ppi and dpi

"alex" wrote in news:w_WdnVdMEdy8DbiiXTWc-
:

are the terms pixels per inch and dots per inch interchangeable?

While they are often used interchangeably, they mean different things.

PPI is only a *sampling* rate for digital images, back when all digital images were created by scanning either a print or a bit of film. Once the file is generated, the only meaningful measurement is strictly pixel dimensions, as in 800×600. Inches have no meaning in digital files, and even when you specify a PPI value in Photoshop, it is simply an index line. You can change it to whatever you want, it will have no effect on the image if the pixel dimensions are unchanged.

DPI, as others have said, refers to print resolution. It does not have to correspond to PPI in any way, either from the original scan source or from what value someone assigned in Photoshop.

Now comes the confusing part. PPI and DPI are often used interchangeably for two reasons. The first is, people don’t know any better. The second is, having a particular aim for detail in the final printout.

If you scan an 8×12 at 300 PPI, for instance, then print it out as an 8×12 at 300 DPI (or higher), you should get something that looks much like the original. 300 DPI output is considered a good guideline for ‘photo quality’, though I’ve seen a lot of differing opinions on this.

You can print out at a much higher DPI than the pixels account for, and the printer or image software will usually ‘extrapolate’ or ‘upsample’ dots to fill in the gaps not defined specifically by pixels from the digital file. This does not add any useful detail, however. Also, printers that have a high DPI capability (like 1440 and up) may be layering in different color dots slightly overlapped to give much better color control, and it does not mean you should have pixels that match the DPI capabilities of the printer. A printer has three to seven different color inks, while a 24-bit image file has millions of colors, so this has to be faked somehow…

Also, it’s usually bad to print at a lower level that the pixel count, since this dumps detail from the original.

In the end, it’s better to try and have pixel dimensions to match your final output. Want an 8×12 image at 300 DPI for ‘photo qulaity’? Then you make sure your digital image measures at least 2400×3600 pixels. This can mean scanning an 8×12 print at 300 PPI, or a 35mm film frame (about 1×1.5 inches) at 2400 PPI.

– Al.

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To reply, insert dash in address to separate G and I in the domain

In article ,
says…

are the terms pixels per inch and dots per inch interchangeable?

Yes, certainly so when you are speaking of image resolution. Both terms dpi and ppi mean "pixels per inch". Exactly same meaning. No difference, use whichever sounds best to you.

Lots of words in English have multiple meanings, used in context.

The term dpi also has another different meaning, used by inkjet printer ratings to refer to paper and carriage motor stepping that position their ink dots. Laser printer ratings use dpi that way too.

Frankly, novices that have recently discovered there are two meanings sometimes become very intolerant with their new-found knowledge. They simply dont know that the printing industry has called image resolution (pixels per inch) by the name dpi for years before we had any inkjet printers, and still do. Pixels are indeed colored dots too (it is simply that the inkjet cannot reproduce them, but sure would if it could, and tries in its way).

Perhaps it is jargon, but DPI is simply the name of image resolution (meaning pixels per inch), and always has been. PPI is also a perfectly fine term for pixels per inch too, that is indeed exactly what it is. Use either term according to your whim. Everyone will understand either way, if the context is about image resolution. We must understand either way, because we will see both terms used everywhere. If you limit your knowledge to thinking dpi can only mean printer ink dots, you will get mighty confused in the real world, because the real world has other notions.

Just to make the very obvious point:

Dye-sub and optical photo paper printers (Frontier/Noritsu/Lambda type), what about them? They are printers too, but they dont print dithered ink dots at all. Instead they print pixels – continuous tones which are dots that can make image pixel colors directly, and their ratings dont say ppi at all, they say dpi, meaning pixels per inch.

Scanners all use dpi for their ratings too, and scanners scan pixels, not ink dots. Scanners think dpi means pixels per inch. For proof, look at any scanner rating, and see the pictures of the scanner software on these pages, which says DPI:
http://www.scantips.com/umaxvs.html
http://www.scantips.com/umaxms.html
http://www.scantips.com/polaroid.html
http://www.scantips.com/acer.html

So who’s wrong? I suspect the manufacturers know a thing or two about it. DPI is simply the name for image resolution. PPI is also very fine as a preference, it has much to be said for it, say if if you choose, but it is merely a preference. It sure aint law or practice <g>

—
Wayne
http://www.scantips.com "A few scanning tips"