[Definition] DPI, SPI, PPI, and LPI

[Ed_L]

Hey Richard! Good to have you back! Thanks very much for the explanation, which will obviously need to be studied a little more by many of us.

Ed

[DJ Dubovsky]

Ditto Ed's sentiments Richard. I too will have to read it several times to get the details worked out.

Also Thanks John for the link. I saved it to my files. I need all the info I can get on this subject as you can tell.
DJ

[Ed_L]

Richard said:

"LPI can be adjusted.

So, say a printer has a resolution of 600DPI. This means it can squeeze 600 dots of information into a linear inch at maximum, or that if you lay the smallest dots the printer can make side-by-side there will be this many in an inch (and none will overlap). At the same time LPI is made up of dot rows (that are different from the DPI dots). If the same 600DPI resolution printer prints with a 60 LPI screen, there will be 60 lines of dots that are 10x10 printer resolution dots in size (for a single color). Each dot in a line can then represent 100 shades of gray. (Um, this gets more complicated when you discuss printer half-steps and additional colors, so I'll skip, OK?)"

Does anyone really understand everything Richard said? I'm trying to get it right in my head, but I want to start out right (before I get too deep into thought) so as not to confuse things. Tell me if my thinking on this part is correct or not: A higher resolution file at a fixed physical size for printing will result in a higher line frequency when the image is printed. Correct? And if that's the case, how is the maximum LPI determined?

Ed

[Doug Nelson]

LPI has nothing to do with inkjet printing. It refers to how fine a halftone screen will be applied before lithographic printing.

[Ed_L]

Thanks Doug. I'll try to get my brain redirected!

Ed

[Jakaleena]

Wow! I just found this thread and it IS a can of worms... Here goes my feeble attempt at explaining DPI and PPI. This is going to be terribly simplistic...

PPI has to do with the ACTUAL resolution of your digital file. DPI is the print resolution. The first thing to remember is that they are not interchangeable - apples and oranges...

PPI

When you were in school in art class, did you ever have to make a pointalist drawing using only dots you made with your pencil? OK, then think of that as PPI. Now imagine you made a drawing like that not on paper, but on a flat sheet of rubber. Now stretch it... No matter how you stretch or contract that rubber, the drawing you made will never have any more or less pencil dots on it. It will only appear bigger or smaller. The more dots you put on it, the bigger you could stretch it before it started looking distorted.

DPI

How many of you have ever done a silk screen print? Ok, good. Same rubber photo above. Same number of pencil dots. But your rubber now has some really neat properties that are completely against the laws of physics. It can liquify on command (think Odo on Star Trek - DS9). You can make a screen print from that drawing by simply laying the rubber on the screen, telling it to liquify, and then squishing it through the screen onto your T-shirt. However, Odo cannot reconnect himself once he has been squished through the screen. However many dots are in that screen are the number of dots he must remain in after being squished. The screen is DPI. Still no change in the original dots on the rubber. But the number of dots the rubber has been squished into on your shirt can be different depending on whether you have squished your Odo-ized print through a screen with holes the size of chain link, or if the holes are more like the ones in your screen door...

Gawd I hope that makes sense...!