Scanning basics

This is from www.scantip.com a great article on the basics and beyond by Wayne Fulton.

The first fundamental concept of digital images

There are two very different ways to use images, printing or video screens.

We scan for the capability of our output device.

We choose the scan resolution based strictly on the needs of the output device that will process that image. At home, that output device is normally a printer or a video monitor.

Video monitors and printers work very differently from each other, and must be discussed one at a time. All of the rules are different for images intended for these two devices. The following material details the significance of these differences. (when I say "video", I mean the video monitor screen).

All of the points below will be covered.
Properties of
Printed Images Properties of
Video Images
Image size is measured on paper in inches or centimeters (paper size is also measured in inches) Image size is measured on the screen in pixels (screen size is also measured in pixels)
Image size does NOT vary with scanned resolution Image size varies with scanned resolution
Image size is modified on paper by scaling Image size is modified on screen by resampling
Image pixels are spaced on paper using specified scaled resolution (dpi) Image pixels are located at each screen pixel location, one for one
Several printer ink dots are used to represent the color of one image pixel One screen pixel location contains one image pixel, and can be of any RGB value

So because of these great and fundamental differences, when this text says "it's this way" or "it's that way", then notice that it also says "for printing" or "for video". Don't get them out of context, because the two modes are very different, with different properties and concerns.

The next page will discuss "What is a pixel?", but before we get started, a note relating to context usage of "dpi":
DPI, PPI, SPI - What's in a name?

Printer ink dots and image pixels are very different concepts, but both use the term dpi in their own way (dots per inch).

Inkjet printer dpi ratings refer to printer ink dots (the four colors of ink), which is NOT AT ALL the same thing as image pixels. These are such different concepts that some people think we should reserve the term dpi for those inkjet ink dots, and reserve use of ppi only for image pixels. Not a bad plan, except that this view fails to recognize real world usage.

We may hear scanning resolution called spi (Samples Per Inch), and that is indeed what it is. We often hear image resolution called ppi (Pixels Per Inch), and that is indeed what it is. The spi and ppi terms are correct. But historical and common usage has always said dpi for image resolution, meaning pixels per inch, and fully interchangeable with ppi. Pixels are conceptually a kind of colored dot too, and resolution has always been called dpi, for years before we had inkjet printers. Dpi is just jargon perhaps, but it is a fact of life. Scanners and scanner ratings say dpi too, meaning pixels per inch (see dialog pictures here, here, here, and here). I habitually always say dpi myself, but I did try to switch to ppi in the book version.

We may use the term of our own preference, but we need to understand it both ways. Some photo editor programs have switched to saying ppi now, which has much to be said for it. But others have not switched, so insisting on conformity for others to only say ppi will necessarily encounter much frustration, because the real world simply isn't that way, and obviously is not ready to switch yet.

My point here is that we must understand it both ways, because we will see it both ways, often, in the real world.

It's easy, not a problem - the idea of printing digital images is always about pixels per inch, so when the context pertains to images instead of printers, all of these terms, spi, ppi, and dpi, are exactly the same equivalent concept - they all mean pixels per inch.

There is no problem understanding any use of dpi if you know the context. It always means the only thing it can possibly mean. If the context pertains to images or printing pixels, dpi means "pixels per inch". If the context pertains to inkjet printer ratings, dpi means "ink dots per inch". There is no other meaning possible. This should be clear and no big deal - the English language is full of multiple context definitions.

So yes, inkjet printer rating dpi is something entirely different, referring to inkjet printer ink dots instead of image pixels. The inkjet printer tries to simulate the color of one 250 dpi pixel by making several ink dots of four CMYK ink colors, which are located perhaps on 1200 or 1440 dpi spacing (See the Printer Basics section). The printer is trying the best it possibly can to reproduce the pixels (pixels is all there is), but inkjets cannot reproduce colored pixels directly. Image pixels and inkjet printer ink dots are NOT the same thing at all.

However, continuous tone printers (dye-subs, and Fuji Frontier types) don't print discrete ink dots of three colors like inkjet printers must - instead they mix the color of the pixel directly. There are no dithered ink dots then. But these printer ratings still refer to the spacing of those image pixels with the term dpi, simply because dpi has always been the name for "pixels per inch". Scanner ratings also always call it dpi, also referring to pixels of course (scanners don't use ink dots). I always say dpi too, simply because that has always been the name for pixel resolution.

Ppi is a relatively recent new term, and we are seeing ppi used more now, and it is a perfectly fine name too. Recent photo editor software often says ppi, while scanner software generally says dpi (but there are exceptions to both, preferences allow this). But either term is correct, and in fact, the long established name for image resolution has always been dpi, for many years before inkjet printers could print photo images. So you will often see either dpi or ppi used, and you must understand both as the same term. It may be a bit confusing at first, but that's simply how things are. Think of this as training to understand what you will see elsewhere.

There is really no problem understanding the two uses of the word dpi if you know the basics, and realize the context. It always means the only thing it can mean in context. This should be no big deal, the English language thrives on multiple context definitions.

If the usage context pertains to images or printing pixels (and it almost always does), then dpi always means "pixels per inch". So does ppi, same thing exactly. It cannot mean anything else, printing is about spacing pixels on paper. The two terms are fully interchangeable, use either according to your whim. If we have a 300 dpi image, both terms mean it will print at 300 pixels per inch (pixel spacing on paper), so 300 pixels will cover one inch.

If the usage context pertains to inkjet printer ink dot ratings, dpi means "ink dots per inch" (but since the ink dots are actually larger than their spacing, the rating is more specifically about carriage and paper motor stepping intervals). If the printer rating is 1440 dpi, it means its motors can space 1440 ink dots per inch while trying to simulate the color of the pixels in that 300 dpi image.

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