Sorry to again post to the mailing list about something that you
know far better than I do (it is like \infty to 0 !):

from
http://www.libpng.org/pub/png/book/chapter11.html#png.ch11.div.8

Physical Pixel Dimensions (pHYs)

* Status: PNG Specification
* Location: before first IDAT
* Multiple: no=20

The pHYs chunk encodes the absolute or relative dimensions of pixels. For=
example, an image scanned at 600 dots per inch has pixels with known, ab=
solute sizes--namely, one six-hundredth of an inch in both x and y direct=
ions. Alternatively, an image created on a 1280 =D7 1024 display will hav=
e nonsquare pixels, and the relative dimensions of each pixel, also refer=
red to as the aspect ratio, may be stored so the image can be displayed a=
s it was intended to be seen.

The layout of the chunk is shown in Table 11-4.

Table 11-4. pHYs Chunk

Field Length and Valid Range
Pixels per unit, x axis 4 bytes (0-2,147,483,647)
Pixels per unit, y axis 4 bytes (0-2,147,483,647)
Unit specifier 1 byte (0, 1)

If the unit specifier byte is 1, the units are meters; if it is 0, the un=
its are unspecified, and only the relative dimensions are known. Currentl=
y, no other values are valid. Note that the format of the chunk precludes=
pixel sizes greater than one meter, which should not be a significant ha=
rdship for most applications, but it allows pixels as small as 4.7 =C5ngs=
troms, which is roughly the size of a single atom.


best regards
Jean-Fran=E7ois