This is getting into a pretty complex realm. I hope what follows is
useful rather than rant...
On 6 Feb 2007, at 13:16, David Bovill wrote:
There are lots of time when you need to make these calculations - due
usually to information or interfaces provided by other software (often
defined primarily for print) - in my case I need to pass inches to the
shell and therefore convert to inches from the rect in Rev,
So the question is exactly what is involved in moving from lets say
640 x 480 image rect to a value in inches that you must supply to a
piece of print orientated software? At one time I thought it was just
72 dpi = an inche - but then I never got straight the use of points
and pixels and dots per inch.
A 'point' is a typographic real-world measurement, and is ~0.3mm. So
12pt type should be around 4mm high.
I know what dpi are in print world - I
know (I think) what pixels are in the screen world. That is i think
I do :)
- Pixels are little glowing spots on your screen
Yes.
- Dots are little non-glowing spots of colour (usually on paper)
Yes, but strictly speaking this is for for inkjet printers - laser
printers and commercial printing are actually measured in *lines* per
inch and the images themselves are measured in *pixels* per inch.
- the rect and loc etc in Rev refer to pixels of glowing colour
In general, ALL screen measurements refer to pixels, regardless of
the program, and these pixels don't have a fixed size.
- inches and other conventional measure refer to fixed distances
in space (relativity not withstanding)
Yes, generally with no relevance to images shown on screen.
- to get from pixels or dots to some real physical measure you need
to know the number of pixels or dots per unit of space on the output
device concerned (paper or screen)
- dpi is this factor and the dpi of monitor screens used to be
72dpi I think all though tis should really be pdi for pixels per inch
or gdpi for glowing dots per inch. 96 is another number entirely.
Puts on pedant hat:
DPI only applies to scanners and inkjet printers.
When you talk about the actual image file you are talking about
*pixels* per inch.
PPI is almost a 'made-up' measurement - the file may have a ppi
measurement, but all it does is tell the program doing the printing
'print it at this size'.
The only 'real-world measurement' that is in ppi is the pixel density
of fixed-resolution displays such as LCDs.
72ppi 96ppi - it *used* to be roughly true that Macs used screen
resolutions that generally worked out at 72ppi (e.g. 1024px across
14 width of screen) and Windows machines generally used screen
resolutions that gave 96ppi. But this is long-gone.
What it left behind, though, is that these are normally the figures
used for working out how big text of a certain 'point' size should be
shown on screen. This is the reason why Macs are assumed to be 72ppi,
as they were heavily geared towards desktop publishing, and the most
common font sizes are multiples of 12 - fitting into 72ppi was easy
to do.
Obviously, as you can run the same CRT monitor at 800x600, 2048x1600
or whatever, those assumptions are just assumptions.
If it's being shown on screen the only sensible measurement is the
total pixels and how much of the screen that takes up. If you have to
show things on screen in a specific real-world size then you have to
get in to all the stuff where you get people to measure distances on
the screen with a ruler, as per one of the recent threads. And it
*will* involve measuring both vertical and horizontal distances,
given how many screen resolutions end up having non-square pixels. :-(
Can someone help me here - I think I am in the wrong tree - how do I
know the dpi factor to use in a conversion?
What are you trying to convert? If you have some specific examples of
what you want to do, I can probably say how to do them - I'm a
photographer and designer who also programs, so this is my home
field. ;-)
Hoping I've not gone overboard,
Ian
___
use-revolution mailing list
use-revolution@lists.runrev.com
Please visit this url to subscribe, unsubscribe and manage your subscription
preferences:
http://lists.runrev.com/mailman/listinfo/use-revolution
