DPI vs PPI

[jmigueldepriego]

What's the difference?, if any

[ellis_vener_photography]

PPI, pixels per Inch, applies to an iamge in its pure digital form: what a camera or scanenr

creates, what your digital darkroom program(s) process. I tis a way of referringto

resolution

 

DPI, dots per inch, is a term that has relevance to images that are created with inkjet and

offset printing where the image you are looking at is quite literally made up of very small

dots of color. It also refers to the resolution.

 

There is no direct connection between these two differing measurements of resolution.

[jmigueldepriego]

Thanks a bunch Ellie

 

Regards

 

JL

[._kaa]

<p><i>PPI, pixels per Inch, applies to an iamge in its pure digital form: what a camera or scanenr creates, what your digital darkroom program(s) process. I tis a way of referringto resolution</i>

 

<p>Will all respect to Ellis, I think this answer is very confusing.

 

<p>Images in "pure digital form" have no resolution -- they only have size, measured in pixels.

 

<p>Think of it this way -- there are two separate realms, physical and digital. In each realm there are units for measuring the objects' dimensions. In the physical realm it's inches, millimeters, miles, etc. In the digital realm it's pixels.

 

<p>Just like a piece of a photo paper in the physical realm might have size of 8x10 inches, an image in the digital realm might have size of 2000x3000 pixels. That's it -- just size, no resolution.

 

<p>Things like PPI -- pixels per inch -- appear when you move an image from one realm into another. For example if you scan an image, you're moving it from the physical realm where things are measured in inches into a digital realm where things are measured in pixels. You need to tranlate one unit of measure into another and the scale of that translation is PPI -- pixels per inch. It works the same if you move an image from the digital realm into the physical realm -- usually called printing.

 

<p>For example, if you are printing a digital image that's 1800x1200 pixels in size and got a physical print that's 6 inches by 4 inches, your PPI was 300 pixels per inch. That's what PPI means.

 

<p>But note that PPI has meaning only when you are transfering images from the digital realm into physical or vice versa. As long as you are purely in the digital realm, just use pixels. As long as you are purely in the physical realm, just use inches.

 

<p>As to difference between DPI and PPI -- when people say DPI (dots per inch) they usually mean a characteristic of an ink-jet or laser printer which work by putting discrete dots of ink (or toner) onto paper. Note -- it's important -- that for inkjet printer one dot is not the same as one pixel. Each pixel when printed by an inkjet printer is composed on many dots.

 

<p>When printer manufacturers quote DPI numbers what they usually mean is the precision of the step mechanism on the printer head. Saying that a printer has 4800 dpi means that the printer head can move in steps of 1/4800 of an inch. It does NOT mean that this printer will be able to print 4800 pixels on one inch of paper.

[barefoot]

Will all respect to .Kaa, I think this answer is very confusing <g>

 

DPI begins with a D and ends with PI whereas PPI begins with a P and ends with PI. Everything

else that matters can be worked out with a little experimentation.

[ellis_vener_photography]

talk about obsfucation.

[godfrey]

Kaa is close, but I'll give this another go as I think that explanation is a still a little too

confusing.

 

PPI vs DPI

 

PPI is an abbreviation for "Pixels Per Inch", where Pixel is an abbreviation for "Picture

Element". Digital images are two dimensional arrays of Picture Elements or pixels, each of

which typically contains a mix of red, green, and blue values (chrominance) along with an

intensity (luminance).

 

Given this context, PPI is a density measure for outputting a digital image to paper. If you

have a picture which is 2000x3000 pixels in size and you output it to paper at 300 PPI, the

image will cover a 6.7x10 inches on paper.

 

DPI is an abbreviation for "Dots Per Inch". A "dot" is typically the smallest spot of ink that a

printer can lay onto paper. So when looking at printer specs and settings, you often see

numbers like 720x720 dpi or 1440x1440 dpi ... What this means is that a 1440x1440 dpi

printer can achieve that number of dots per inch, horizontally and vertically, with its head

and paper feed mechanisms,

 

Now, a dot is not the same as a pixel. A dot is simply a micro-droplet of ink of some color

applied to the paper. A pixel itself has no dimension, it's just a set of numbers describing

chrominance and luminance. When a pixel is printed, however, it has to be assigned a size

... that's done with the PPI value you've set your image for output. The printer also has to

be told how finely its mechanisms must step ... in some printers, that's listed as dpi

values, in other it's coarsely listed as 'quality' values ... which determines how many drops

will be integrated in the print to make a pixel.

 

In general, for quality rendering of photographic images, you want your dots per pixel

ratio to be high so that many very small dots of different color add up to an accurate

representation of the pixel's chrominance and luminance. With a 1440x1440 DPI printer,

you normally set it to "Highest" quality (maximum DPI) and set your pixel density to

somewhere above 200 PPI. This means that you are printing 200 pixels per inch, and each

pixel is an aggregate of about 7x7 or 49 dots of ink... that allows for a pretty good

rendering of the pixel values.

 

The problem is that in many spec sheets and manuals, the terms PPI and DPI are used

interchangeably, which leads to confusion. You have to understand that the terms mean

and the context of their use to truly understand what is meant. When looking at printer

spec sheets, dpi means "dots [of ink] per inch laid on paper", where when you look at a

scanner spec sheet, dpi means "pixels per inch of capture resolution".

 

I hope that is clearer and useful.

 

Godfrey

[maiken]

<p>Here's a stab at a very concise answer, at the expense of some nuance:

 

<p>PPI and DPI are the same when a pixel is the same as a dot. On a computer screen, each "dot" that you see is a complete pixel*. In a DSLR, each captured "dot" is a complete pixel**. But when you print, the printer can only use its limited number of inks, and it needs many dots to reproduce a single pixel. To see why this is so, think of what the printer has to do to print a middle-blue. It has only solid blue available, so it has to lay down a little grid of widely-spaced solid-blue dots, that at a distance look like light-blue.

 

<p>So if you want to print your image at 200 <i>pixels</i> per inch (to show reasonable detail), but your printer has to lay down a grid of 4x4 <i>dots</i> to reproduct one <i>pixel</i>, then you need to print at 800<i>dpi</i> to get 200<i>ppi</i>.

 

<p>Hope this helps at least a little.

 

 

<small>

<p>*Yes, I know this ignores the fact that LCDs are composed of side-by-side primary-color bars, and that the individual phosphors are distinguishable on CRTs, too.

<p>**Yes, I know this ignores the Bayer sensors that most DSLRs use.

</small>

[bwcombs]

Whoa!<div></div>

[awindsor]

<i>talk about obsfucation.</i> <P>

I can't. Obfuscation maybe ?<P>

While pixels per inch (PPI) is well defined you will find dots per inch (DPI) used in many places where PPI might be more appropriate (in the decription of the output resolution of continuous tone devices for example). Even many imaging applications use DPI and PPI as synonyms. <P>

PPI is terminology that Adobe has tried to popularize (and which Photoshop uses). A reasonable discussion from a practical point of view is <A HREF="http://www.adobe.com/support/techdocs/331327.html">here</A>. <P>

 

The main thing to know is that you need around 300 PPI for detailed output designed to be viewed from close up but that you would want to output this at 1440 DPI on your inkjet.

[kelly_flanigan1]

"DPI" was used in scanners; fax machines; before Photoshop or Photostyler was born; or many experts in Photoshop were born. "DPI" is used with scanners; in patent literature for scanners; in enngineering info; for many many many decades; going back to the 1970's and before. Scanners use "DPI"; because they have alwys been spec'ed like this. This seems to nug greenhorn Photoshop writers; whos frail egos want to reinvent old terms; developed before most were being potty trained. "DPI" goes back into ancient FAX development; over 1/2 century ago. "DPI" is used in formal bid packages for scanning for the government; for over a decade.

[roger krueger]

I'm with Mark and Kelly

<br><br>

Additionally, Ellis' statement

<br><br>

<i>DPI, dots per inch, is a term that has relevance to images that are created with inkjet

and offset printing where the image you are looking at is quite literally made up of very

small dots of color. It also refers to the resolution.

<br><br></i>

is wrong about dpi in offset printing. Halftone dots have always been spec'd as lines per

inch (lpi). The only offest-specific dpi relevance is imagesetter resolution, where

2400 dpi is enough to get a 16x16 grid--and thus 256 possible tones--from a single

halftone cell at 150 lpi.

[maiken]

I might be misunderstanding what Kelly is saying, but it sounds as though she (he?) is implying that PPI is a bogus term.

 

IMHO, the distinction between PPI and DPI, although maybe difficult to appreciate at first, is important and legitimate. Specifically, it lets you understand why a 300dpi color printer is completely inadequate for reproducing photographs, if you're trying to print them at 300ppi.

 

I would also add that the fact that DPI is a much older term almost certainly has much to do with the fact that most of the technologies it was applied to in the 70s and 80s were monochrome. When a pixel can only be black or white, and you're using black ink on white paper, a pixel and a dot are the same thing.

 

I'll be the first to admit, though, that I don't understand offset printing at all; it sounds like Roger can teach us something there.

[beauh44]

"PPI" = "Perplexing Photonet Information"

"DPI" = "Dat's Pretty Insane!" ;-)

[christian_mozetic]

So what about when printing to photographic paper? Is there any point in distinguishing between PPI and DPI if (as I presume to be the case) one pixel equals one dot on the paper?

[jcuknz]

I don't think so Christian becuase some printers make more than one ink droplet per pixel I understand.

 

Basically dpi is what printeres used before the PC was conceived since photos where printed on paper while ppi is a computer term.

 

To get the required dpi for an inkjet print, commercial magazine print, you often have to interpolate in the editing programme to increase the number you have.

[leonard_evens]

A digital image consists of pixels. In color each pixel is a triple of numbers, 8 bits or 16 bits depending on the color depth. The pixel dimensions of the image are two numbers, such as 2000 x 3000, which give the short and long dimensions in pixels. If you then also specify an image size in inches, which you would do when printing, then you divide each of the pixel dimensions by the dimensions in inches to get pixels per inch or PPI. Almost always these are the same. So if you specified 10 x 15 inches for a 2000 x 3000 pixel digital image, the result would be 200 ppi.

 

Dots per inch or DPI are used to describe the resolution of inkjet printers. Theoretically, the number should give you the number of dots laid out by the printer in each linear inch. But modern printers use a complicated way of laying out dots, so the number is usually just some sort of average. Also, there is no simple relation between DPI in the printer and pixels in the image sources. Each dot will be just one color, and generally it takes many dots to represent the color in a simgle pixel. It is generally recommneded that you send an inkjet printer at least 240 ppi to provide adequate color fidelity and image quality in the print. Most people use 300 ppi or higher and some claim they can see a difference up to 600 ppi.

 

Unfortunately, the term DPI has also been used in discussing scanning resolution in scanners and in some other contexts such as resolution of computer monitors. For scanners, the scanning resolution tells you the number of samples collected per inch by the scanner in each line. It would be better to call this PPI, but we have to live with what is conventional in the industry.

[ellis_vener_photography]

<I>Is there any point in distinguishing between PPI and DPI if (as I presume to be the case)

one pixel equals one dot on the paper?</I><P> At what point in the process?

[ken_burns1]

PPI. That begins with P, and that rhymes with T, and that stands for TROUBLE!

[jason_simpson]

I'm only new at this type of thing, so bare with me. I've been asked to reproduce some of my digital images to 300DPI. Now I've got a 4MP Digital Camera. What size (In PPI) should I resize the images to to make it 300DPI for printing in a colour magazine ???

 

Kind Regards,

 

Jason