Steve758 wrote:
Sorry but I must disagree. Here's a quote from a color and printing web site.
"What is the difference between PPI and DPI? PPI describes the resolution in pixels of a digital image whereas DPI describes the amount of ink dots on a printed image. Though PPI largely refers to screen display, it also affects the print size of your design and thus the quality of the output."
If you have a 20x30 inch image at 300 ppi (pixels per inch) and you print at 720 dpi your printed image will be 20x30. If you print the same image at 1440 dpi your printed image will still be 20x30. However if you reduce the ppi to 150 your image would then be 40x60.
Sorry but I must disagree. Here's a quote from a c... (
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I'll add that:
dpi (yes, all lower case ONLY) is a scanner term as well as a printer term. It may refer to the physical cells a scanner divides each inch into. A 300dpi scan (relatively low resolution for line art, but decent for photo prints to be reproduced at the same size) divides every inch of the scanner bed into 300 dots or samples per inch, both horizontally and vertically. Then the scanner driver software makes a FILE at that resolution. (i.e.; 300 dpi "send" 300 pixels from every linear inch of the copy into the file of pixels). Finally, the scanner resolution is stuffed into the file header as 300 dpi. That tells graphic arts industry software to "flow" the image onto a software layout at 300 pixels (yes, pixels) per inch.
Printers have two resolution factors of concern:
First is the input resolution to the printer driver, as expressed in PPI. How many pixels per inch of output are you going to turn into dots that reproduce the image? You can feed the printer driver what it uses, or let it interpolate. Either way, it will turn pixels into dots when printing. A higher number may or may not make a visible difference, depending on a number of different factors. It will certainly increase the printing time!
Printer dots are unique to the brand and printing process. They could be laser light spots on photo paper. They could be halftone dots on a letterpress. They could be ink spots spattered on paper with "frequency modulation" screening. Given appropriate input pixel density, it is entirely possible for different printers to produce THE SAME APPARENT RESOLUTION with differing numbers of printer dots per inch. Printer dpi always refers to the OUTPUT resolution, which may be completely different from the INPUT resolution.
Dots are physical. They have dimensions. They are like sound going into a microphone or out of a speaker. Just like a microphone and speaker are TRANSDUCERS, a scanner and a printer are transducers. Scanners turn light photons into electron voltage into a number array (pixels). Printers turn number arrays into ink spots on paper, or exposed spots of light on silver halide paper, or halftone dots for a letterpress.
Digital cameras have "dots" called sensor elements, or sensels. Their output voltages are digitized and THEN processed into pixels using some very complicated math. We don't have finished pixels in raw files, just unprocessed data — latent images. It takes an image processing chip in the camera, or a piece of post-processing software, to develop pixels. The matrix math for that is fairly advanced...
Monitors work similarly, but not exactly in the same way. Put a 10X loupe magnifier on your monitor and you will see a pattern of red, green, and blue cells. These merge together at a distance because our eyes do not have sufficient resolution to separate them when we back up. So at 100% magnification, one pixel in a file is represented by three monitor cells. At any other size, there's interpolation or downsampling of the input.
Pixels are just numbers — triplets of numbers representing the intensity of red, green, or blue light. They have no size, but can be represented at any size, via different output media. Exactly like digital sound files, they require transduction to reveal them. In the case of silver halide photo paper, RGB becomes CMY. On a printing press, it's usually CMYK. High end inkjet printers may use several different densities of K (black) and magenta and cyan inks, plus red or orange or green, to produce more life-like color.
I hope that helps. It's fundamental to our understanding of HOW to use this stuff!