Dallas Genealogical Society Technology Special Interest Group

Thursday, April 2, 2015

Tony Hanson Webmaster

Technology Special Interest Group Leader Director - Education

Demystifying Digital Images

Tony Hanson Webmaster

Technology Special Interest Group Leader

What we will be talking about • What is a Digital Image • How Digital Images are Created • Numeric Representation of Colors • Histograms • How Digital Cameras and Scanners Work • Digital File Formats • Image Resolution and Size • Printing Images • Metadata • Resizing Images

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What is a Digital Image?

• A numeric representation of an image • Comprised of an organized collection of Pixels

(Picture Elements) – Rows & Columns – Pixel a.k.a. ‘Dot’

• When viewed on a suitable display (or printed) it appears to be an exact copy of the original image

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02/2015 5 “Prairie Passage” by Carl and Sandra Bryant

Eagleville, MO Welcome Center

Mosaic Image

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Notre Dame Cathedral, Montreal Canada

How Are Digital Images Created?

• ‘Born Digital’ – Created using software that directly creates the

digital file – File -> Save As ->

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This is an image file created directly from Publisher

Graphic Images

• Has relatively Few colors • Usually a (relatively) small file • Some file formats are customized to support

this type of image

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How Are Digital Images Created?

• Converted to a digital format – Scanner – Digital Camera – These both use a Charge Coupled Device (CCD) to

convert the optical image into an appropriate numeric representation

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CCD: Converts Light to a Numeric Value

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Memory CCD

Color values are represented by three components: • Red • Green • Blue

Light

x

y z

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Color Palette

Red

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Memory CCD

255

0

0

Green

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Memory CCD

0

255

0

Blue

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Memory CCD

0

0

255

White

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Memory CCD

255

255

255

Black

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Memory CCD

0

0

0

Purple

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Memory CCD

136

109

249

Gray

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Memory CCD

179

179

179

Note that the color values are all the same for Gray….

Numeric Representation

• Color • Gray Scale • Black and White

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Numeric Representation: Color

24 Bit Color Depth • Each component value (Red, Blue and Green)

can range from 0 to 255 – In Binary, the range is

• 0: 00000000 • 255: 11111111

• 3 Colors x 8 bits each = 24 bits

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255 in Binary Notation

1 2 6 3 1 8 4 2 6 8 4 2 1 1 1 1 1 1 1 1 1 128+64+32+16+8+4+2+1 = 255

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48 Bit Color

48 Bit Color Depth • Each component value (Red, Blue and Green)

can range from 0 to 65,535 – In Binary, the range is

• 0: 00000000 00000000 • 65536: 11111111 11111111

• 3 Colors x 16 bits each = 48 bits • Provides more colors

– More steps between Black and White 02/2015 25

65,536 in Binary Notation 3 1 2 6 8 4 2 1 7 3 1 0 0 0 5 2 1 6 8 9 9 4 2 1 5 2 6 3 1 8 4 2 6 8 4 2 6 8 4 2 6 8 4 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 32268+16324+8192+4096+2048+1024+512+256 = 65280 128+64+32+16+8+4+2+1 = 255 65280+255 = 65535

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Black is Still Black

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Memory CCD

00

00

00

White is still White

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Memory CCD

65535

65535

65535

More shades of Blue (and Red, Green…)

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Memory CCD

0

0

65535

Gray Scale

• Ranges from White through various shades of Gray to Black – Has equal Red, Green and Blue values – Since each of the three values is the same it is

only necessary to record it once

• This limits the colors available, but may produce a high quality image that requires less storage

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Grayscale Palette

Gray Scale Color Depth

• Usually available in 8 bit and 16 bit color depth

• The end points in both cases are still White and Black

• 16 bits gives you more granularity – More shades of Gray

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Black and White ( a.k.a. ‘Line Art’)

• Each Pixel can have one of two values: – 0 => White – 1 => Black

• It is usually possible for you to modify the threshold between what is considered Black and what is considered White

• These values (0 or 1) can be stored in a single bit, significantly reducing storage requirements

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Size Matters!

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Source: ‘A few scanning tips’ http://www.scantips.com/basics1d.html

How many Pixels at each color?

How Many Pixels

0, 0, 255 0, 255, 0 255, 0, 0 0, 0, 0 255, 255, 255

“Black” “White”

27

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26

“Blue” “Green” “Red”

A Histogram!

0, 0, 255 0, 255, 0 255, 0, 0 0, 0, 0 255, 255, 255

“Black” “White”

How Many Pixels

Count of How Many Pixels at each color

s

Histogram

• Shows a count of each ‘color’ in the image • Helps understand how much contrast the

image has – Probably most useful to photographers (useful for

judging exposure) – Also useful when adjusting scanner threshold

settings

s

Digitation Tools

• Camera • Scanner

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Digital Camera

Image Lens Charge Coupled Device

Light

Shutter

Aperture

s

Image on a Charge Coupled Device (CCD)

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Pixels

CCD Characteristics

• How sensitive it is to Light • Its Size • How many Megapixels • Aspect Ratio

– Ratio of Width to Height (Pixels) – A square would have an aspect ratio of 1 – Typical values for a camera are 1.3 or 1.5

(rectangular)

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• Records images in a 3264 by 2448 format – 3264 x 2448 = 7,990,272 Pixels (8 MegaPixels) – 3264 / 2448 = 1.33 to 1 Aspect Ratio

2448

3264

iPhone 6

Scanners

• Unlike a camera, a scanner has only a single row of CCD sensors

• The sensor is moved across the document to be scanned and takes many samples in the process

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Flatbed Scanner

Document Light

Charge Coupled Device

Mirrors

Glass

Document

My Simple Document

Tony.

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Each Line scanned as a series of Pixels…

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Scanner Specifications

• Optical Resolution – The Pixels per Inch capability of the CCD (Horizontal resolution)

• Hardware Resolution – The resolution of the mechanism that moves the CCD across the document (Vertical Resolution)

• Interpolated Resolution – Using software to increase resolution by predicting what might be in between two scanned pixels

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How are digital images saved?

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Digital File Formats

• A digital file format defines the way that the digital information will written into (and read from) the file

• Many file formats have been created over the years to address different needs and concerns – Wikipedia has identified 262 different graphics file

formats!

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https://en.wikipedia.org/wiki/Comparison_of_graphics_file_formats

Image File Format Characteristics • Raster vs. Vector • Compression Type • Color Depth • Color Indexing • Transparency • Metadata • Interlacing • Animation • Layers • Color Management

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Raster vs. Vector

Vector formats deal with lines and shapes • These scale (adjust to different sizes and

display formats) very well • Useful in graphic and scientific applications Raster formations deal with pixels/dots • The focus of our discussion

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Compression

• Not all formats perform compression • Those that do fall into two classes:

– Lossless • No loss of resolution or color information • Results in larger files

– Lossy • Some irreversible loss of resolution or color information

in the compression process • Results in smaller files

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Color Depth

• Defines the number of bits available to encode color – Usually can be modified – Ranges from 1 to 48 bits

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Color Indexing

• Some formats limit the number of discrete colors that can be utilized

• They provide a fixed size matrix where the colors utilized in the image can be specified

• During the encryption, each pixel’s color is referenced to this table – Advantage: Smaller File – Disadvantage: Limits the color granularity of the

image

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Transparency

• Not supported by all formats • Allows the background to be viewed behind

the image • Achieved by designating a specific color in the

image • When displayed, image pixels with the

designated color are discarded and replaced the appropriate pixel from the background image

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Metadata

• Not supported by all formats • Descriptive information about the image • Appended to the image file • There are a variety of metadata types/standards

that can be used • The most common are:

– Exif – Photographic Information – IPTC – Descriptive Information – XMP – ISO Standard

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Popular Image File Formats

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TIFF

• Compression: None • Color Depth: 1,2,4,8,16,24,32 bit • Color Indexing: Yes • Transparency: Yes • Metadata: Yes • Comments

– Can be compressed with external/3rd party compression tools

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JPEG

• Compression: Lossy • Color Depth: 12/24 bit • Color Indexing: No • Transparency: No • Metadata: Yes • Comments

– Amount of compression can be varied – Repeated saving compounds color loss

• a.k.a. ‘JPEG Artifacts’ – Good for photos, bad for text or graphics

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JPEG vs TIFF

• Original TIFF File – 5036 x 8326 Pixels – 600 dpi – 25,667 KB

• Large JPEG – 5036 x 8326 Pixels – 600 dpi – 15,360 KB

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• Medium JPEG – 5036 x 8326 Pixels – 600 dpi – 3,146 KB

• Small JPEG – 5036 x 8326 Pixels – 600 dpi – 1,567 KB

JPEG Artifacts

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Loss of: • True Color • Correct Intensity • Resolution

GIF

• Compression: LZW • Color Depth: 1,2,3,4,5,6,7,8 bit • Color Indexing: Yes • Transparency: Yes • Metadata: Yes • Comments

– LZW compression had licensing/legal issues that limited its widespread use

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PNG

• Compression: Lossless • Color Depth: 1,2,4,8,16,24,32 bit • Color Indexing: Yes • Transparency: Yes • Metadata: Yes • Comments

– Much better choice than JPEG – Some consider it to be nearly as good as TIFF – W3 endorsed replacement for GIF

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JPEG 2000

• Compression: Lossy or Lossless • Color Depth: Color up to 48 bit • Color Indexing: No • Transparency: Yes • Metadata: Yes • Comments

– Popular with governments and institutions – Not yet widely supported in the consumer market – Eventual replacement for TIFF

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Metadata

• Data about something • Digital Metadata is Embedded in each file

Pre-Digital Metadata • Date (Aug 61) • Description

(Hanson – Evans – Tollief’s Fam)

File Format

Image Data

(Pixels)

Camera Data (EXIF)

MetaData

Exchangeable Image File Format

Format Specific (TIFF, JPG, PNG, etc…)

IPTC: International Press Telecommunications Council

• The most widely recognized standard for Metadata

• Established in 1965 by a group of news organizations

IPTC Defined For Many Image File Types

• You can define IPTC metadata for the following image formats: – .tif – .jpg – .png

• … But NOT for these image formats: – .bmp – .gif

Viewing or Printing an Image

• Requires software that understands the file format

• Extracts the data and re-creates the image • Computer monitors and printers present

images in very different ways – This may impact how the image appears to you

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How Big Is That Image?

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• Digital images have no ‘size’ • However, they do have characteristics that

may limit how large you want it to be when it is displayed or printed

This is a stamp

• The original is 1 ¼ inched tall and 1 inch wide • Scanned at 300 dpi

– 300 x 375 dpi

s

1920 x 1080 Resolution

A 1 ¼ inch tall stamp scanned at 300 dots per inch

1280 x 720 Resolution

A 1 ¼ inch tall stamp scanned at 300 dots per inch

800 x 600 Resolution

A 1 ¼ inch tall stamp scanned at 300 dots per inch

Pixels and Colors

• Computer Monitors will always display images with truer colors when compared to most home printers

• This is because each pixel on a monitor can display the entire possible range of colors – A printer can only approximate the color using a

mixture of colors

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Printing Images

• Printers used at home (generally) can only print dots – Black and White printers can only create black

dots (or no dots at all – a.k.a. ‘white’) – Two modes of printing:

• Line Art • Shades of Gray

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Line Art

• Dot for dot representation of the image • Good for text & some (limited) types of

images

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Shades of Gray

02/2015 79 HP 1320 Laser Printer: Prints at 1200 dpi

Shades of Gray

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Grid Sizes

• 6 x 6 – 37 Shades of Gray, reduces resolution by a factor of 6 – 600 dpi -> 100 Line Per Inch (lpi) – 1200 dpi -> 200 lpi

• 7 x 7 – 50 Shades of Gray, reduces resolution by a factor of 7

• 8 x 8 – 65 Shades of Gray, reduces resolution by a factor of 8

• 10 x 10 – 101 Shades of Gray, reduces resolution by a factor of 10

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Printing Color Images

• Printers used at home (generally) can only print dots – Color printers can only print the colors for which

they have ink • Cyan, Magenta, Yellow and Black • Cyan, Magenta, Yellow , Black, light Cyan, Light

Magenta

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Printing Colors

• Colors are created by placing different colors close together – One color image (pixel) requires many printer ink dots – The human eye cannot distinguish the individual

colors and will perceive the mixed color – Called Dithering

• This is why (for example) we need a 1200 dpi printer (ink dots) to print an image at 250 dpi (pixels).

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Printer dpi Specifications

• Printer specifications are real, accurate and meaningful, but are not to be confused with image resolution

• Printer ink dots and image pixels are very different things

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Rule of Thumb

• The effective resolution (image pixels per inch) should be in the range of 150 – 300 dpi (for the actual printed dimensions)

• Sending images at higher resolutions is counter productive – Files are larger – Most of the pixels will be thrown away – May actually decrease the quality of the printed

image

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1 ¼ x 1 inch Original

• Scanned at 300 dpi • OK to print it at its original size

– Effective resolution would be 300 dpi

• Could print it twice as large (2 ½ x 2 inches) – Effective resolution would be 150 dpi

• Larger images would probably not look good – If desired, scan again at higher dpi setting

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Image Resizing

• Scaling • Cropping • Resizing

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Image Resizing: Scaling

• Scaling – Specifying a dpi value to use when printing – Just a numeric value that is saved with the image

and used if/when it is printed – The original image remains unchanged – Defined for TIF and JPG

• Not all applications pay attention to this value

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Scaling

• Stretching or shrinking the image to fit a specified area

• Can be done while scanning or afterwards..

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Scaling: Photoshop 12 Image -> Resize -> Image Size…

Image Resizing: Cropping

• Selected part of the original image • The original image remains unchanged

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Image Resizing: Resizing

• Resizing – Alters the dpi of the image – The original image IS CHANGED

• Best to work on a copy!

– Works better when you DECREASE the dpi value • You can increase, but you are asking the software to

make an educated guess about the pixels it adds

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Resizing: Photoshop 12 Image -> Resize -> Image Size…

Other Modifications

• Orientation (Portrait / Landscape) • Contrast

– Modifying what is considered ‘white’ and what is ‘black’

• Color Balance • File format conversions • Much more…

– Ask Sandra Crowley to give her Image Editing seminar again….

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More on Size and Resolution

• Let’s assume we want to digitize a document that is 8.5 x 11 inches (the entire document) – Using an 8 Megapixel Digital Camera – Using a Flatbed Scanner

• Questions: – What is the best resolution I can obtain using the

camera? – What dpi setting would I need to use on the

scanner to obtain better resolution?

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Best Case

• We want to maximize the resolution of the image by using as many pixels as possible – Use all of the horizontal pixels (2448) – Determine how many vertical pixels we can use…

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3264

2448

Math!

2448 / 8.5 = x / 11 x = ( 11 * 2448) / 8.5 x = 3168 Unused Pixels: (3264 – 3168) * 2048 = 96 * 2048 = 196,608 (2.5%)

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3264

2448 Pixels 8.5 Inches

96

3168 Pixels 11 inches

What is our effective resolution?

• 2448 Pixels / 8.5 inches = 288 Pixels/inch • 3168 Pixels / 11 inches = 288 Pixels/inch

To obtain a higher resolution image using a scanner we could scan it at 300 dpi.

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Another Example

• I have a slide (1 inch High x 1.5 inches Wide) • I want to print a 4 x 6 image • What dpi setting do I need to use on the

scanner to scan the slide?

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Solution

• I want to sent a 4 x 6 image at a density of 300 dpi to the printer.

• A 4 x 6 image is exactly 4 times the size of the slide (they have the same Aspect Ratio) – 1 x 4 = 4 – 1.5 x 4 = 6

• Since I am enlarging the image by a power of 4, I will also be decreasing the resolution by a power of 4 – 300 dpi (printed) = dpi (scanned) / 4 – dpi(scanned) = 4 * 300 = 1200 dpi

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1.5 inches 1800 Pixels

1 inch 1200 Pixels

4 inches 1200 Pixels (300 dpi)

6 inches 1800 Pixels (300 dpi)

What if I wanted an 8 x 10?

• (6 * 1 x 1.5) = 6 x 10 – Not big enough!

• (8 * 1 x 1.5) = 8 x 12 – Too big! – Need to crop 2 inches off the width – Could take 1 inch off each side – Let’s assume that this will work for the image…

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Solution

• We still want an effective density of 300 dpi • Enlarging it by a factor of 8 • 8 * 300 = 2400 dpi • Scan the slide at 2400 dpi, crop to 8 x 10 and

print! – Check:

• ( 1 * 2400 ) / 8 = 300 dpi • ( 1.5 * 2400) / 12 = 3600 / 12 = 300 dpi

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References/Resources

A few scanning tips Wayne Fulton

http://www.scantips.com/

• Invaluable reference! • Also good tips on photography…

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Scanning 101 – The Basics * START - The First Fundamental Concept Video Resolution - How much to scan?

Say No to 72 dpi - It's a false notion Images for television or for PowerPoint

Printing Resolution - Scaling and Resampling Finding the Scaling and Resampling Menus

Printer Resolution - How much to scan? Line art and Threshold - Copy and OCR, Printed Text

Line art and Grayscale Scanning for Fax Descreen to remove Moiré Interference Images in magazines/books/newspapers Interpolated Resolution - 9600 dpi? Photo Resolution - How much can we scan? Image File Formats - Which format? Video Boards Transparent Media Adapter - 35 mm slides? What about Film Scanners? Dynamic Range - 24 bit or 36 bits?

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Wikipedia

Comparison of graphics file formats

https://en.wikipedia.org/wiki/Comparison_of_graphics_file_formats

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Epson Technical Brief

• Discusses Image Quality, Performance and Flexibility of Epson Scanners

• A good overview of technical aspects of scanner design and operation https://files.support.epson.com/pdf/exp16_/exp16_ts.pdf

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Questions?

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Thank You!

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Tony Hanson Webmaster

Technology Special Interest Group Leader

http://www.dallasgenealogy.org