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Re: Pixels per Inch [message #37382 is a reply to message #37371]

Mon, 15 December 2003 13:46

JD Smith
Messages: 850
Registered: December 1999

Senior Member

On Mon, 15 Dec 2003 13:20:08 -0700, Craig Markwardt wrote:

> JD Smith <jdsmith@as.arizona.edu> writes: [ ... ]
>> It depends on how many "inches" you intend to output the image to. This
>> is a basic and common mis-understanding of how image resolution works.
>> A bitmap image like a PNG file has a specific size in pixels, nxm say,
>> but does not have an associated "physical size". The
>
>
> JD, I'm not sure that's always true. I believe that TIFF images have an
> inherent (desired) dot pitch. Perhaps other formats do too, but PNGs
> definitely do not, so your comment is appropriate for the original
> poster.

Actually, PNG also has an optional header chunk called "pHYs" which gives
the pixels per meter. Other image formats have something similar. My
main point was that, no matter what you put in the header regarding the
"physical size" of the image, this is only a suggestion, and has no
bearing on the resolution or detail present in your image. Many programs
explicitly ignore these data (especially for on-screen display). You have
to separate the notion of resolution as "convenient suggestions for
printing and print layout programs" vs. resolution as real physical detail
present in the image.

The program pngcrush can add, change or remove the pHYs resolution
information from PNG files (http://pmt.sourceforge.net/pngcrush). But,
for example, if I take a PNG file and give it resolution 300dpi, 50dpi,
and no resolution, the three version display exactly the same in all
browsers, and the Gimp.

JD

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[Message index]

		Re: Pixels per Inch By: sdj on Tue, 16 December 2003 09:03
		Re: Pixels per Inch By: JD Smith on Mon, 15 December 2003 13:46
		Re: Pixels per Inch By: Craig Markwardt on Mon, 15 December 2003 12:20
		Re: Pixels per Inch By: JD Smith on Mon, 15 December 2003 10:00
		Re: Pixels per Inch By: R.Bauer on Mon, 15 December 2003 05:52
		Re: Pixels per Inch By: William Thompson on Fri, 19 December 2003 10:46
		Re: Pixels per Inch By: JD Smith on Tue, 16 December 2003 10:49

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