Coming to terms with terms

Adobe Community Professional ,
Apr 13, 2018

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To maintain constancy and eliminate the current confusion in definitions, I would like of offer a standard regarding terms for use in Forum posts. They refer to dpi, lpi and white balance. If I am lucky, in the future these terms will be as universally treasured as the flag, home and the loyalty of a fine dog. 

PPI: Pixels – picture elements per inch viewed on a monitor. I think we are safe here.

DPI: The number of inkjet printer ink droplets. It is measured in picoliters and may appear among a given machine’s specs. When printing, the number of droplets varies throughout the tonal scale with few ink dots deposited on a surface to create the lighter tones and more of them deposited to create the darker tones. We might refer to this variation in the number of dots as Frequency Modulation (FM) and it is the only time the term DPI should be used. Promise me.

LPI:  Dots deposited on a surface by offset lithography or letterpress printing. When viewed from a distance, the dots seem to fuse and create a smooth gradation of tones. On the printed page smaller dots create the lighter tones and larger ones create darker tones, but the number of dots throughout does not vary. Since only dot size varies, we might refer to this as Amplitude Modulation (AM)

Why LPI?

Some halftone history: Before the introduction of computers in the graphic arts, the device used to create halftones consisted of two sheets of glass, each with fine parallel opaque black lines -- perhaps 150lpi – glued together (one vertical, the other horizontal) to create a grid of small clear windows. When this device, called a glass halftone screen, was positioned in front of high contrast film (not in contact with the film) in a camera and exposed to an image, it broke the light reflected from the subject being photographed into an array of various sized dots -- a halftone negative that was then used in plate-making.

Today, glass screens and their successor -- plastic contact screens of a checkerboard-like design (which are placed in contact with the film) -- are gone but the term lines-per-inch (lpi) -- a vestige of the glass halftone screen era -- remains to designate the number of dots to the linear inch in the image on the printed page. That is why a 150 dots-to-the-linear-inch image on a printed sheet, for example, is referred to as a "150 line halftone." To repeat: a halftone is an example of Amplitude Modulation (AM), different from an inkjet’s FM.

Now, on to White Balance (and don’t even try to talk me out of this) is defined as the neutral quality of only the highlight values in an image. The neutral quality of tone elsewhere in an image -- sidewalk, schnauzer, slate roof -- should rightly referred to as Gray Balance. These are not two arbitrary definitions: they are terms that have been used in the graphic arts for about a century. The camera manuals are wrong. There. I said it and I'm glad.

If you concur, raise your hand.

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Most Valuable Participant ,
Apr 13, 2018

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out of date and pointless but also correct

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Adobe Community Professional ,
Apr 14, 2018

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Ussnorway  wrote

out of date and pointless...

I resemble that

Nancy O'Shea, ACP
Alt-Web Design & Publishing

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Adobe Community Professional ,
Apr 15, 2018

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Yes, I'm all for it - with one reservation:

PPI: Pixels – picture elements per inch viewed on a monitor.

That's obviously pixels per inch on paper, not pixels per inch viewed on a monitor? As we all know, the ppi number means nothing on screen, where the image pixels relate directly to the screen pixels, either 1:1 or some scale ratio.

It also has a derivative meaning, when calculating font sizes (a physical size unit), as well as smart objects. But this is a secondary use.

But yes, I'm getting a bit tired of people talking about dpi when they mean ppi. And it's not just splitting hairs either, it's really an important distinction. The term "pixels per inch" is actually totally self-explaining once you stop and just...look at the words. It's all there. Pixels per inch. What's unclear about that?

Dpi, on the other hand - that's what people say when they mistakenly think it's some kind of abstract integral file property. As if the file was born with it.

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Adobe Community Professional ,
Apr 15, 2018

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Agree! Where do I sign?

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Adobe Community Professional ,
Apr 15, 2018

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Monitor Resolution in PPI does have meaning when you set up View > Print Size

That's horizontal pixels/horizontal physical size of the screen.  1280 px/11.5 in = 113.7,which when rounded out (pixels are whole integers) is 114 ppi in my Macbook.

This value is entered under Preferences > Units and Rulers > Screen Resolution.

That sets up the proper zoom level for View > Print Size if for some reason you wanted to see what the physical size might look like.

Before CS, that value was glued to 72 ppi and couldn't be changed.

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Adobe Community Professional ,
Apr 17, 2018

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So…font-pixels have an actual size? Doesn't that get confusing?

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Most Valuable Participant ,
Apr 18, 2018

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yes it did back in the day but most people don't care any more

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Adobe Community Professional ,
Apr 18, 2018

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Fonts have an actual size. This is from back in the day with lead type and real, living typesetters. One point is 1/72 inch - or 72 points to an inch.

It's not confusing when you know about it. It gets very confusing if you don't

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Adobe Community Professional ,
Apr 18, 2018

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One more heads-up: As you see from the illustration below, a point size can be visually misleading because it includes the entire top surface of the type. In typesetters jargon, a font that is large-on-the-body (relatively short ascenders and descenders) will appear larger than one that is small-on-the-body (with long ascenders and descenders). As a result, two fonts of the same point size will appear to be of a different size. (As a teenager redistributing foundry type back in the proper case after use, it was a lesson I learned the hard way.)

     bi

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Adobe Community Professional ,
Apr 20, 2018

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Font: one typeface, one size, one style

Examples:

Helvetica 12 pt Bold

Helvetica 10 pt Bold

Helvetica 12 pt Italic

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Adobe Community Professional ,
Apr 20, 2018

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I'm not seeing how any of this is relevant to pixels. I know what a point is, I just don't see how a pixel can describe an absolute size, unless you know pixels per unit measure.

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Adobe Community Professional ,
Apr 20, 2018

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tromboniator  wrote

I just don't see how a pixel can describe an absolute size, unless you know pixels per unit measure.

That was exactly the, er...point. By bringing physical size units into a pixel based editor, you suddenly find yourself in need of a ppi figure to connect the two.

In a vector based application, operating by physical dimensions to begin with, this is unproblematic.

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gener7 LATEST
Adobe Community Professional ,
Apr 20, 2018

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Photoshop is famous for importing legacy darkroom, graphics and typography terms to make traditional users feel at home, but it is not an easy fit.

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