Why use DPI (not ppi)? (image properties)

[Yichuang驿窗]

The location labeled on the attached screenshot uses the DPI.
I looked at the wiki, and the WIKI also mentions that it should be DPI.
Why use DPI instead of PPI?

The screencapture is the Content Properties of image frame.

[Nermander]

PPI would be the correct term, but for digital media the PPI equals the DPI. I am not aware of any digital format where a pixel is made up of several dots.

[Yichuang驿窗]

I also think the correct one should be ppi. but I see that both Content Properties and the Scribus wiki are dpi, so that creates confusion.

[Aleks100]

DPI, for sure. Google dpi vs ppi

[Yichuang驿窗]

I think it should be PPI, not DPI.

First look at the ppi.
The sole purpose of ppi is to determine the physical print size of a pixel. Assuming an image is 600x900 pixels, what is the size of that image when printed or printed to paper?

This is when ppi is used. if the resolution of an image is 300 ppi, that means 300 pixels spread over the length of an inch, then the physical print size of each pixel is 1/300 of an inch = 0.003 inches. a 600*900 pixel image with a resolution of 300 ppi translates to a print size of 0.003x600*0.003x900. That ends up being 2x3 inches.

No dpi is used in this process, only ppi.

Look at dpi.
dpi initially describes the performance of the printer, said the printer in the length of an inch of paper can print how many dots.

Let's say the resolution of a printer is 300 dpi, which means that this printer can print 300 dots on a length of one inch of paper.

Or, to describe a scanner, it means that the scanner can read information about how many dots on one inch from the paper. There is no ppi involved here, only dpi.

Now we have a look at Scribus.

In Scribus we need a ppi to tell printer how much the physical print size of the image pixel is. For this reason, we need only ppi, not dpi.


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DPI is for printer, like laser heads for laser printers, or printheads for inkjet printers. The dpi only reflects the level of accuracy of the printer itself and has nothing to do with the image.

[RodneyLee]

A DOT is a PIXEL

[a.l.e]

"""
In simple, rudimentary, terms...

• A dot is the smallest possible spot of ink on paper.
• A pixel is the smallest possible spot of light on a screen.

Dots are never on a screen and pixels are never on paper.
"""

https://graphicdesign.stackexchange.com/a/154690

this is the short version of the fine explanation by yi chuang.

[Yichuang驿窗]

A Dot is a Pixel, maybe YES, maybe NO.

Example for YES:
----------------------

An image file is 600x300 px，resolution is 300 PPI，it was printed on a paper with a laser printer, the accuracy of the laster printer is 300 DPI.


The printout result is，an image on paper that is 2 inches in horizontal dimension and that 2 inches consists of 600 pixels. The image is 1 inch in portrait dimension and the 1 inch consists of 300 pixels.

"2 inches in horizontal dimension" is because of 600px÷300PPI, "1 inches in portrait dimension" is because of 300px÷300PPI.

Because the printer's accuracy is 300 DPI, the printer will print 300 Dots on a one-inch length of paper. Corresponding to the image printed above, the image is 2x1 inches, so 600 Dots are printed horizontally(2 inches x 300 DPI); 300 Dots are printed vertically(1 inch x 300 DPI).

Summary:
An image on paper has 600 Pixels in the horizontal direction, and these 600 Pixels are rendered by the printer with 600 toner Dots; in the vertical direction, there are 300 Pixels, and these 300 Pixels are rendered by the printer with 300 toner Dots.

Here, the pixels and toner dots correspond to each other, and the result is what we call "A dot is a pixel".



Example for NO:
----------------------

Same image file, same laser printer. Before printing, we change the resolution of the image from 300 PPI to 600 PPI, then print the image to paper and see what changes.

(Note that the image file is now 600x300 pixels, the resolution is 600 PPI, and the printer precision is 300 DPI).

Since the image has a resolution of 600 PPI, the image printed to paper is 1 inch (600px÷600PPI) horizontally and 0.5 inches (300px÷600PPI) vertically.

While on paper, the printer still uses 300 toner dots to render an inch long image content, that is, 1x0.5-inch image, horizontal 1 inch, there are 300 toner dots; vertical 0.5 inches, there are 150 toner dots.

The result is that printing a 1x0.5-inch image on paper with 600 pixels an inch horizontally requires only 300 toner dots; printing a 0.5-inch image with 300 pixels vertically requires only 150 toner dots.

Summary:

An image on paper has 600 pixels in landscape, which are rendered by the printer with 300 toner dots, and 300 pixels in portrait, which are rendered by the printer with 150 toner dots.

Here, the pixels and toner dots are not one-to-one and become "One dot is (rendered with) Two pixel".


======================
In the above examples, if the precision of the printer changes(300DPI to 600DPI, or 300DPI to 150DPI), it will likewise affect the toner_dot-to-pixel correspondence.

So, exactly how the dots correspond to the pixels is mainly affected by two elements, one is the resolution PPI of the image and the other is the precision DPI of the printer.

However, the only element that determines the size of an image on paper is the PPI, not the DPI.

[Nermander]

A DOT is a PIXEL

No, for most printing techniques a pixel is rasterized into many smaller dots.

For example most 2400 DPI inket printers print 300 PPI, each pixel is represented by 4x4 dots.

[Aleks100]

For practical reasons IMHO in printing it is better to look at LPI value as analogue to PPI on screen.
Nice & short explanation is here:
https://apmbags.com/what-difference-between-dpi-ppi-and-lpi

[Nermander]

It depends on what you are printing on.

Since the digital image has pixels, and for print you define the size (whether in mm, cm, inch) you get a PPI.

The DPI and LPI values are only relevant for the printing technique, and not for how big the image becomes in the print.

A 300x300 pixels image "scaled" to 300 PPI becomes 1x1 inches. No matter how many DPI or LPI the printer has.