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dpi vs ppi???
Really stupid question...what is differnce between dpi and ppi? Is ppi the pixel/inch used for resolution? DPI for printing?Thanks...YP..

Comments (10)

That's pretty much it. They are considered different because a printer usually requires a matrix of dots (at least 3x3) to recreate a single image pixel...

Comment #1

When I first became involved in the design/print business, over 15 years ago and therefore pre-digital cameras, the only term I ever heard in the trade was dpi. It was used for scanning resolution - and scanned images are equivalent to digital camera images of course. The same term was used for laser printer resolution, for dye sublimation output (which is continuous tone), for slide recorder output (also contone), for colour inkjets, and for imagesetter (litho film) output..

'Pixels per inch' seems to be a more recent and arguably unnecessary/pedantic alternative to distinguish between RGB pixels and single colour dots of ink/toner etc..

So while 'dots per inch' is always ok - the context will make it clear what you mean - 'pixels per inch' is only correct for, well, pixels...

Comment #2

The term pixel is interesting...its a Rosetta Stone of sorts. Physically theres no such thing as a pixel. A pixel from a camera is a combination of the values from four photosites on the sensor. A pixel on the screen is a combination of light from three electron guns (for CRTs.) A pixel in an image file is an X,Y position and three or four numbers that represent a color. A photograph pixel on a printer, even a laser, is a matrix of dots..

The difficulties of working in these different physical representations are removed by simply working in pixels..

I think a dye-sub printer is the only case where a pixel is actually physically manifested...you get one dot and one color. Thats a pixel...

Comment #3

Graystar wrote:.

The term pixel is interesting...its a Rosetta Stone of sorts.Physically theres no such thing as a pixel..

I don't agree that there's no such thing. Not all pixels are created equal, that's for sure, but I have no problem with calling three phosphor dots grouped together "a pixel". Nor with any of the other ways of creating a coloured spot, or the illusion of one..

A pixel from a camerais a combination of the values from four photosites on the sensor..

That is a misunderstanding of how a Bayer mosaic works. Each pixel corresponds to one photosite, with it's value adjusted by interpolation of data from surrounding pixels. There's loads of info on the web - just Google "Bayer interpolation" or "Bayer demosaicing"...

Comment #4

Ed Grenzig wrote:.

See this link on ppi versus dpihttp://www.photoresampling.com/ppi_eng.php.

That's a good explanation of the concepts for people who are unfamiliar with them..

However I still say it's perfectly ok to talk of printing a 300 dpi image on a 1440 dpi printer...

Comment #5

Digital cameras sensors have photosites. Your computer monitor has pixels. Prints have dots of ink...

Comment #6

Re>Really stupid question<.

Nope..

Lots of people are confused, and as you can tell by the variety of answers, even those who understgand have different opinions, at least to some extent, from each other..

Dots is an old printing term, and if you take a magnifying glass and a black and white newspaper photograph, you can see the dots of ink that make up the image..

Pixels are inside the camera or inside the monitor, and are the clusters of colors that make up the image that gets collected on the memory card, or viewed on the monitor..

Fuji used to have two measurements of pixels, one of which pretty much matched Canon and Nikon and Pentax,and one of which was double that number. So a Fuji camera could be 7 MP and a comparable Nikon only 3.5 MP..

Dots as used in printing presses for magazines and newspapers are very different from dots as used on ink jet printers. And with printing presses, those dots may be referred to as lines, as in 80 lines per inch is the measurement used for a mid-quality newspaper, and 150 lines per inch is a very good magazine page..

But there there's 1440 dots per inch on an ink jet printer a different measuring system with a similar name..

That's part of the confusion..

Mostly, PPI and DPI and lpi does not meanmuch to most photographers, because software does all kinds of adjusting and translating for you..

One time it matters is if you are combining images, such as putting two cropped pictures and some words on a 4x6 print, to have printed at a lab. To do this, you need to make sure the pixels per inch of each photo match, so that when you put them onto the new background they'll take up the proper amount of space, each..

REMEMEWBER, SIFTWARE C"HANGES" T"HINGS" automatically, so that on your screen an image may fit into a box you draw, regardless of the file resolution, because the software adjusts the resolution automatically. You se this in Photoshop and Photoshop Elelemts, where it may say 12.5 percent or 33 percent above an image that's been reduced to fit into a certain area..

Bottom line:.

Pixels inside a camera and on a memory card; pixels as in Megapixels for an image size recored into the camera. Pixels per inch on an image file when it is still electronic, even when viewed on a monitor..

Dots, and dots per inch, when printed on a printing press, (but not a photo lab printing machine like Costco or Wal-Mart, and maybe, depending on the terminology adopted by the manufacturer, on as ink jet printer).

BAK..

Comment #7

BAK wrote:.

Dots as used in printing presses for magazines and newspapers arevery different from dots as used on ink jet printers. And withprinting presses, those dots may be referred to as lines, as in 80lines per inch is the measurement used for a mid-quality newspaper,and 150 lines per inch is a very good magazine page..

Lines per inch is correct, but you will notice that I didn't mention this in my earlier post and that is because the resolution is *never* referred to as 'dots per inch' - always 'lines per inch'..

However, the individual dots *are* referred to quite correctly as 'dots'!!! Unlike any other printing process the tone is determined not by the number of dots but by their size. The size can be varied because these dots are in fact themselves made up of smaller dots of a fixed size/frequency which together make up the one larger dot. These small dots, produced by firing a laser at photosensitive litho film, are analogous to those in a (for example) 600 dpi laser printer - and yes, they too are called dots..

Oh, and another thing worth mentioning is that the 80 lines per inch and 150 lpi that you mention are examples of 'screen resolution' - but this has nothing to do with what most forum readers would think of as screen resolution. Nope, here the word 'screen' is screen as in a mesh of a certain size, and is similar in origin to the screen in 'screen printing'..

No wonder people get confused .

But there there's 1440 dots per inch on an ink jet printer adifferent measuring system with a similar name..

Not really, because as I said above the dot screen resolution is measured in lines per inch...

Comment #8

Steve Balcombe wrote:.

Graystar wrote:.

A pixel from a camerais a combination of the values from four photosites on the sensor..

That is a misunderstanding of how a Bayer mosaic works. Each pixelcorresponds to one photosite, with it's value adjusted byinterpolation of data from surrounding pixels. There's loads of infoon the web - just Google "Bayer interpolation" or "Bayer demosaicing"..

If each pixel corresponded to one photosite then why do sensors have more photosites than pixels in the images they produce?.

I did the search and here is one of the results. This page graphically demonstrates how the 2x2 matix of photosites is used to create a single pixel.http://www.cambridgeincolour.com/tutorials/camera-sensors.htm..

Comment #9

Graystar wrote:.

Steve Balcombe wrote:.

Graystar wrote:.

A pixel from a camerais a combination of the values from four photosites on the sensor..

That is a misunderstanding of how a Bayer mosaic works. Each pixelcorresponds to one photosite, with it's value adjusted byinterpolation of data from surrounding pixels. There's loads of infoon the web - just Google "Bayer interpolation" or "Bayer demosaicing"..

If each pixel corresponded to one photosite then why do sensors havemore photosites than pixels in the images they produce?.

They don't, I'm not sure where you have picked up that idea..

(There is a small unused border but that has nothing to do with this issue.).

I did the search and here is one of the results. This pagegraphically demonstrates how the 2x2 matix of photosites is used tocreate a single pixel.http://www.cambridgeincolour.com/tutorials/camera-sensors.htm.

Yep, I've read that. The method which Sean McHugh describes does position the output pixels in the gap between four photosites, but can you see that each photosite is used four times, so you still have a net 1:1 relationship between photosites and pixels? Other algorithms place the output pixels at the same locations as the photosites, making the 1:1 relationship more obvious...

Comment #10

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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