Resolution - DPI, PPI & LPI (was: Wide PNG's displaying only coloured stripes?)

[Ian Wood]

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