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Why do we read so much of dpi and ppi for the same? 
When are it dpi and when ppi and above all: 
What is the difference ?

 



PPI


PPI (Pixels per Inch) are clearly defined color pixels (pixels) in a fixed grid. Each individual pixel can take color values from 1 bit (2 colors) to 48 bits (280 trillion colors) depending on the product (sensor / light source). 

- Fixed size per pixel 
- Area-filling color per pixel 
- Any color value 1 - 48 bits 

Use of the name ppi : all devices that can display many colors per pixel: 
- Digital Cameras 
- Scanner 
- Monitors / Screens 
- Projector 
- Display 

Any color values pixels


Here a gray bar (gray pixel) with a digital camera recorded 


It is thus not possible to display each ppi with a dpi.



DPI


DPI (dot per inch) or (dots per inch) are limited color points, usually from printers. 
The color points are thus directly dependent on the number of color sources of the devices used. 
A simple inkjet printer delivers 4-6 colors (known as ink cartridges) 
However, in contrast to the PPI, the color points can vary in size and thereby 
The Farcharakter in the color intensity influence. 

- Different dot size 
- Reduced number of base colors per dot (eg 4 -6 colors for printers)
 
- The PPI pixel colors must be generated by several DPI points
 
- A 1200 dpi printer with 6x6 matrix can only represent a 200 PPI pixel
 

Use: Usually only printers that have to deal with a few basic colors per point: 
- Laser printers 
- Inkjet printers 

Here a 4-color printer CMY + Black 


The colors can only be achieved by mixing the basic colors and by the size Of the dots (dot). 

Here the same gray bars as in the example above, but printed. 
In the matrix 6x6 Dot's is represented by the different black spots of the  Distant seen a gray generated.

The mixture of the primary colors C (cyan) M (manganese) Y (yellow) and K for key (black fraction)