A Digital image consists of pixels. A pixel does not have well defined size. To represent the size of a pixel, it is necessary to know the resolution used to represent this pixel . The resolution defines the number of pixels per unit of length (Centimètre or inch). The resolution of an digital image is expressed in pi (Pixel Per Inch) or PPP (Pixels Per Inch). The resolution of impression of a Imprimante is determined in DPI (Dowry Per Inch) or PPP (Points Per Inch)

In French, it is dangerous to make the amalgam between Pixel Per Inch and Points Per Inch (PPP >< PPP). The nuance is that the number of pixels useful for the image is used to manufacture the points of the printed screen.

Example

With the screen: Previously, when the screens existed only in one gauge, one said, wrongly, that a screen of computer had a resolution of 72 PPP, which wants to say that for each inch of screen (horizontal), there are 72 pixels. But really, the resolution varies according to the possibility of posting of media. Currently thus, the screen resolution depends on the standards of posting on the user.

With the impression: The pixels posted with the screen are broken up into points of ink to form a screen. From where need for having more pixels on surface represented. In general, one standardizes the resolution of an image with 300 Pixels Per Inches (PPP). The eye not making it possible to see the difference in quality to the top of these 300 PPP, the printers thus generalized this rule.

In truth, the resolution must be calculated according to the quality of the peripheral of exit. The quality of impression of a printer is noted in " Dowry Per Inch" (not by inch) or " Line Per Inch" (line per inch). Pi (pixels) are transformed into DPI (points) by a software upstream of the printer or in the printer.

If the number of pixels per inch of the image is too small, the printer will restore the staircases formed by contrasts between the pixels of an image. It is what generally arrives with screens impressions like the impression of pages Internet or captures of screen.

For the numerical cameras, one speaks wrongly about resolution when one says: “This apparatus has a resolution of 8,3 mégapixels.” As one speaks about definition of the image, it would be necessary to say: “This apparatus is equipped with a sensor of a definition of 8,3 mégapixels.”

Which resolution to adopt?

It is necessary to choose the final resolution according to the result concerned and of the capacities of its material.

• Impression on paper: if the impression on paper is the had aim, to adopt a resolution of 300 pi or PPP for a correct quality. A resolution lower than 150 pi is insufficient for impressions of quality. It can however be sufficient for the informative daily press and reviews. To know the Linéature printer would be useful to know the number of pi necessary for your document.
• Visualization on screen: in this case the resolution is the real size of the posted pixel is defined by the material used. The impression of a screen will have by defect a resolution of 72 pi (Apple) or 96 pi (Microsoft).
• Visualization on Vidéoprojecteur and " Datashow": the resolution will depend on the distance between the projected image and representation surfaces it. It will also depend on the possibility of posting of the material.

See too

• Definition of screen

• Measuring unit in data processing
• Graphics card
• Pitch (data-processing)

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