19“pixel” is actually often used as, and interchangeable with, other units of measurement. The most prominent example being in CSS. Dimensional analysis plays no role in these applications but if it did, we’d probably indeed (have to) use pixel². – Konrad Rudolph – 2018-02-13T18:38:52.610

15pixel is already an measure of an area (like acre or football field). It is defined by the smallest controllable area of a display. This area is small on a monitor and can be large on a billboard. Assume a pixel is 2mm wide and 1mm high -> pixel=2mm^2 A screen 800x600 is 8002mm600*1mm 960000mm^2 but always 460000 pixel – Joachim Weiß – 2018-02-14T07:49:14.483

8I'd say it's not a one-dimensional unit of measurement. Every object can be used as unit of measurement for a quantity in its own dimension, or lower. Pixels are two-dimensional, so we can use it to count out areas (but also lengths, by abbreviating "as long as the sides of 1080 pixels"). – Raphael – 2018-02-14T10:13:48.273

@JoachimWeiß I tend to agree that it should be so, however the widespread CSS use of the unit px is actually a measure of length, not area. – leftaroundabout – 2018-02-14T11:18:47.293

@leftaroundabout not quite... an area of 10 px height and 10 px length must not be rendered as a square. Its more like Lego: a px is a Lego tile in the best case a 2x2 but it can be a 2x4 brick. If you define 10 height it is 20 "spots" height but it has always a width. – Joachim Weiß – 2018-02-14T12:13:21.440

+1 for the brick analogy and putting that picture in my head! Made me laugh :D – CodingInCircles – 2018-02-14T21:23:19.587

20The London Bus is also a popular unit of width, height, length, and volume, but no-one would ever speak of a cubic Bus. – Michael Kay – 2018-02-14T22:03:56.853

4@MichaelKay Weight, rather than volume. Volume is measured in Olympic swimming pools. :-P – David Richerby – 2018-02-14T22:10:58.907

3

@DavidRicherby: obligatory XKCD

2Right, a pixel is a sample or a physical light emitting unit, depending on if your looking at abstraction or hardware. So an object, not a unit. And this causes a lot of problems since people also think its a good candidate for a unit like in CSS, which its not. – joojaa – 2018-02-16T06:50:45.333

1Short and to the point with one exception: Bricks would actually be voxels, not pixels. – Trilarion – 2018-02-16T14:40:49.840

2IMHO, this explanation is the simplest, and therefore the best.

@Trilarion - If you remove the z axis when you look at a brick wall, you wind up with pixels. Consider that the brick wall is 1 brick wide, then you can for the purposes of this example, look at them like pixels. – Scuba Steve – 2018-02-16T21:01:14.013

Why is it not appropriate to view Pixels or bricks from the unit perspective? Why do they have to contradict? – JFFIGK – 2018-02-19T16:49:40.103

@JFFIGK "brick-squared" has no physical meaning. You're not measuring something in bricks: you're counting a number of bricks. In contrast, you don't count the number of meters; you measure the wall in meters. – David Richerby – 2018-02-19T16:59:49.923

@DavidRicherby I disagree: For instance, I can measure the volume of the wall in bricks. Indeed, Bricks^2 makes no sense for me neither at this time. But the idea of measuring the length of a wall in brick widths makes sense again. – JFFIGK – 2018-02-19T18:28:52.843

@JFFIGK You can measure the volume of the wall and express that as a multiple of the volume of a brick. Or you can count the bricks in the wall. But brick-volumes and brick-widths aren't bricks. – David Richerby – 2018-02-19T18:48:21.553

If I express the volume of sth with bricks, would that not induce that I measure something with bricks? – JFFIGK – 2018-02-19T19:08:52.050

1@leftaroundabout The CSS length unit px is not (or no longer) related to the pixel = picture element. Meaning: For some devices, a screen area that is 10px wide and 10px high may take more than 100 RGB data points. – Hagen von Eitzen – 2018-02-21T11:43:05.040

4

I think you’re historically correct: see for instance the usage of twips in VB6, which was a device independent unit that converts to device dependent X- and Y-pixels (and the twips/X-pixel and twips/Y-pixel conversion factors depended on the canvas scale and were generally not identical).

17In fact the unit of width and height is *√pixel*, multiplying together gives you *pixels* :o) – Will Crawford – 2018-02-13T19:47:39.383

27@WillCrawford You have fallen into the assumption that pixels are square! Or else you don't want the square root. =) – Daniel Wagner – 2018-02-13T20:46:46.723

I love the idea of Polar Pixels/Voxels for when you want to map onto/into a sphere, as opposed to Cartesian Pixels/Voxels. In fact, checking, I see that Polar Voxels can be useful for tomographic image reconstriction.

18Pixels aren't a distance or an area per se. They're discrete objects. – Shufflepants – 2018-02-14T15:24:49.453

5Too bad this is so heavily upvoted when it's simply wrong. A pixel is a single, unresolvable, element of a sampled image. No more, no less. The fact that pixels when realized physically, such as LCD elements or CRT colorbars, have an area, or an equivalent image solid angle, is secondary. When a display is K*J pixels, the entire display has that many pixels, but you do not (yet) know anything about its physical dimensions. (yeah, yeah, images are 2-dimensional, so pixels must have two dimensions unless you're into Peano Curves) – Carl Witthoft – 2018-02-14T18:49:00.230

6@CarlWitthoft I'm not sure I understand which part of my proposed answer you're disagreeing with, exactly. It is common for there to be varying conversion factors between units; e.g. if I have $100, how many euros do I have? Dunno until I go to exchange them -- could be different today and tomorrow. Same with pixel dimensions and physical dimensions: it's true that I don't know how many square inches a pixel is, but it is still sensible to talk about how many pixels I have. And anyway no part of this answer discusses physical dimensions, so what conflict has been set up by your statements? – Daniel Wagner – 2018-02-14T18:54:29.567

For one thing, it's acceptable to describe the minimum resolvable region (square, rectangular, or whatever) of a continuous image as a pixel. For another, a physical implementation of a pixel has identified length and width, but its basic definition is as a portion of an image of undefined size. – Carl Witthoft – 2018-02-14T20:10:45.613

3@CarlWitthoft I still don't see where are you are disagreeing with this answer. – mbrig – 2018-02-14T20:27:08.180

@DanielWagner if you have $100 you don't have any euros. What happens at a Bureau de Change is a transaction, not a simple change of unit of measurement. The exchange rate has units of $/€, which is not dimensionless. – bdsl – 2018-02-14T23:12:29.473

3@bdsl Right. And the exchange rate of pixels/square inch is also not dimensionless. – Daniel Wagner – 2018-02-15T00:41:20.867

I'm still a little confused about this answer... Is it the same for other currencies, like Yen or Rupee? I'm traveling to Japan in May and I need to bring my laptop along. – Brad Werth – 2018-02-15T22:02:18.820

This answer is just wrong on so many levels and the logic behind your comments is so broken.. units for areas or dimensions need to be absolute, consult physics or math. Money exchange is not conversion of units. Pixel is short for Picture-element which is literally an element of 2D raster - it has nothing to do with areas or units. Do not bring your philosophical "natural-language" nonsense into exact sciences. – Jerryno – 2018-02-19T13:12:24.943

E.g. when the PAL analog TV signal is digitized, the resulting pixels are not square (per standard). – Pablo H – 2018-02-19T18:57:34.923

@Jerryno I think you are overstating it. Cf. the mole for a somewhat analogous situation in the "exact sciences".

@duplode mole is atomic weight divided by a constant - so an absolute unit. Pixel is not an unit, nor is pixel-width or pixel-height. It has nothing to do with dimensions. – Jerryno – 2018-02-21T15:10:49.580

@Jerryno A mole is an amount of substance that contains a certain number of particles. There is a conversion factor to kilograms (the molar mass), but it is substance-dependent. In fact, there are plans to remove the link to the kilogram from the definition of mole. Also (and going back to pixels, lengths and areas) cf. the second part of Nat's answer.

@duplode If that happens to mole, it will no longer be valid unit, but a substitute for count. Nat's answer is wrong, don't link that, units must be absolute in the coordinate system we measure in. When people realized every feet is different it was no longer units. That's why pixel is not an unit (which Nat writes), but dip is. The dev deals with dips and the underlying OS uses displays ppi to display correct size. A Dip has ppi assigned to it, and the OS makes corrections based on it's device ppi. How much IQ does it take to understand this easy matter? Probably more than I first thought.. – Jerryno – 2018-02-22T08:54:02.227

Where can I find the criteria for a valid unit? – JFFIGK – 2018-02-22T10:18:29.907

@Jerryno [1/2] (1) No, the mole will keep being a valid unit for amount of substance, which will continue be a meaningful quantity. No chemist is going to lose sleep over that. (2) There is nothing relative in saying "this PNG image is 168 pixels wide", as we are dealing with an image size (a valid quantity on its own), and not a physical size (a different quantity, which only comes into play when e.g. displaying the image in a screen). (3) People always knew that every feet is different -- it's just that, in older times, the need for an universal standard was not as pressing. – duplode – 2018-02-22T13:19:17.943

[2/2] (4) "Where can I find the criteria for a valid unit?" -- @JFFIGK. I think we could do worse than quoting International Bureau of Weights and Measures documents. Cf. the brochure The International System of Units (SI) (p. 103: "The unit is simply a particular example of the quantity concerned which is used as a reference"; cf. Nat's "anything we use to express measurements"), and the definition in the International vocabulary of metrology (p. 6).

@duplode Quantity has nothing to do with units. I give up, there is no point in this, keep living your life good sir. Similarly like a flat-earther would. – Jerryno – 2018-02-22T15:01:00.103

@Jerryno it is not helping at all if you strictly express your disagreement without a logical driven explanation based on generally accepted facts. – JFFIGK – 2018-02-22T16:21:13.347

@Jerryno By quantity, I mean "property of a phenomenon, body, or substance, where the property has a magnitude that can be expressed as a number and a reference" (as in p.2 of the second IBWM document I linked to above). I can see how there might be some confusion between that and the ordinary meaning of "quantity" -- in Portuguese, for instance, there is a quite different word ("grandeza") for this technical sense, and so such mix-ups are unlikely to arise. – duplode – 2018-02-23T02:47:49.103

@JFFIGK Yeah I know my last comment is not helping and it is by design, because I decided no longer to try to help.. isn't it obvious Sherlock? Or has every comment have to help? The other party is not capable of logic. I already explained why you can't measure in pixels and why it's not the same as measuring in feet. Isn't it weird on it's own that pixel² is not used if it's a dimensional unit? All the academics and scientist who created it must be wrong, right? Try it yourself to explain why and you will also see that the effort is futile. – Jerryno – 2018-02-23T09:08:13.643

12I really think this is the authoritative answer: pixels are two-dimensional. Note also that, historically and currently, pixels are not always square - or even identical on a unit level (for instance, PenTile displays muddle what a single pixel really is). – KlaymenDK – 2018-02-14T11:17:31.120

2pixels are not an "abstract concept", they are a physical object. Centimeters and other units of measure are the abstract concept. – Stephen Ostermiller – 2018-02-15T12:39:03.917

3@StephenOstermiller this is incorrect. Centimeters have a fixed physical definition as 1/100th of a meter. A meter is defined as the length of the path travelled by light in a vacuum in 1/299,792,458th of a second. Originally it was meant to represent 1/10,000th of the distance between the north pole and the equator but this was difficult to gauge. By contrast, the pixels on my current screen are of both a different size AND shape as the pixels on my phone. Software interprets them, and hardware renders them to a local environment. Centimeters are a physical standard, pixels are abstract. – Skoddie – 2018-02-15T17:18:37.080

3"Physical" mean something that is actually there that you can see. It doesn't mean "standard". It would be more accurate to say "Since pixels vary in size, and are not a strict unit of measure..." – Stephen Ostermiller – 2018-02-15T17:51:06.143

3@StephenOstermiller This is an issue of semantics. A software developer would often interpret a pixel as being more abstract than a centimeter because the measurement of one is known, while the other varies based on device. The correct use of verbiage here will vary depending on the individual's perspective, and I wrote the answer from my own. – Skoddie – 2018-02-15T18:34:44.807

5I think the 'brick wall' analogy is best - nobody uses K*J bricks^2 to build a wall:-) – Carl Witthoft – 2018-02-14T18:46:36.870

@CarlWitthoft that's true, and I agree that analogy is better, but if someone is set on thinking in terms of units, it's helpful to realize that "10 pixels high" is really shorthand for something like 10*(1 pixel-height) – reffu – 2018-02-14T19:27:31.123

1But the foot (as used for linear measurement) isn’t an object! It’s defined to be some fraction of a meter and the meter is defined in terms of the wavelength of light. In Britain, the foot has been officially defined with respect to a reference bar for over a thousand years, and anything based lengths of actual feet has been just a local approximation; likewise, the ancient Greeks defined the foot independently of actual feet. – David Richerby – 2018-02-14T11:57:28.370

1@DavidRicherby They've been redefined in terms of of the speed of light in a vacuum (the wavelength of light varies, e.g. blue light has a different wavelength than red light), but the original definitions were just use-based. Folks needed to express lengths long before modern standardization, so things could be expressed in terms of body parts, paces, common objects like barley grains (to grab a random definition of "inch" from Wikipedia), etc.. Measurement units and counting objects aren't separate concepts; the notion of "length" itself is basically a concatenation of things. – Nat – 2018-02-14T12:46:12.423

@DavidRicherby Hah I checked Wikipedia; you're right, they did define a "meter" in terms of a particular type of light's wavelength from 1960 to 1983! Actually kinda fun to read up on the history of this stuff. – Nat – 2018-02-14T13:01:32.500

Well, actually, I was mistakenly referring to the current definition! I was confusing it with the second, which is defined as a fixed number of cycles of a specific frequency of light. Anyway, the general point stands: the meter is defined in terms of some property of light. – David Richerby – 2018-02-14T13:14:42.620

What new insight does this answer add that hasn't already been said? Multiple answers already noted that pixels are individual objects, most often already of dimension 2. – Discrete lizard – 2018-02-15T08:14:53.537

4Simple: it adds scanline as a unit which still plays nicely with dimensional analysis. – Ruslan – 2018-02-15T09:11:10.850

I thought the scanline bit added new, relevant and useful information. This makes much more sense to me than the brick wall analogy. – peaceoutside – 2018-02-16T01:58:36.317