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Electronic Image Capture and Manipulation

Ch. 14 of the Offset Lithographic Technology Book

Laura Roberts

on 12 June 2013

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Transcript of Electronic Image Capture and Manipulation

File Formats
OPI is a work-flow protocol in which high resolution images are stored on a server and low resolution files with corresponding names are placed in the layout.

The low-resolution files are considered FPO (for position only) images. When the page is output to an imagesetter, the high-resolution image replaces the low-resolution image.
Electronic Image
Capture and

After images are captured, they usually require some preparation before they are ready to be integrated into the publication.
Some of the most commonly used programs are Adobe Photoshop, Macromedia Freehand, CorelDRAW, Corel Paint Shop Pro, and Adobe Illustrator programs.
A technique called trapping is used to create a small
area of overlap, or trap, between adjacent colors and
prevent gaps of white between objects that need to be
printed in tight register.
Image Manipulation with Vector Graphic Programs
Drawing programs, such as Freehand, adobe Illustrator and CorelDRAW, generate vector graphics.

Instead of bitmap art of image-editing programs, vector graphics are based on mathematical formulas. Therefore they can be enlarged without losing resolution.
Spread Trap
A spread trap is created by expanding the foreground into the darker background color.
Choke Traps
Choke traps are performed by expanding the background under a darker foreground image.
Image Manipulation with Bitmap Programs
Using Image Manipulation Programs
Image Manipulation
Within the digital work flow, the capture, creation, and manipulation of images usually precedes the integration of these images with page composition software.
After an image has been captured with either a scanner or
a digital camera, it can be manipulated in several ways.

Several programs can perform image manipulation and the
desired effect determines which software is appropriate.

Basically, there are bitmap (or paint) programs and
vector-graphic (or draw) programs.

Image manipulation programs typically allow the user to crop,
change color and contrast, combine images, and add or remove
visual information.
Learning Objectives
When you have completed this presentation and the assigned activities related to this chapter, you will be able to do the following:
Explain different methods used to capture images digitally.

Explain the advantages and limitations of different types of digital cameras.

Name various types of scanners and the light gathering devices used in them.

List and define several software applications used for image manipulation and some of their functions.

Define and explain the different types of traps.

Know what Open Prepress Interface (OPI) and the use of for position only (FPO) images.
Images that are captured with either a digital camera or a scanner are usually manipulated with Photoshop, CorelDRAW, or Paint Shop Pro.

This presentation will examine many of the options available to prepress operators in capturing and manipulating images.

Either a digital camera or a scanner is used capture images to input into a computer
Digital Cameras
As digital cameras become less expensive and more popular, they are playing an increasing role in prepress production.
Digital cameras capture light images and convert them into electrical charges and then into digital data.

The wide range of digital cameras is extensive because they vary in sensor technology, color-capture mechanisms, image resolution, image storage, zoom capabilities, and cost.
Sensor Technology
Digital cameras use rows of light sensors called photosites to transform light energy (photons) into electrical energy (electrons).

Increased light energy striking a photosite results in increased levels of electrons which are then transformed into digital data.

Most cameras use the same charge-coupled device (CCD) technology used in flatbed scanners for their photosites.

A charge-couple device (CCD) is a solid-state chip that converts light into electrical charges, which are, in turn, converted to an analog/digital (A/D) adapter into digital data in the form of pixels.
Color-Capture Mechanisms
No photosites are able to distinguish colors of light. Therefore, photosites must be used with filters to isolate the color being measured.

Digital cameras use a beam splitter to send the image to three sensors, each behind a different colored filter, to measure the amount or red, green or blue light passing through the filter.
Image goes in here
Bayer Filter Patterns
Less expensive and less bulky digital cameras use a system that permanently places a blue, green, or red filter over each photosite.

A Bayer filter pattern alternates horizontal rows of red and green filters with rows of blue and green filters.
Behind each filter is a photosite, which received the amount of light passing through the filter. Through interpolation the image is interpreted using neighboring photosites to represent the image.

Interpolation is the increase of image resolution by the addition of new pixels throughout an image.
Image Resolution
The image resolution of a digital camera is a measurement of the amount of detail that can be captured, and image resolution is determined by the number of pixels, which is determined by the number of photosites.

Resolution is expressed by the number of pixels arranged horizontally, followed by the number of pixels arranged vertically.
Image Storage
After the image is captured, it can be stored within the camera's built-in memory and then transferred to a computer, or it can be stored on a removable storage device.

Greater image capacity is available with a removable storage device.

Although these devices come in many formats and capacities, they share the function of transferring digital images by being removed from the camera and plugged into the computer.

Compression allows a digital image to be converted to data requiring considerably less memory, thereby permitting more images to be stored.

Joint Photographic Experts Group (JPEG) is a file format that allows the compression and is used in most digital cameras.

Tagged image file format (TIFF) is a bitmap file used for exchanging raster images between applications.
Zoom Capabilities
Many conventional cameras can be fitted with a telephoto or zoom lens that enlarges the subject by extending the focal length (FL).

Similarly, most digital cameras can extend the FL with a feature called optical zoom.
The amount of optical zoom is indicated with a number revealing the maximum power of the optics.

For example, 2x zoom will double the size of the image, and 3x will triple it.
Digital cameras often have a feature called digital zoom, in which interpolation is used to create new pixels after the original pixels are moved apart to enlarge the photo.
Digital cameras can cost less than $200 or more than $4000.

The least expensive digital cameras are compact, have low resolution and have no optical zoom. These are great for people who wish to email photos or post them on the internet because they seldom require high resolution photos.
Photographers who wish to print their images in sizes such as 5"x7" or 8" x 10" will be better served by investing in cameras with optical zoom and a resolution of 5.0 megapixels or more.
Electronic Scanners
A scanner is an electronic device that can capture analog images,
such as type or other line art, photographs, art, or film and convert them to digital data.

This device can also create color separations by measuring the color densities of an original image, and using special software to manipulate the data to create RGB or CMYK images.
The term megapixels refers to the number of millions of pixels constituting an image.

For example, 7.7 megapixels is 7,700,000 pixels.
The main differences among scanners are the resolution capability and the quality of the light-gathering sensors.

To understand how scanners function and to judge their capabilities require a grasp of some terms. These terms are resolution,
dpi, bit depth, and dynamic range.
Resolution determines the ability
to show fine detail.

The resolution quality of a scanned image is
determined by the number of dpi and bit depth.

The dpi of an image represents the number of pixels
making up the image. The greater the dpi at the time an image is scanned, the greater the amount of detail and the better the resolution.
Bit Depth
The bit depth of an image is defined by the number of bits used to make up each pixel. A pixel can be made up of one bit or
several bits.

A pixel consisting of only one bit can be either black or white, while a pixel consisting of more bits can combine various combinations of their being into more tones of gray or color.
Dynamic Range
A Scanner's bit depth defines its capability of recording tonal variations, but its pixels cannot record more than the scanner's dynamic range can see.

Dynamic range is the difference between the Dmin and Dmax (whitest and blackest) tones a scanner can capture.

The relationship between bit depth and dynamic range, a scanner's bit depth is its ability to record the tones and colors the dynamic range can recognize.
Optical & Interpolated Resolution
A scanner's optical resolution is a measure of the maximum resolution at which the scanner can capture an image using its optics alone.

Optical resolution is properly specified in dpi in both horizontal and vertical direction, but is commonly given in horizontal direction only.

Very often, images are scanned at one size with the intent of being greatly enlarged before being printed. Unfortunately, the overall image is made larger by enlarging the individual pixels, with the result that they can become visible and greatly impair image quality.
One solution is to increase the number of pixels with interpolation software to maintain resolution.

Interpolated resolution is a measure of the maximum resolution at which a scanner can capture and image using its optics in combination with interpolation software.
Light-Gathering Sensors
The light-gathering sensor determines
the bit depth, and dpi a scanner can produce.

The most common light-gathering sensors in use today are the photomultiplier tube and CCD.
A photomultiplier tube is composed of highly sensitive photocells.

Drum scanners are the only scanner that use photomultiplier tubes.

Flatbed, handheld,
and film scanners
use CCD.
Light Sources
The light source is generally held in a sealed compartment to shield the other parts of the scanner from heat to eliminate flare.

There are three common types of light sources used in scanners.
Xenon Lamp
A xenon lamp is a
gas-discharge lamp
that emits a constant high-intensity light.

This lamp provides a high concentration of light in a small area.
Tungsten-Halogen Lamp
The tungsten-halogen lamp is a low-voltage, high-current tungsten light. The halogen gas prevents the vaporized filament material from depositing on the walls of the lamp and dimming its output.
A laser produces consistent, parallel rays of light that can be easily controlled. This light source offers a high concentration of light and is much cooler than other light sources.
Types of Scanners
There are four types or categories of scanners.
Drum Scanner
A drum scanner, or rotary scanner, scans images mounted on a rotating drum.

The drum spins quickly in front of a sensor unit or analyzing head.
Flatbed Scanner
A flatbed scanner uses a flat glass bed and a linear CCD array to digitize images.

A linear CCD array allows the scanner to capture and image in a single pass.
Handheld Scanner
Handheld scanners are small devices you move across the paper or image by hand.

The scanners are typically used for scanning small amounts of text and for position only (FPO) images.

For position only (FPO) image is a copy of a graphic image placed on a mechanical or in an electronic layout to indicte placement, but not used for reproduction.
Film and Slide Scanner
Film and slide scanners are designed specifically for digitizing images from various types of film.

Film scanners use a CCD array to capture images.

First-generation images (slides or film negatives) should be used for scanning.

For every photographic generation produced, a little of the integrity, sharpness, and tonal quality of the original is lost.
A magnetic image holder can be used on some scanners to eliminate the glass between the image and optics.

This helps preserve sharpness and reduce distortion and Newton's rings.

Newton's rings are undesirable color pattern resulting from interference between the exposure light and its reflected beam from the closely adjacent surface.
Scanner Maintenance
The buildup of ink and other chemicals on the scanner
components can cause permanent damage and poor image quality.

It is important to keep your scanner clean and maintain it properly.

Most scanner manufacturers offer cleaning kits, which include special cleaning fluids and lint-free cloth to remove buildup.
Feeding Documents into the Scanner
All paper clips and staples should be removed from documents before
feeding them into the scanner.

These sharp objects can scratch optics and tear rubber components
along the paper path, causing streaks, noise, and skews.
A Scanner's
Software Applications
The electronic scanner has become the primary
means of image generation for use in a digital
work flow.

With the proper software, a scanner can perform many of the image-correction and image manipulation processes highly skilled technicians once performed.
Optical character recognition (OCR) is an imaging process that electronically reads typewritten or printed pages and save the copy in its ASCII format.

An imaging process known as intelligent character recognition (ICR) is used to recognize handwritten material.
Unsharp Mask
Unsharp masking (USM) is the process of adjusting tonal contrast where light and dark tones come together at the edges of an image to create a sharper image.
Although it is best to perform scans with continuous tone originals, it might be necessary to scan a printed image.

If you scan a printed halftone image, the final image might contain a moire pattern.

Descreening is an electronic means of eliminating or minimizing the halftone-dot pattern of a scanned printed image.
Working With Layers
Layers in an illustration program can be though of as a stack of clear acetate sheets. Each sheet contains one or more objects. The sheets are transparent wherever there is no object, and you can see through to the sheet below.
Applying Filters
The same types of special effects created with special halftone screens and screen tints can be created electronically with a wide array of filters.

Filters are used to apply textures and patterns to images.
Painting and Erasing
Most image manipulation programs provide paint features similar to those provided in paint and draw programs.

The user can create and add color to images. Other tools allow the user to erase portions of the image, adjust color values, and create variations.
Applying Masks
Masks protect certain areas of an image from being changed.

Semitransparent masks enable the user to partially affect an area of an image.
Determining and indicating which portion of the photograph will be printed is cropping.
Enlarging or reducing a photograph is traditionally referred to as scaling, but sizing is the term used in the digital world.
If an image is scanned at a resolution twice the number of lpi that will used during platemaking, the file will contain excessive data and use unnecessary storage space.

Resampling can save file space by reducing the image resolution to 1.5 times the lpi of the printing plate.
Tone compression
The tonal range of an original photograph is likely to be greater than the tonal range that can be achieved on an offset press.

Tone compression must be applied to the image to bring the tonal range closer to the press capability.
Cast Removal
Color photographs taken under unbalanced light sources, such as incandescent and fluorescent lighting, often acquire an orange hue or a green hue.

Cast removal can
remedy the off-color
nature of the
photograph by
creating better
color balance.
Color-Level Adjustments
The process ink colors CMYK are referred to as color channels in most photo-editing software.

Color levels allow
color modification
by either adjusting
one channel at a
time or adjusting
all four collectively.
Curve Adjustment
The diagonal line representing a photograph's density, from the darkest shadow through the midtones and highlights, can be altered with curve adjustments.
Posterization is the conversion of a continuous tone image to a high-contrast image with one or middle tones.
Photo Retouching
Artifacts are visible imperfections in a digital photograph or other image.

Photo retouching can be performed with various tools.
Gray Scaling
The process of converting a color photograph to a monochromatic image is gray scaling.
The cloning stamp is used to cover a target (unwanted) area of a photograph with pixels from a source area.
The healing brush uses a print source to identify the area to be carried to another part of the image.
There are three types of traps: spread trap,
choke trap, and automatic trap.
Automatic Traps
Automatic traps require one or more colors present in both the foreground and background
Trapping can be done automatically in some illustration programs by applying a trap filter.
Graphic files are divided into two types - object oriented (vector) and bitmap (raster)

Object oriented graphics (vector) are often used for line drawings, logos, and any image requiring smooth, crisp edges.

Bitmap (raster) graphics are usually pictures or photographic-type images, such as those captured with
a scanner.
The tagged image file format transfer (TIFF) is a device dependent format that incorporates high-end capabilities for describing demanding four-color documents, including specs for printing presses.

The EPS format is used to transfer PostScript files between PostScript devices and is one of the most stable file formats used in outputting to an imagesetter or a platesetter.

Other common file formats include the Windows metafile (WMF), picture (PICT), Desktop Color Separation (DCS), and the Graphics Interchange Format (GIF). JPEG and GIF are appropriate for the internet because of their compression.
Open Prepress Interface (OPI)
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