[sgiroux]

Hi Everyone,

The picture I've attached to this post is 180 dpi.
However, Gimp says the ppi of the image is 300 (Gimp also used this information display the printed size, so it should be "dpi", but this is not my problem).

I fully understand the difference between dpi and ppi.

My question is: From where is this information coming from?

My default image resolution in Gimp is 96 ppi (under Preferences > Default image > Advanced option).

Edit: Since I have some difficulties with attachment, here's the image: http://postimage.org/image/th0vspkx3/

This post has been edited by sgiroux: 22 February 2012 - 03:45 PM

[billps]

When I download it, the image is 300 ppi.

When I re-save it as 180ppi, and open it again, it is still set to 180ppi, so I can't replicate what you are getting. This is what I get after re-saving and opening: http://i.imgur.com/emG4L.jpg Perhaps it's being changed by the site you have uploaded it to. Are you sure you have the most up-to-date stable version of GIMP (2.6.12)?

By the way, the image resolution is not the ppi. ppi is the output resolution. The actual resolution of a digital image is measured in the number of pixels - what you have the ppi set to is irrelelvant. If you want to understand more about why ppi/dpi is irrelevant check out this link: The Myth of DPI http://www.rideau-in...os/mythdpi.html

This post has been edited by billps: 24 February 2012 - 03:03 PM

[sgiroux]

billps, on 24 February 2012 - 02:54 PM, said:

When I download it, the image is 300 ppi.

When I re-save it as 180ppi, and open it again, it is still set to 180ppi, so I can't replicate what you are getting. This is what I get after re-saving and opening: http://i.imgur.com/emG4L.jpg Perhaps it's being changed by the site you have uploaded it to. Are you sure you have the most up-to-date stable version of GIMP (2.6.12)?

By the way, the image resolution is not the ppi. ppi is the output resolution. The actual resolution of a digital image is measured in the number of pixels - what you have the ppi set to is irrelelvant. If you want to understand more about why ppi/dpi is irrelevant check out this link: The Myth of DPI http://www.rideau-in...os/mythdpi.html

Thank you for taking the time to write at my post.
To answer your question: I'm using Gimp 2.6.11.

...and the behavior you described is exactly what I get; I displays 300 ppi, but... to me, it does not make sense, since the DPI is 180, and I can't find from where this 300 ppi comes from, weither it is calculated in Gimp or it is stored in the picture.

[ofnuts]

sgiroux, on 22 February 2012 - 01:48 PM, said:

Hi Everyone,

The picture I've attached to this post is 180 dpi.
However, Gimp says the ppi of the image is 300 (Gimp also used this information display the printed size, so it should be "dpi", but this is not my problem).

I fully understand the difference between dpi and ppi.

My question is: From where is this information coming from?

This information is encoded in your image JFIF header (which is what is used by JPEG decoders):



("12C" in hexadecimal is "300" in decimal).

The 180DPI you mention is in the EXIF data, but from a JPEG handling point of view, EXIF/XMP/IPTC metadata have no specific meaning. So the question is what software sets the EXIF version but not the JFIF one. Cameras usually set the EXIF one to "72" which really means "I don't know", because the PPI of a photo would be meaningless: in the picture field you can see objects with very different physical sizes take up the same number of pixels (think about all the tourist pictures with someone leaning on the Pisa or Eiffel towers).

This post has been edited by ofnuts: 27 February 2012 - 04:02 PM

[sgiroux]

ofnuts, on 27 February 2012 - 03:54 PM, said:

This information is encoded in your image JFIF header (which is what is used by JPEG decoders):

("12C" in hexadecimal is "300" in decimal).

The 180DPI you mention is in the EXIF data, but from a JPEG handling point of view, EXIF/XMP/IPTC metadata have no specific meaning. So the question is what software sets the EXIF version but not the JFIF one. Cameras usually set the EXIF one to "72" which really means "I don't know", because the PPI of a photo would be meaningless: in the picture field you can see objects with very different physical sizes take up the same number of pixels (think about all the tourist pictures with someone leaning on the Pisa or Eiffel towers).

Thank you ofnuts for this clear and precise answer!