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

Post a reply to this message