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Input/Output/Workflow Scanning, printing, color management, and discussing best practices for control and repeatability

[Definition] DPI, SPI, PPI, and LPI

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Old 02-01-2002, 10:50 PM
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[Definition] DPI, SPI, PPI, and LPI

I'm a little fuzzy on some of the terms. As I understand it, SPI refers to samples per inch when scanning, DPI (dots per inch) refers to printer resolution, PPI (pixels per inch) refers to the resolution of a digital image, and LPI (lines per inch) refers to something to be concerned with for pre-press work (?). I'm *real* fuzzy on the last one. How do these resolutions interact with each other? An image scanned at 300 SPI results in a digital image with a resolution of 300 PPI at 100%(right?). We usually think of 300 PPI as being about right for printing, but how does that interact with the printer, whose setting might be 320 - 2800 DPI?

What a great place to learn!

Ed
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Old 02-01-2002, 11:22 PM
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Can't wait to see someone try to explain this. Boy did you open a can of worms with this one Ed. I have a good scanning book that tries to explain it and after reading it, I'm still confussed. All I do know is they don't work out to be the same. And on that note, I leave it to better qualified individuals to explain it further.
DJ
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Old 02-01-2002, 11:25 PM
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Oh man...this is one of those that is almost easier to explain using visual examples! Let me see if I can find my notes...
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Old 02-02-2002, 08:06 AM
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PPI is pixels per inch, which is pretty self-evident. The more pixels you have per inch, the higher the resolution of your file (although not necessarily of your image. Due to resampling, extrapolation, etc., you can bloat a 300ppi image to fill a 2000ppi file, but you'll have no more actual resolution than you started with)

DPI is ink dots per inch, an inkjet term frequently misused. An inkjet printer can lay down many ink dots per pixel, or even represent several pixels with one ink dot, so this is not useful as a determinant of image resolution.

I'll leave LPI to a pre-press person. And I've never even heard of SPI.
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Old 02-02-2002, 08:47 AM
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Would it be fair to say that ___ number of pixels (with true information) per inch is necessary to produce an image with ____ many actual tonal (or color) differences? And would it also be fair to say that ____ dots per inch are necessary to produce the same number of tonal (or color) differences? In other words, if the two resolutions would not support the same number of tonal differences, one of the resolutions should have been higher or lower in order to properly relate with the other without having overkill? Or am I completely misunderstanding the concept? This is an area I need help in understanding.

Ed
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Old 02-02-2002, 09:24 AM
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I'm not positive I understand your question, but here goes, anyway:

Resolution has nothing to do (directly) with tonal differences. Any one pixel is capable of being set to any value within that color mode's spectrum. And the pixel right next to it is capable of being the exact opposite, or anything in-between.

However, if there is a gentle tonal gradation, with more tonal variations than there are pixels within a given distance, some of those variations will necessarily be dropped, and you'll never get them back.
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Old 02-02-2002, 09:59 AM
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Here's what the definitions of each are according to a book I have.

SPI = Samples Per Inch (Input Frequency of Scanned Image)
DPI = Dots Per Inch (Resolution of Imagesetter (Output))
LPI = Lines Per Inch (Halftone Frequency for Printer (Output))
PPI = Pixels Per Inch (Resolution of Computer Screen (Output))

My trouble is why don't they all represent the same thing? They are all basically one little portion of a whole image whether that be on a printer, monitor or printing press or even a sampled image of a scan.

You really did it Ed.
DJ
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Old 02-02-2002, 02:52 PM
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SPI seems to be an outdated term and most people use PPI to describe the same thing. Part of the confusion is that PPI is also used to describe monitor resolution. (eg. mac monitors display images at 72ppi). It gets even more confusing when people begin to interchange the term DPI with PPI!

LPI, as far as I can remember, refers to the number of halftone dots per inch. I think the reason why there is not a 1:1 ratio of PPI to LPI is because of the angle of each halftone screen. I have a good book on this (maybe the same one as DJ!), but I seem to have misplaced it. I do know the correct ratio is about 1.5 ppi : 1 lpi.

(anyone in prepress- feel free to correct any errors!)
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Old 02-02-2002, 04:28 PM
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I guess I'm having a hard time finding the right wording. I have an older book "Real World Scanning" that I haven't read in some time. I'll dig it out, and see if I can make any sense of it. I remember it as being over my head before, but maybe I've learned enough to make some sense of it.

Yeah Debbie, I guess I really did it this time. But maybe after all is said and done, we'll all understand it a little better (hopefully).

Ed
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Old 02-03-2002, 10:43 AM
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I look at it this way to avoid alot of grief. DPI and PPI in my mind are interchangeable and LPI and SPI I can do without.

I know what they all stand for but until someone can explain just how the numbers in all 4 change in comparison to each other, I give up and only deal with the 2 important ones and they're basically the same to me.
I may be adding to your can of worms after saying that Ed.
DJ
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