Oh come on! It is just a hang over from the early days
of off-set printing. I remember getting black and white
photos printed through screens to get the resolution (in
dpi) that the off set printer could handle without smudging.
In that context it meant a lot. Today it's just dumb.

It should be irrelevant today. You can digitally resize
images to whatever dpi limits your printer has. As you say
your OUTPUT hardware has limitations defined in dpi, just
as it did 30 years ago. So yes, printers have to deal with
dpi, but there's no reason to ask for digital images defined by
both x-y dimensions and "ppi". To do so is either irrelevant
or redundant. The artist doesn't need to know how many square
inches you plan to cover with the pixels, only the number of
pixels.

If I have a 3000x2000 image it means nothing to give
it an internal tag of 72 dpi. If you resize it that internal
tag still means nothing.

The whole thing is so irritating because you need an
additional, and confusing piece of information. What
really upsets digital artists is that print oriented people
will ask for something by only specifying dpi. Completely
meaningless without the number of x and y inches. Then
you say, "yeah, but how large" and repeat the dpi! Augh!

> it makes it easier to deal with the image once it comes into our page 
> layout application

Only because the software was designed for old school
printers who still think in terms of "dpi". Layout workers and
the software could completely disregard "dpi" until the finished
product is sent to the printer. In the old days it was difficult
or sometimes impossible to change an image to a different
resolution. In the analog days if you took a photo screened to
300 dpi and tried to rescreen it to a different odd dpi you would


I say it is an just an old school tradition that print media people
can't get over. It gives them a little convenience (even that
could be handled automatically by software anyway)
at the cost of confusing everyone else with an irrelevant
specification.

But I could be completely wrong. :)
I don't do much print layout :)

"Rick Measham" <rickm*at%isite.net.au> wrote in message 
news:432cdb5e$1@news.povray.org...
> Paul Bourke wrote:
>> The problem is that the printing industry is so steeped in the use of DPI
>> that they often don't understand what's really going on. So when they 
>> give
>> you a DPI value and an image size you simply multiply them to give the
>> right number of pixels. So for your example 300DPI at 213mm is
>>    300 * 213 / 25.4 pixels
>> or about 2500 pixels.
>
> As a member of the pre-press industry, I'm going to take extreme umbridge 
> at that! We do understand exactly what's going on. After all, it's our 
> livelyhood.
>
> The problem comes from non-printing people not understanding what we mean 
> by DPI (and therefore by default, it is our fault for not explaining 
> ourselves fully).
>
> These days the correct term is PPI (pixels per inch). It defines the 
> number of pixels of data we get in every inch in some output format.
>
> We ask for an image to be at 'at least 300ppi' meaning that we need the 
> image to be at 300 *effective* pixels per inch (eppi). That means that the 
> internal file tag may say that the resolution is 300ppi but the data is 
> all still pixels.
>
> We LIKE the image to have this internal tag as it makes it easier to deal 
> with the image once it comes into our page layout application. If you 
> provide an image tagged at 72ppi (or untagged), and we need to lay it into 
> the page at an eppi of 300dpi, we need pull out our calculators and work 
> out that 72/300 ... = 0.24 .. so we can't place the picture any larger 
> than 24%.
>
> Later we come back to the layout to make the picture larger (for some 
> design reason) and notice that it's at 24% .. so, forgetting that it's 
> untagged, we figure we can make it 4 times as large without any problems. 
> But of course, we can't. That's why we want the image at the correct ppi 
> from the start.
>
> Cheers!
> Rick Measham
>

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