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Posted by Oli Filth on 12/06/05 21:34
Onideus Mad Hatter said the following on 05/12/2005 23:45:
> even if you up yer res, it's still 72 pixels per inch,
> altering the res essentially just changes the size of the pixels, but
> it's STILL 72, you fucking moron.
I'll step you through the complicated maths for this...
For any given on-screen image:
Pixels per inch = P / W
where P = Number of pixels across,
W = Number of inches across (physical width as measured by a ruler)
If you alter the resolution of screen (let's say you increase it), you
reduce the size of the pixels, therefore the on-screen image shrinks,
i.e. W reduces. As P stays the same, the result is that PPI (as viewed
on screen) increases.
It can easily be shown that the same value occurs for *any* image on a
given screen, i.e. it's a constant for a given monitor and given resolution.
Which is kind of obvious, because PPI of an on-screen image is entirely
determined by the number of pixels the screen is displaying per inch
(note: this is the reciprocal of "dot pitch").
As an example, in a previous post when you stated in an earlier post:
> http://www.backwater-productions.net/_images/mine3.png
>
> That image has EXACTLY 72 pixels per inch, count them out if you're
> feeling especially stupid, MORON.
That image is 400 px wide, and on my current resolution (1280x1024),
displays at 4.125" wide. Which works out at 400/4.125 = 97.0 PPI.
Definitely not 72 PPI as you claimed.
And look, almost magically, my (viewable) screen is 13.2" wide, which
works out at 1280/13.2 = 97.0 PPI. How convenient.
In the case where the software attempts to compensate for screen
resolution (e.g. in "Actual size" Print Preview), there's no longer a
one-to-one mapping between image pixels and physical screen pixels; i.e.
all it's doing is applying an extra zoom factor.
If it's an integer mapping, e.g. 3 screen pixels to 1 image pixel (in 1D
for simplicity), then an interpolation method is used to fill in the
gaps. For a non-integer mapping, e.g. 1.38 screen pixels to 1 image
pixel, the interpolation results in very few (potentially none) of the
original image pixels actually being displayed.
Either way, the number of physical pixels being displayed in an inch is
still determined by the screen resolution and size.
So when you ask:
> What, do you think your computer
> can just MAKE UP pixels that don't exist? LOL
The answer is: yes! How else could it display a resampled/scaled/zoomed
form of the image? Whether its using nearest-neighbour (zero-order),
higher-order polynomial (linear, cubic, etc.), spline, or
bandwidth-limited interpolation, your computer is displaying pixels that
aren't specified in the original image file. By definition,
interpolation is the process of making up values that don't exist.
This applies whether you've manually resampled the image, or your
software is doing it on-the-fly for scaling or zooming purposes.
--
Oli
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