Printing

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Printing

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Origin of Printing

• Invented by Chinese c.1000 AD
• Entire page printed at once
• wood cuts
• metal type
• Reinvented in Europe, Johannes Gutenburg
• Entire page printed at once
• movable type, could reset the plate

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Traditional Printing Processes

• Intaglio
• Letter press / Relief
• Silk screen / Porous / Stencil process
• Lithography / Planographic

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Lithography/Planographic Process

• Image and non-image areas are on the same plane.
• Difference Receptivity of ink
• Image areas accept ink
• Non-image areas repel ink
• The plate image is the same as the print.

Plate repels ink
Lithography
Absorbs ink for copying
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Intaglio Printing

• The image is etched or depressed into the plate
  as small wells, which contain ink.
• Non-image areas are the surface of the plate.
• The plate image is the reverse of the final print.

plate
final print
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Letter/Relief Press

• The image area is raised and inked.
• The non-image area is depressed.
• The plate image is the reverse of the final print.

plate
final print
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Silk Screen/Porous/Stencil Process

• Image areas are porous allowing ink to pass
  through.
• Non-image areas are non porous.
• Metal, nylon, silk screen, or fibrous material is
  used.
• The plate image is the same as the print.

T-Shirt
Ink
Screen
Ink goes through
Screen
Ink does not go through
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Binary Printing

• The traditional printing processes described
  maintain only two tones

All printing methods from this point on, unless
otherwise noted, are binary.
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Binary Printing

• How do printers produces gray tones between the
  traditional ink and no-ink areas without many
  gray inks?

• Challenge
• Find a way to print continuous tone images

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The Halftone Idea

• Make a grayscale
• with discrete levels of gray.

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The Halftone Idea

• Early artists used the Halftone Idea and
  alternatives to make gray with dots or lines
• Stippling, lithography, mezzotint, etc.

A work by Albrecht Dürer employs both lines and
dots.
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The Halftone Idea

• Levels of Gray
• Decrease the amount of black concentrated in an
  area to make it look lighter
• spaced lines and dots
• Increase the area covered in black to darken it
• condensed lines and dots

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The Halftone Idea

• The spacing and size of markings used to simulate
  gray depend on
• Human Visual System
• Dont want people to distinguish lines / spots
• Roughly, people see up to 60 lines per inch (lpi)
  at 2 ft.
• Viewing Distance

Zoom farther out and the dots appear smaller.
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The Halftone Idea

• Two Types of Halftone Spots
• AM Amplitude (or Area) Modulated spots have a
  fixed screen ruling and vary in size
• FM Frequency Modulated spots have a fixed size
  and vary in line spacing.

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Impact Printers

• Impact Printing
• A needle/head physically hits an ink ribbon onto
  paper leaving an impression behind.
• Typewriter, 1874
• QWERTY keyboard becomes standard
• Text only No halftones
• 1 color ribbon

• Dot Matrix Printers
• Set of 9 or 24 pins form spots
• Text and low quality imagery FM halftones
• 1 or 4 color ribbons

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The Halftone Idea

• Structures of Digital Halftone Cells

Conventional/ AM/Clustered
Stochastic/ FM/Dispersed
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Spatial Resolution

• dpi dots per inch
• Each dot that makes up a halftone spot
• Associated with optical resolution of scanners
  and addressability of digital printers

• lpi lines per inch
• Each spot or entire halftone cell
• Usually associated with printer capabilities

14 dpi
2 lpi
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Tone Resolution

• Digital Halftone Cells
• Based on a grid of spots (halftone cell)
• Change the number of dots in a spot to make
  different tones of gray

• A 2x2 matrix can produce 5 tones.
• A 3x3 matrix can produce 10 tones.
• A NxN matrix can produce N2 1 tones

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Lots of Dots

• People cannot see dots greater than 133 lpi at a
  reasonable reading distance.
• Publications set in 133-150 lpi
• magazines
• books

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Lots of Dots

• People can see dots less than 133 lpi at a
  reasonable reading distance.
• Publications set in 65-100 lpi
• newspapers

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Summary

• The Halftone Idea
• The creation of gray levels utilizing dot
  formations of color on a white background.
• Dot size and spacing are varied to create
  different levels of gray.