to use for print and the images are “too small” the odds are the resolution wasn’t high enough.
The image might look huge on your computer but still print out quite small.
To add to the confusion,
your screen resolution will also determine how big the picture appears to you when viewing it on your computer.

A monitor set to 1024 x 768 will show an 1024 pixel wide by 800 pixel tall image as a full screen image.
On a monitor that is 1920x 1080 the image will only take part of the screen.
Long story short,
the image will look much smaller on that screen,
even though the image is the same size because the screen has higher resolution.

Here are some examples to show you the difference,
no matter what your monitor resolution,
it’s all relative!

The first example below has a lot of detail.
The second example is at 72dpi but scaled up to the same size so you can see the difference in detail.
The actual image would be about 1/4 the size when you go from 300dpi to 72dpi,
but at the same height and width is where you can actually see the difference.
------300dpi example------------------------72dpi example
[ Image ]----[ Image ]

Hopefully this has helped you get a little clearer on the differences between DPI and PPI,
resolution and why if you have someone do something for you in print,
there will be different requirements than for the web.
It’s also why that digital camera with higher megapixels takes better pictures than one with lower,
(lenses and other factors being equal),
because it gives you more resolution to capture more detail.

Another important note about monitors,
even though 72dpi is standard for the web,
monitors have slightly different resolutions depending how you have the monitor set and how big the monitor is.
For example,
a 19″ monitor set to 1024×768 will show 70ppi
(pixels per inch, monitors use pixels which are square not round,
but pixels and dots for the sake of this conversation are otherwise analogous).
By comparison,
a 19″ monitor set to 1280×1024 will have a resolution of 87ppi which means you fit more on the screen and get more detail,
but everything looks smaller.


Side bar:
image files with higher resolution,
(more dpi),
will also have a bigger file size because they contain more data.
Start with the biggest images you can,
but when putting images on the web they should be set to 72dpi,
it’ll save a ton of bandwidth and they’ll load faster.
Yes,
they’ll be smaller than the original,
but should in most cases be plenty big,
because of monitor resolution (ppi) sizes.


One last thing,
don’t confuse “image size” with “file size.”
Image size refers to the dimensions of the image,
while file size is how much space the image takes up on a hard drive (kilobytes or megabytes).



Thanks Wallace. There's a lot of good information in there. Cleared up a lot of things.

_________________
"When people agree with me I always feel that I must be wrong." --Oscar Wilde


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 Post subject: Re: Help explaining changing DPI in IMAGE>SCALE IMAGE
PostPosted: Thu Jul 14, 2016 8:01 pm  (#5) 
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Joined: Sep 21, 2015
Posts: 174
Location: Itasca, Illinois
Odinbc wrote:
Wallace made some excellent points. The main thing to remember is 'start' your design with a 300ppi canvas.
Here's one example of why; importing the same 3048x1536 image to a 72ppi and a 300ppi canvas, both 11x8.5", landscape. If the top image had been scaled down to fit the 72ppi canvas before the canvas was changed to 300ppi, it would appear much smaller than the image currently on the bottom 300ppi canvas.

[ Image ]
to scale


Thanks Odinbc. I tried your experiment. I opened a 3048X1536 digital photo in a 300PPI GIMP "new" screen and a 72PPI GIMP "new" screen and the results were the exact same size.

I don't think the PPI in GIMP does anything at all. Back to my original questions:

Question #1. What is the difference between a 72DPI 1800X2700 ebook cover and a 300DPI ebook cover? In my mind--nothing. They both have 4,860,000 pixels. Am I wrong?

Question #2. If I have an image in GIMP and I go to the Scale Image feature and it says 72 DPI and I change it to 300DPI, what does that do? It can't add pixels.

I scaled the same image at 1PPI and 5000PPI and it made no difference whatsoever in image quality.

Maybe the PPI (changing it to whatever) will be in the meta data (I don't know) but I don't think it has anything to do with the image quality. As far as I can tell it's all about how many pixels there are. Then it's just a question of needing to have more pixels to maintain image quality for larger prints.

Back to Wallace's post. The 72 PPI version of the lips would be just as detailed only at a smaller size.

_________________
"When people agree with me I always feel that I must be wrong." --Oscar Wilde


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 Post subject: Re: Help explaining changing DPI in IMAGE>SCALE IMAGE
PostPosted: Thu Jul 14, 2016 9:14 pm  (#6) 
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Joined: Nov 16, 2011
Posts: 5130
Location: Metro Vancouver, BC
iggy wrote:
Thanks Odinbc. I tried your experiment. I opened a 3048X1536 digital photo in a 300PPI GIMP "new" screen and a 72PPI GIMP "new" screen and the results were the exact same size.
If you opened an existing 3048x1536 image (like my example), your result should not be the same at both PPI. I'll attach my example image below. Try again using it. Something doesn't seem right.

  1. New from Template, US-Letter (300ppi), landscape and Open as Layers my image.
  2. New, Image size 11x8.5", landscape and Open as Layers my original image.

There's great info I believe would be helpful at All About Digital Photos start with, The Myth of DPI.


Attachments:
obc-image.zip [1 MiB]
Downloaded 75 times

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 Post subject: Re: Help explaining changing DPI in IMAGE>SCALE IMAGE
PostPosted: Fri Jul 15, 2016 12:47 pm  (#7) 
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Joined: Sep 21, 2015
Posts: 174
Location: Itasca, Illinois
Odinbc wrote:
iggy wrote:
Thanks Odinbc. I tried your experiment. I opened a 3048X1536 digital photo in a 300PPI GIMP "new" screen and a 72PPI GIMP "new" screen and the results were the exact same size.
If you opened an existing 3048x1536 image (like my example), your result should not be the same at both PPI. I'll attach my example image below. Try again using it. Something doesn't seem right.

  1. New from Template, US-Letter (300ppi), landscape and Open as Layers my image.
  2. New, Image size 11x8.5", landscape and Open as Layers my original image.

There's great info I believe would be helpful at All About Digital Photos start with, The Myth of DPI.


Odinbc. I did it again. Same results. Same image size in GIMP. (I even went to the 'advanced' option and made sure it was 72 dpi in your #2.)

Thanks for the links. That guy has it right in The Myth of DPI.

"It doesn't matter what number is parked in the DPI/PPI setting of the photo"

_________________
"When people agree with me I always feel that I must be wrong." --Oscar Wilde


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