1999.12.06 10:14 "Re: X & Y Resolution", by Ivo Penzar
I've tried to understand what Xresolution and Yresolution are from the tiff specification, but I still don't understand how I would go about deciding the values for these fields based on the number of pixels I have in my image. I know the value of the width and height, but is there any correlation between these and the above values?
E.g., if your document was scanned as 300 dots per inch, X and Y resolutions would be both 300/1 (or 600/2, 900/3, 1500/5, etc. - TIFF stores X and Y resolutions as rational numbers, either as number of dots per inch or per centimeter). If that document was 3 by 4 inches (original), the corresponding width and height would be 900 x 1200 pixels. Now, the smart viewer (or printer driver) knows how to recalculate the original size (3 by 4 inches) from the width and height (in pixels) and from X and Y resolutions.
It is mostly useful for technical documentation, maps, etc. Most images do not have meaningful X and Y resolutions: paintings, photos and alike; sometimes this information is not written (some image formats do not provide fields to store them), or is just stored as "default" value 72 or 75 dots per inch.