: What is the purpose of the resolution property of images (when creating/editing)? I want to check something I don't quite understand with the size/quality properties of an image. OK, as much

@RJPawlick971

I'll try a short-n-sweet answer:


What is the purpose of the resolution property of images (when creating/editing)?


The purpose of the PPI is to tell certain software what size the image should be when printed on a piece of paper. That is all. And most software doesn't care anyways.

A 100x100px image at 72dpi is the exact same image as 100x100px at 300dpi. The only difference is that certain software will print them at different sizes by default.

I believe you're getting confused because you are connecting PPI to 'interpolate' in PhotoShop's set up screens. In this situation, PPI is merely a calculation tool. And does confuse lots of people.

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@Cofer715

For typical images, a pixel is a single byte. This is the smallest atomic unit. It is a number from 0 to 255 and is interpreted as a grey value, 0 being black, 255 being white, and 254 values between. For a greyscale image, there is only one channel called grey, for RGB color, there are three bytes stored "per pixel": one value per color channel per pixel. (CMYK has 4 greyscale channels; any alpha channels adds another greyscale channel etc.)

For your 1024 x 1024 example, the smallest unit is a pixel without any regard to the ppi value, which is not relevant. The smallest editable block will always be a pixel. In your example, there are no inches. Therefore, Pixels per Inch cannot be calculated and has no meaning, neither philosophically nor existentially.

You may have seen people say that screens have 72ppi etc. This is a "lie for children." It is obvious that two 1024x768 screens, one which is 20 inches, one which is 13 inches will have a different ppi.

PPI is the same sort of number as MPH or KPH.

360 miles, 60mph means 360/60 = 6 hours
360 miles, 100mph means 360/100 = 3.6 hours

360 pixels, 60ppi means 360/60 = 6 inches
360 pixels, 100ppi means 360/100 = 3.6 inches


It is important to notice that, just like "60mph" tells you nothing about the time or the distance to any individual destination, 60ppi tells you nothing about the physical dimensions of any individual image.

Resolution has multiple definitions.

In the case of making images it simply means the number if pixels. When referring to the resolution of a screen or image, this is the typical definition. This is the actual pixel data. Altering this alters data. Given the huge variety in screen sizes and native resolutions, for screen and web, inches generally do not exist and so ppi is irrelevant. (It is probably better to say "inches are undefined.")

In the more general case, it means the number of physical units represented by an individual pixel. When referring to ppi, this is the meaning intended. For image manipulation and storage on disk, this is merely a tag. Altering this does not alter pixel data. (Notice in the pixel example above that the inches change without any alteration in the pixel count).

In many programs, the user interface will ASSUME you want to alter the pixel data when altering the ppi flag. I think this is a source of confusion for many people. In photoshop it can be turned off, but in the image resize dialog, if you change the ppi field, it will alter the pixel size automatically. If you reset the pixel dimensions back to what they were before hitting "ok" you will see that the file size estimate shows no change.

Note that if I set an image to some ginormous ppi value in photoshop, and set it to 3 inches in DTP software, the RIP software which makes the plate is probably going to resample it to something else based upon the line resolution of the CMYK halftone screens.

One final point is that if you keep the ppi (or MPH) fixed (say 100) and you want to a specific inches (time) factor (say 4), then the only number left to manipulate is the pixels (miles):

400 miles, 100mph means 400/100 = 4 hours
400 pixels, 100ppi means 400/100 = 4 inches


P.S. For printing, there is a rule of thumb that you want to provide 300ppi. All this means is that you want to provide 300 pixels for every inch in size you want it printed, the dpi flag means nothing only the number of pixels matters.

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@Cugini998

Здрасти! Hello. :D

It's not that hard to understand these measures. You want to know how big the area is and why? Let's take a picture of 1000px by 1000px. The area for these pixels is calculated by multiplying (number of pixels) * (pixel pitch of the display). So if we assume a pixel pitch of 0.25 mm, the area for this square would be 250mm by 250mm. This area would vary depending on screen density. Higher density -> smaller pitch -> smaller area.
What is the smallest area? The smallest area you can control is 1/3 of a pixel. If we keep the pixel pitch example of 0.25mm / 3 = 0.083mm.
PPI is pixels/points per inch, so 1inch = 25.4mm. When we divide by the pixel pitch of 0.25mm, we get 101.4ppi. So if we draw 101px by 101px square it would fill one square inch on the display.

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@Kristi927

PPI does not have any meaning for an image without the concept of a physical media (like printing on paper). Some raster formats can have metadata for DPI/PPI that is used by printer software when printing the image. Vector formats have no concept of resolution because it has no concept of pixels or dots. The resolution you set, only have meaning when exporting to a raster format that supports resolution metadata.

From Wikipedia:


A digitally stored image has no inherent physical dimensions, measured
in inches or centimeters. Some digital file formats record a DPI
value, or more commonly a PPI (pixels per inch) value, which is to be
used when printing the image. This number lets the printer or software know
the intended size of the image, or in the case of scanned images, the
size of the original scanned object.
(...)
For vector images, there is no equivalent of resampling an image when
it is resized, and there is no PPI in the file because it is
resolution independent (prints equally well at all sizes). However
there is still a target printing size. Some image formats, such as
Photoshop format, can contain both bitmap and vector data in the same
file. Adjusting the PPI in a Photoshop file will change the intended
printing size of the bitmap portion of the data and also change the
intended printing size of the vector data to match. This way the
vector and bitmap data maintain a consistent size relationship when
the target printing size is changed.


72 PPI is often used as default resolution if you don't need a specific resolution to match a specific size when printed.

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@Gail6891361

PPI is a technical conversion factor from pixels to physical dimensions so a image pf 1024 x 1024 at 72 PPI would be 14.22 inches by 14.22 inches or 36.12 mm by 36.12 mm in size.

PPI has no bearing on how big the images are displayed on computer screens. Computer screens show pixels. It only has meaning in very special circumstances such as printing.

Simply divide the pixel quantity with PPI for physical units. You can also figure this out with dimension analysis. So the formula is:

Pixels/PPI = distance in inches

but you were asking the area of the pixel. Individual pixels are dimensionless so they have no area. But if you want to interpret the pitch as an area then

area of pixel = (1/PPI)^2

This is offcourse meaningless since pixels are not squares just samples. See A Pixel Is Not A Little Square.

However, in a image file PPI is a piece of non-authoritative metadata. That is to say its some sort of guide for some purpose or it might have no purpose. In fact 72 PPI and 75 PPI values are typical for systems where the PPI has no meaning whatsoever. 72 and 75 is quite often the default guess of systems as to what the resolution of a monitor is. This off course has not been true for a long time if ever. Its often filled in by the system if no PPI has ever been specified.

So in a image file the information is a technical tool for suggesting what the size of the image is going to be if printed, or for doing fast calculations for quality etc.. This information may or may not be used by the actual printing process. Even in cases where it is used its more of a guide than a actual statement of fact.

Also, PPI is the better descriptive sibling of DPI. And these two are sometimes synonyms to each other. Also used PPM and DPM which is the metric equivalent of PPI an DPI respectively.

TL;DR PPI is a technical conversion factor from pixels to physical units

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