PPI vs. DPI confusion (not a duplicate! NOT the same question that's been asked!) [duplicate]

Question

I was also confused about the meanings of PPI vs. DPI and so I read a few tutorials on Google (including the top posts on StackExchange network addressing this). I think I know why people are confused even after reading the tutorials:

• The term "DPI" is pretty consistently used to refer to printers and printing

• However, different tutorials seem to define "PPI" to mean one of three different things:

(a) the meta-information stored in an image file, describing how many pixels should be printed per inch when the image is printed; or

(b) the number of physical LEDs in one inch of distance on the monitor screen -- a physical, unchangeable property of the monitor (or analogously for non-LED screens, whatever other building blocks of light are used for the display); or

(c) the number of image pixels displayed per inch on a display screen; hence, the horizontal screen resolution divided by the width of your screen in inches (which, of course, can be changed)

And this is not casual misuse of the term; this is from people who are writing entire tutorials trying to clarify the correct usage.

(I included examples at the bottom, so you can see the different tutorials really do say different things and I'm not just stupidly misinterpreting them.)

Moreover, when each tutorial gives this definition, they almost never mention the other definition(s) that are in widespread use, which, of course, is probably where the confusion comes from. So doesn't that mean that the tutorials which don't mention the alternative definitions, despite being written with the best of intentions, are part of the problem?

If I wanted to give someone one quick summary that would sort it out once and for all, is the following correct?

• Be aware that even among people who are being careful, the term "PPI" is sometimes used to mean any of the 3 things above. (And among people not being careful, all bets are off.)

• How an image is displayed on screen by default, is determined by its pixel dimensions -- a 960x540 pixel image will by default take up half the screen in each direction on a 1920x1080 display. It has nothing to do with the image's PPI property and nothing to do with the density of physical LEDs on the screen. In particular, an image with the property of "96 ppi" probably WILL NOT display 96 of its pixels in one inch of the physical screen.

• How an image is printed is determined by its pixel dimensions and PPI property, so that an image with the property 96 ppi that is 96x96 pixels will print to exactly one square inch by default.

Now I tried posting the links to the different sources, but StackOverflow said "You need at least 10 reputation to post more than 2 links." So here instead are just the quotes from the different sources about what PPI means (which you can Google if you really want to find the original page for some reason). I'm only posting these to show that the online explanations really do say conflicting things, and it's not just a conspiracy theory I dreamed up:

Votes for "physical, unchangeable property of the screen":

• "Pixel stands for “picture element”. It’s the smallest physical element of a digital display device that the eye can discern... Note that pixels are physical things of a fixed size... Hence, the number of pixels per inch (PPI) on your screen is a fixed quantity."

• "PPI. Pixels per inch. That’s how many points of light live on an inch of screen."

• defining PPI "Same principle: It counts the number of pixels your screen can display per inch." [Note "CAN display", rather than "does display" -- "CAN" display implies it's a physical property of the screen]

• [saying that PPI is a property of the monitor] "The images you see on your computer are the result of thousands of tiny colored and illumined squares that physically make up the screen. Those squares are called pixels."

Votes for "image metadata that pertains to printing":

• "All that PPI does is affect the print size of the image."

• "Pixels Per Inch is a description of the logical number of pixels from your original image that will be used to tell the printer to print one inch on paper."

• PPI: "This setting is a single number stored as part of the JPG file, and is used my most programs in determining the scale at which to print the image"

• "PPI or "pixels per inch" is the term you will see most often when selecting a resolution for your images in photo editing software."

Answer 1

There are multiple definitions of a pixel and this is unfortunate.

The problem with the physical LCD pixel descriptions you provided is that they are trivially wrong (the vast majority of displays use multiple light elements to produce a single "pixel"); they are involved primarily with specific implementations (of which there are many and various); and they are not relevant to the majority of producers of image content (especially those who are actually confused about the distinctions).

Regardless, a physical LCD pixel has a dimension since it is a physical object that can be measured with a ruler. This is one definition of a pixel. But this is the wrong definition for image editing and manipulation.

The problem with DPI or printing definitions is again, an implementation problem. For printing there is a tighter target, but commercial printing can be stochastic or FM screening, and the size of the screens vary. In addition each color screen is sampled at a different angle, so the screens themselves to not have a direct correlation to the apparent grid-like nature of the dominant raster-image storage methods.

Again, these are physical dots and they are not one-to-one representations of pixels stored in an image file. Obviously, this is not a definition of a pixel (yes they are picture elements, but the etymology of the word only tells us where it came from, not the specific meaning).

The thing is: the pixels people are concerning themselves with are the ones we manipulate in e.g. photoshop or GIMP etc.

These pixels have no size. They are dimensionless numbers. To ask what size they are is to misunderstand what they are. To ask what color they are is a misunderstanding. When you ask someone how big they want the image and they say "300ppi" they might as well say "purple monkey dishwasher". If someone wnats to know the size of a raster image pixel, the only answer (aside from a blank stare) is mu.

When we talk about resizing an image, we usually mean resample: add or remove data. This is where image quality degradation comes from. If we are not actually re-sampling a physical object with a higher density of samples, then we are making up data.

When we say there are color channels, we forget that using the data for color information is a convention, not a requirement. The red channel might be depth data, the blue channel might be brightness, the green channel might be time of day in seconds.

When we make an image "300ppi for print" what we are doing is: deciding on a target physical size on the paper we want for the finished item; considering the expected line screen halftone process being used by our supplier; providing twice as much data so that the when the plates are made, the resampling process can accurately reproduce the original signal. We can usually assume that the print providers have equipment designed around 100-133 line screen, and we like to give a little more data, so the rule of thumb works and we don't need to calculate.

We usually get pretty good results with this rule so it gets spread around, and it works. But the key here is to focus on the data, not the PPI. The PPI is mutable and derived and has very little value without a physical target.

NASA has an inverse relationship with with imagery. Ignoring that they use different intermediary image storage methods etc, the products they put out to the public are using typical 3-channel raster images and often have a resolution quoted with them. Only they quote in "meters per pixel." They can do this because, again, the pixels are dimensionless point samples that are fixed at the time of creation. The resolution of "3 meters per pixel" is the threshold below which the depicted objects are smaller than 2 point samples and therefore suffer from aliasing

For more reading on 2x data sampling ( Shannon-Nyquist Sampling Theorem ). Note that this is the reason "CD quality" is 44kHz: the human ear has about a 21kHz range and so music files sample at double that in order to properly reproduce the original signal without noticeable aliasing.

For more reading about pixels as point samples see (A Pixel is not a Square)

Answer 2

PPI is just a conversion factor. It is essentially same thing as DPI. The primary difference between DPI and PPI is that PPI describes image elements that can have more than one color per channel (printers dots are either there or not there).

PPI and DPI are ONLY meaningful if you manufacture/buy displays or prints. Since prints are always manufactured its allways somewhat relevant. But if you are using monitors, other than using one specific monitor, then the value is almost never relevant!

The problem is that people want PPI to be more fundamental than it is.

Notes:

There is no conflict between PPI being a feature of the physical device and it being a metadata in the image file. The other qualifies the device the other is a indication what the designer was wishing when creating the image. The other is a statement of fact and the other is a statement nore like: " I designed this picture using a ruler unit of X so that the image is 20cm wide". This makes sense from a manufacturing point of view, ie print, or if you happen to specify what monitor to buy. But a mobile phone user or a projector user really has no way to comply with your wishes in many cases. So they will just ignore you as stupid.

Adobe in their infinite wisdom never considered that a image file could actually have "no size", forcing you to think about this even if its irrelevant to you. Image files quite often have no specifier for the PPI or DPI value. 1 Pixel being different size on different devices, but that is ok and how it should be do.e in many cases.