What do we mean by "Image Resolution", and how is it related to printing on paper?
Is resolution related to displaying the image on computer screens too?
What is the meaning of "High Resolution"? "High" relative to what?
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A good way to understand resolution is thinking in terms of the grid of pixels that you have on your screen, rows and columns. More lines, smooth curves, better images. Less lines, more "squares" on your image.
High Resolution is relative. It's more of a recommendation than a standard. For instance, photography cameras offer higher resolution than the video/cinema high-resolution have. Video monitors, and computer screens use 96 dpi by default. For printing, 300 dpi is a good starting resolution.
These resources should be helpful:
In the digital era, resolution is just a number. This number represents the count of cells which are in you image (file). The measure is pixels, MP (megapixels), kP (kilopixels), GP (gigapixels).
And because your image is rectangular, you have two numbers and multiplying them you get the number of pixels of your image. Let's suppose we have an image with dimensions 6000 pixels by 4000 pixels. This example means you have a 24 MP image.
The other way to get the dimensions of your image is to have the resolution and aspect ratio between two sides. For example, if you have a 24 MP camera with an aspect ratio of 3:2, you can calculate the long and short edge dimensions (in pixels) by:
dim1 = √(24000000/(2*3)) * 3 = 6000
dim2 = √(24000000/(2*3)) * 2 = 4000
If the aspect ratio were 4:3 (same 24 MP), then:
dim1 = √(24000000/(4*3)) * 4 = 5600 (average)
dim2 = √(24000000/(4*3)) * 3 = 4200 (average)
When we talk about printing, we have take into consideration the density of your print. The usual measure is DPI (dots per inch). The usual starting point for print for wall is 300 DPI. But this depend on a lot; for example, for road ads it can be from 3 to 15 DPI (depend of distance of observation).
Lets give some example with print. You have 24MP 3:2 image and you want to print it in high quality at 300 DPI. So the dimensions of the print will be:
dim1 = 6000/300 = 20" (or 51cm avg.)
dim2 = 4000/300 = 13.3" (or 33.8cm avg.)
I understand "high resolution" as strictly marketing expression. This changes over time (higher resolution sensors, etc) and it's personal. For someone 10MP can be high resolution because he/she wants to add this image as background on his/her Full HD screen (~2 MP). For others, a 10 GP image will be high resolution because he/she want to see every possible detail in this image zooming and zooming....
Relative, it is all relative. This is a non-standard answer, and can potentially confuse the reader more because I am exploring in depth some terms and I am inventing some more.
There is not only one "resolution", but there are also several "resolutions" involved in photography, camera, lens, image size, printer, printed size, screen. And you can some more terms if you add video to the list...
The same way velocity is a relationship between some units of dimension (km, miles, m, etc.) over one unit of time (hr, seconds, day), in general terms:
Resolution is a relationship between some units of information over one unit of dimension.
Then you need to define what are the units you want to use, and this can vary.
A printer uses dots to generate an image. If we use inches as a dimension unit we have DPI. Dots per inch.
A digital image is made of pixels, we can stretch, squish and squash an image to be printed larger and smaller, having bigger pixels will fit less of them on an inch. We have PPI Pixels per inch.
We can have some amount of pixels on one image, in one photo normally we have millions of them so it is a good base, millions or mega. Mpx per image Pixels per image. The per-image sounds silly, but the definition applies.
We can have some amount of the wrongly so-called pixels on one screen, I am starting to call them "display units" or dixels (my neologism) on one screen, this can be called Mdx per screen. And on a camera, I am starting to call the sensor's capture units capxels (neologism here)... se-xels sounds catchy tho... (It seems there is a word for this Sensels https://en.wiktionary.org/wiki/sensel)
And also we can have roughly 2000 of this dixels on the base of a screen (1920 on a full HD) or 4k of them on the base of the screen (4kilo dixels) or 4k.
Youtube, for example, uses the vertical value of a video, 480, 720, 1080 pixels per video. This are in fact pixels)
On a video, we have frames per second, and that is a time resolution. 24 fps, 30, 60.
On audio, we have samples per second... etc.
High is relative to low.
I can make a big list here, but when applying this to an image, it can be high starting from about 2Mpx on images to be viewed on screen, (FullHD) or lets say above 6Mpx when printed. (3000x2000) But this is relative...