graphic information

[bconner]

Hi,

I'm a bit fuzzy on attributes of graphic files in general. They have pixels in the x and y direction, color depth, perhaps other metadata. Is pixels per inch also part of the attributes? That seems odd as I would think it would be conditional based on the device they are displayed on or printed to.

But I've seen some information displays from programs list a pixels per inch. ?

Thanks!

--Ben

[mwenz]

Yes, but DPI/PPI is also just meta data. It's an aid for many/most application to size the image on the page at import so that there are X number of pixels per inch per the meta data.

In the absence of this DPI/PPI meta data, most all (probably all) applications import the image at the OS' default DPI/PPI. So for Windows that is 96 DPI and for a Mac it is 72 DPI.

[bconner]

I guess one of the questions I have then is how is it assigned when resizing? Do different graphics packages have default values? Is it even used by anything?

I have an image I resized. The original had 330 dpi but the resized one had 200. There was no option I saw during the resizing to pick the dpi value.

--Ben

[mwenz]

DPI/PPI is simply a mathematical construct of the physical number of pixels versus the physical size. This is known as Effective PPI.

So in your example, you increased the size and so the effective DPI/PPI drops. A 1200 pixel image that reports it is 300 dpi is only 300 dpi at 4" in width (1200 / 300 = 4).

The only way of maintaining the DPI when scaling larger is to also resample the image. Please note that means whatever application is resampling during resizing is adding pixels to the image. Depending on the application, the resampling method used and the degree of enlargement, the quality can range from pretty good result quality to absolutely unacceptable.

[bconner]

Found out the root cause of the original motivation for the post. The images I was using were being included in a document going to a PDF file. Apparently PDF files like their images at 150 DPI. These were way over that. So resizing them should take care of the issue.

Never realized PDF files had such low resolution.

Thanks so much for going in to detail about image metadata!

--Ben

[mwenz]

PDF has no such resolution restriction on its own. Whatever makes the PDF uses a library for making the PDF. This library has the ability to downsample to a given resolution if it is told to.

[bconner]

Interesting. I will inquire as to how they create them and see what the issues are.

Thanks!

--Ben