On Wed, Mar 23, 2005 at 11:17:51AM +1100, Malcolm Tredinnick wrote:
> On Wed, 2005-03-23 at 01:00 +0100, cedric wrote:
> > When creating a new document, we have two fields. I've always used the 
> > same value but if they are 2 it is certainly because there is one. So 
> > does anybody know in what cases X and Y should be different ?
> Think about printable media (not just images for the screen). There are
> many printers that can print at different densities in the horizontal
> and vertical direction (e.g. 720 x 1440 dpi or 1440 x 2880) as there
> highest dot density. So if you were trying to work precisly, pixel-for-
> pixel, you might want to set the resolutions appropriately.

Another simple case is after deinterlacing an image, the y pixel density
will be half of what it was before, while x stays the same.

In fact, it is very rare for monitors/LCD panels/CCD cameras/etc to really
have square pixels.  Pretending it is square may be close enough for the
purposes of the Web, but for print media, medical equipment (my work), GIS
(my old work) and other endeavours, x density = y density is inadequate and
incorrect.  In fact, even linear mapping is often insufficient. (I've had to
use some pretty wild transforms in the course of work on images)


Computer Science is as much about computers as astronomy is about telescopes
        -- Edsger Wybe Dijkstra (1930-2002)

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