Opening a 300dpi JPG changes to 72ppi?

[Ilpoxi]

I make 300dpi JPG-s from CorelDraw. After export from CorelDraw I check the resolution once again in the Win Resource Management from file's properties and it's 300dpi. When I open this JPG in GIMP and check the Properties from Picture> (or Image>) Properties, the resolution is 72x72PPI. Why? I have done this 10 times to check it.

I have configured in GIMP program properties that a new document will be 300x300DPI. But this doesn't help.

If I open a photograph taken with 300dpi cellphone camera in Gimp, the resolution seems to remain 300DPI.

So, somehow my 300 DPI CorelDRaw Graphics are changed to 72PPI. Frustrating!

[Blighty]

A digital image only has size in pixels. It does not have a resolution in dpi. The dpi is just a dormant value stored in the image and is only used when printing.

For example, if an image is 900 x 900 pixels and the dormant resolution is 300 dpi, then the print size will be 3" x 3" (900/300 = 3).

In Gimp you can set the dpi using Image > Print Size. But this will only change the indication of the print size, the image itself does not change at all.

If there is no dpi set in an image then the software opening the image will assume some value. But again, this does not affect the image itself. The image size in pixels remains unchanged.

[Ofnuts]

Gimp uses (and sets when saving) the fields defined by the JPEG standard (the XDensity and YDensity fields)(and I just checked this by patching a JPEG with bogus values in these fields)(see attachment, which should show 336/288 DPI, you can also use it to check what CorelPaint sees).

It is possible that this information is also kept elsewhere (EXIF data) and that CorelPaint overlooks the fields from the JFIF fields.

[Ilpoxi]

Ok, thanks both for answers. Now I remembered I have Corel PhotoPaint that wes mentioned above I opened there a 300dpi JPG exported from CorelDraw, and surprise surprise, both Y and X dpi-s are 300. So, now I'm convinced. And I might continue using Corel PhotoPaint instead of Gimp since I have to be sure about the image quality. Besides, when exporting a JPG from Gimp, there is a checkbox "Optimize" under Advanced Settings arrow, which is "hidden" and checked. So the quality will be optimized what I don't like. But now I know the checkbox.

[Ofnuts]

(04-11-2018, 07:08 PM)Ilpoxi Wrote: Ok, thanks both for answers. Now I remembered I have Corel PhotoPaint that wes mentioned above I opened there a 300dpi JPG exported from CorelDraw, and surprise surprise, both Y and X dpi-s are 300. So, now I'm convinced. And I might continue using Corel PhotoPaint instead of Gimp since I have to be sure about the image quality. Besides, when exporting a JPG from Gimp, there is a checkbox "Optimize" under Advanced Settings arrow, which is "hidden" and checked. So the quality will be optimized what I don't like. But now I know the checkbox.

The image quality is not the DPI... The image quality is the size in pixels. The DPI is merely an "intent" for the print size. The only case where it really counts is when the image comes from a scanner, since it lets you print the image at the original size.

Optimize has nothing to do with the image quality, from the doc:

Quote:optimize:
If you enable this option, the optimization of entropy encoding parameters will be used. The result is typically a smaller file, but it takes more time to generate.

The options are all explained here.. Besides Quality the options that have an impact on the image accuracy are:

• Smoothing: quite a lot
  
• Subsampling: quite a lot too on some images, but also the setting that has the biggest influence on file size.
  
• DCT Method: a little
  

Can you tell what subsampling your software is using? It is rarely set to "None" in most image editors.

Any setting can be made the default.