[Definition] DPI, SPI, PPI, and LPI

[Ed_L]

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

[DJ Dubovsky]

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

[G. Couch]

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...

[Doug Nelson]

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.

[Ed_L]

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

[Doug Nelson]

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.

[DJ Dubovsky]

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

[G. Couch]

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!)

[Ed_L]

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

[DJ Dubovsky]

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

[john_opitz]

Here's an article from Dan Margulis' book "Professional Photoshop 6", at his web site.

Resolution

The article is helpful for desktop printers as well.

[Sam]

That is such a useful and informative article I've printed it out, so thanks for posting that link. One big mistake I've made often in the past is to think that the higher the resolution, the better - Not any more!!

One more painful crash from falling off a vertical learning curve has been avoided.

Long Live RetouchPRO!!!

[Richard_Lynch]

I'll stick out my neck a little here.

I think the consensus is pretty much correct:

LPI - Lines per inch (printer resolution, line frequency of dot rows)
PPI - Pixels per inch (display size)
SPI - Samples per inch (scanner resolution, samples -- not pixels)
DPI - Dots per inch (also a printer resolution, dots of ink, fixed size printer dots that make up screening dots)

I think you might want to add Total Pixels in there too.

While DPI is technically a printer term, it has varied meaning and is often used as a more-or-less universal term for SPI, PPI and DPI. I would probably consider this a casual use, and not get too uppity about it. Some people get downright upset. While it can be confusing if you just throw it out at random when you really mean something specific, it is also probably just as confusing to try and be specific consistently. I must admit, I use DPI incorrectly with great frequency as a generalization for resolution. In reality it is the wrong term, but it is one that people understand. However, I'll try and use them all correctly in the discussion that follows.

LPI and DPI are not interchangeable at all, and I don't believe they are ever interchanged – which is pretty curious as they are probably the most closely related. SPI and DPI are used interchangeably when referring to scans, but probably should not be, as your SPI can be different than the number of pixels produced by the sampling. PPI and DPI are used interchangeably for screen resolution, and probably should not be as DPI technically refers to printer dots. While there is a relation between the PPI and DPI in their effect on one another, the two terms are decidedly different because of what they inherently represent. PPI is essentially filtered into DPI via LPI.

Perhaps this is best looked at in context of a workflow.

An image is scanned with a scanner that has the ability to read at a specific number of samples per inch, or SPI. This is the number of details looked at in the object being scanned. The image resulting from the scan can be any PPI, really, as the samples can be used to create an interpolated (larger than the sample size), equivalent, or downsized result. In other words, the samples may be used differently to determine the pixel result. A similar thing happens in a camera. Once the sample data is turned to a digital file as pixels, the reference should probably technically be PPI, but really this is not quite right. The PPI actually matters less than the total pixels, as the number of pixels can be rearranged to cover different areas without actually changing image information. In other words, depending on how the scanner or camera handles sampling and turning the information into a digital file, the sample the file is not really SPI or PPI, but total pixels (that is stored information which does not necessarily have specific dimension). PPI is not really relevant to an image until there are display parameters in the file which give it dimension. When an image is displayed using the same information the pixel information is confined to dimension via PPI parameters. For example, on a monitor using 72PPI or 288PPI, the image will appear as 25% smaller at the higher PPI. This effects the presentation on screen and the eventually result in print due to the way it defines the image dimension -- not because PPI is equivalent to DPI.

Dots are pretty confusing as there are actually 2 types. There is an absolute dot, which is the smallest unit of ink the printer can print (the 'dot' in the DPI), and the screening dot (the size of the dot or 'line' in the LPI). The LPI dot may vary in shape and size, and is made up of a relative number of render dots which make up half-tone screening. LPI will define the number of rows of rendering dots (screens) used for creating the final image in print. DPI is the maximum resolution of the printer, that being the number of fixed-size printer dots it can make in a one-inch increment. These DPI dots are a fixed size for the printer (expressed as the resolution for the printer, in DPI). The maximum actual resolution is the LOWER of any 2 numbers reported by the manufacturer. 1200x600 DPI is really 600 DPI with a half-step sizing for the rows (which allows them to overprint). The number of DPI dots is really always the same, but the number of rows that can represent the dots and the dot shape is controlled by the number of dot lines (screen frequency, or LPI). 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?)

If you followed and are clear to here, this may clarify why you get noticeably better results printing to an image setter (with 2540 DPI+). If you want to get 256 levels of gray in a print, you have to have dots that are 16x16 (printer dots): 16x16 = 256 (possible gray levels for EACH dot). With 2540 printer dots at your disposal, you can run up to 150 linescreen (158, really) and still get 256 levels of gray. To get 256 levels of gray on a 600 dpi printer, you have to run at about 38 LPI...which is a pretty big screening dot (most laser printers will default a bit higher than this, which limits the gray levels in output). You see there is a trade-off between getting more levels of gray and having a tighter screening (smaller screen dots); the only way to get the full gray-level depiction and shrink the screening dots is to have higher resolution in the printer (higher DPI).

To summarize: The pixels in the image are derived from samples (SPI) provide information to the image setter based on dimension (PPI) which is then essentially fitted to the screening frequency (LPI) and recorded as printer dots (DPI) for output as the result.

I hope that didn't prove more confusing than clarifying. If I muddied that up, please give me the opportunity to clarify.

RL

[G. Couch]

WOW!

I am going to have to read over this a couple of times...still a bit confused.

[john_opitz]

Hello Mr. Lynch.

If anyone who is really into P.S., knows Mr. Lynch(the author) for "Special Edition using Adobe Photoshop 6".
It is nice to have the authors of the P.S. books to get involved more into the forums.
His web site here is for all P.S. 'ers, as well for the people who buy his book. But too dowmload the images for his book. Your going to need the password from the book.