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Pixels, PPI, DPI, and LPI for Scanning, Printing, and Web Publishing. CSC361/661 Digital Media Spring 2002 Burg/Wong. A Pixel . A pixel on your computer monitor is a point of light What is the maximum resolution of your monitor? Probably 1024 X 768

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Pixels ppi dpi and lpi for scanning printing and web publishing

Pixels, PPI, DPI, and LPI for Scanning, Printing, and Web Publishing

CSC361/661

Digital Media

Spring 2002

Burg/Wong


A pixel
A Pixel Publishing

  • A pixel on your computer monitor is a point of light

  • What is the maximum resolution of your monitor? Probably 1024 X 768

  • How many pixels per inch is this? ~ 91 ppi

  • What is standard for computer monitors?


A pixel1
A Pixel Publishing

  • A pixel in an image file is a piece of data.

  • It could be represented in 1 bit, 1 byte, or 3 bytes

  • How many colors can you represent if a pixel is

    • 1 bit 21 = 2

    • 1 byte 28 = 256

    • 3 bytes 224 = ~16 million


Resolution
Resolution Publishing

  • Resolution of your computer screen is measured in pixels per inch (ppi).

  • Resolution of an image file is measured in pixels per inch.

  • What’s the relationship? There’s a one-to-one relationship between pixels in the image file and pixels displayed on the computer screen. However, this does NOT mean that the picture, when it is printed out, will be the same size as the picture displayed on the computer screen.


A dot
A Dot Publishing

  • Resolution of a scanner is measured in dots per inch (dpi). This is essentially the same as pixels per inch (ppi) in the sense that it is pieces of information per inch.

  • If you scan at a resolution of 72 dpi (or ppi) you’re getting 72 pixels of information about the picture in each direction for each inch of the picture. (5184 pixels per inch2)


A dot1
A Dot Publishing

  • The resolution of a printer is measured in dots per inch (dpi).

  • Each dot is a spot of color output by the printer.

  • The dot is the smallest dot of color the printer creates.

  • Your printer is probably 300 dpi, 600 dpi, or 1200 dpi. Note that this is really dots per inch in both the x and y directions.


Inkjet printers
Inkjet Printers Publishing

  • The dot of ink sprayed out by an inkjet printer is wet. One dot blends into another.

  • In general, when you’re scanning a picture to be printed out on an inkjet printer, a resolution of no more than 1/3 the resolution of the printer is sufficient.

  • Thus, for a 600 dpi inkjet printer, 200 dpi resolution should be enough.


Laser printers halftone dots and lpi
Laser Printers, Halftone Dots, and LPI Publishing

  • A dot is the smallest printable unit for a laser printer.

  • A line is created from a group of dots. The line is a series of half-tone dots. (p. 59, Avoiding the Scanning Blues, p. 87, The Non-Designer’s Scan and Print Book.)

  • Thus, laser printers are characterized by the lpi at which they print as well as the dpi.


Half-Tone Dots Publishing

Each line printed out by a laser printer is a series of half-tone dots. For a grayscale picture, a half-tone dot looks more or less gray depending on how many primitive dots it is composed of.

From p. 59, Avoiding the Scanning Blues by Taz Tally, Prentice-Hall Publishers, 2001.


Laser vs inkjet printers
Laser vs. Inkjet Printers Publishing

  • Laser printers use AM screening (amplitude modulation). That is, the size of the halftone dot changes to change the color.

  • Inkjet printers use FM screening (frequency modulation). That is, all dots are the same size, but the number of dots in the picture changes.

    (p. 44, Avoiding the Scanning Blues)


Common dpi and lpi for laser printers
Common DPI and LPI for Laser Printers Publishing

  • 300 dpi 60 lpi (5 pixels2 per halftone dot)

  • 600 dpi 85 lpi (~7 pixels2 per halftone dot)

  • 1200 dpi 110 lpi (~11 pixels2 per halftone dot)


Resolution when printing from a laser printer
Resolution When Printing From a Laser Printer Publishing

Experience shows that a good rule of thumb is to scan at a resolution that is 1.5 to 2 times the lpi of the laser printer you’ll be printing on.

  • e.g., for a 600 dpi laser printer, scan your picture at about 130-170 ppi


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