Dean Pentcheff

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• Dean Pentcheff
• NHMLAC MBPC/Crustacea
• 17 April 2006

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Digital Imaging

• Image types (realworld, bitmap, vector)
• Bitmap specifics
• Vector specifics
• Guidelines

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Digital Imaging

• Image types (realworld, bitmap, vector)
• Bitmap specifics
• Vector specifics
• Guidelines

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Image types

• Realworld what you see around you
• Bitmap grid of pixels (GIF, JPG, PNG, )
• Vector list of curves (AI, Postscript, PDF, )

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Image types Realworld

• Continuous resolution
• Infinitely zoomable
• No limits on rotation

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Image types Bitmap

• Grid of pixels
• Scaling is limited
• Examples
• JPG/JPEG
• GIF
• PNG
• BMP
• TIF/TIFF

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Image types Vector

• List of curve and shape postions
• Infinitely scalable
• Examples
• Adobe Illustrator
• Postscript
• PDF

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Digital Imaging

• Image types (realworld, bitmap, vector)
• Bitmap specifics
• Vector specifics
• Guidelines

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Bitmaps Grid of pixels
A bitmap is a list of colors, each corresponding
to a dot on a grid
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Bitmaps Pixel dimensions

• Bitmaps are best considered as a grid of dots
  (pixels)
• Every bitmap has a width (X dimension) and length
  (Y dimension)
• A bitmap has no intrinsic size (except in pixels)
• So what is resolution?

width 30 pixels
height 22 pixels
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Bitmaps Resolution and DPI

• How big is it on the screen or on the page?
• how many pixels per inch (dots per inch DPI)?
• Sizes
• screens 72 pixels per inch (DPI)
• printing 300 1200 pixels per inch (DPI)
• But remember you have a limited bag of
  pixels size only matters at final rendering

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Bitmaps Resolution and DPI

• Example image 30 pixels wide x 22 pixels high
• how wide will that be on screen (72 dots per
  inch)? width of image x ( 1 inch / 72
  dots) 30 pixels x ( 1 inch / 72 pixels
  ) 0.4 inches
• And the result is?

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Bitmaps Resolution and DPI

• What about making it bigger with a lower DPI?
• example put it on a billboard
• how wide will that be (10 dots per inch)? width
  of image x ( 1 inch / 10 dots) 30 pixels x
  ( 1 inch / 10 pixels ) 3 inches

• Notice that theres no more information

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Bitmaps Scaling and rotation

• Implications of the grid
• scaling a picture means sampling the image and
  synthesizing a new grid of pixels
• rotation similarly means sampling and
  resynthesizing the grid
• Why does the grid of pixels have to be sampled
  and resynthesized?

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Bitmaps Scaling example
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Bitmaps Scaling example

• Minor rescale 29 pixels wide (from 30 pixels
  original)

• To rescale, new pixels have to by synthesized by
  interpolation from the original pixels

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Bitmaps Rotation example

• Similarly small rotation 3 degrees

• Same situation new pixels have to be synthesized
  by interpolation from the original pixels

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Bitmaps Scaling and rotation

• Scaled or rotated images will be blurrier
• Avoid rescaling/rotating if you can
• If you have to do it, do it after any other
  manipulations or image enhancements

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Bitmaps Color formats

• Common versions of bitmaps
• RGB (red, green, blue) color monitors/Web
• CMYK (cyan, magenta, yellow, black) commercial
  printing
• How are colors represented?
• looking at RGB

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Bitmaps RGB colors

• Usually 24-bit color
• 3 numbers (one each for red, green, blue)
• each between 0-25510 (8 bits)
• Representations
• red,green,blue as decimals 255,0,0 0,0,255
  127,0,127 255,127,0 100,50,0
• rrggbb as hexadecimals ff0000 0000ff 800080
  ff8000 643200

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Bitmaps Color modes

• Truecolor
• 24 bits per pixel
• Indexed
• 8 or 12 bits per pixel, which select one of a
  limited palette of colors
• Grayscale
• Similar to color, but only one channel

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Bitmaps Storage and compression

• On disk, bitmaps are a string of numbers
• To save space, compression is used
• Lossless compression every pixel preserved
• PNG, GIF, TIFF
• Lossy compression some information lost
• JPG (efficient and effective for photographic
  images only not for line art)

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Bitmaps Storage and compression

• Example of JPG over-compression

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Bitmaps Image sources

• Original (digital) photographs and SEMs
• Photos and SEMs scanned from printed material
• Original digital art (Photoshop-created)

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Bitmaps Original photos SEMs

• Recall the scaling/rotating issues
• Since the target is (usually) a bitmap, theres
  not too much to be said here

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Bitmaps Scanned photos SEMs

• Printing does violence to images
• When scanning, we need to recover the original
  qualities of a digital image

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Bitmaps Scanned photos SEMs
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Bitmaps Scanned photos SEMs
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Bitmaps Scanned photos SEMs

• Naïve rescaling leads to trouble

Moiré distortion
original very close up
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Bitmaps Scanned photos SEMs

• To avoid Moiré problems
• scan at high resolutions (600 1200 DPI)
• blur the image until the dots just disappear
• scale image down to a reasonable size

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Bitmaps Scanned photos SEMs
Photoshop start
Make sure youre seeing the full resolution on
the screen
Set up Gaussian Blur
Not enough blur
Too much blur
Just about right
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Bitmaps Scanned photos SEMs
Naïve rescaling
Better rescaling
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Bitmaps Original digital art

• Created in Photoshop or similar program
• Like original photos, observe scaling issues

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Bitmaps Summarizing

• Bitmaps are a bag of pixels with no intrinsic
  size(dots per inch and resolution matter
  only to a final output device)
• Scaling and rotating mean resynthesizing the grid
  of pixels (hence blurring)
• Scanned photographs need special care

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Digital Imaging

• Image types (realworld, bitmap, vector)
• Bitmap specifics
• Vector specifics
• Guidelines

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Vector images Lines and shapes

• Reminder vector images are descriptions of line
  and shape positions and sizes
• Hence they can be scaled to any size with no loss
  of information or resampling
• ExamplesAdobe Illustrator, Postscript, PDF

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Vector images Lines and shapes
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Vector images scaling rotating

• There is no loss of information or resynthesizing
  when a vector image is scaled or rotated
• Programs may save a target size or page size
  with the vectors but that doesnt constrain
  scaling

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Vector images With bitmaps

• Vector graphic formats can contain bitmaps (e.g.
  photos in a PDF or a scan of a microscope sketch
  in Illustrator)
• Those contained bitmaps have all the properties
  of bitmap images rescaling rotating imply
  resynthesizing the grid of pixels

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Vector images Storage

• .8 setgray
• clippath fill
• -110 -300 translate
• 1.1 dup scale
• 0 g
• 0 G
• 0 i
• 0 J
• 0 j
• 0.172 w
• 10 M
• 0 d
• 0 0 0 0 k
• 177.696 715.715 m
• 177.797 713.821 176.973 713.84 v
• 176.149 713.859 159.695 761.934 139.167 759.691 C
• 156.95 767.044 177.696 715.715 V

• Always lossless
• Numbers and shape descriptions
• Very compact, vs. bitmaps

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Vector images Rendering

• Vector images are descriptions of lines
• Display devices are bitmapped (screens,
  printers, etc.)
• Converting the geometric descriptions into an
  output bitmap is rendering

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Vector images Rendering
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Vector images Rendering

• How rendering is done depends on the output
  device
• resolution screen 72 DPI, printer 3001200
• size number of pixels size x DPI
• color or BW
• Hence rendering may differ for printed paper,
  screen, PDF, etc.

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Digital Imaging

• Image types (realworld, bitmap, vector)
• Bitmap specifics
• Vector specifics
• Guidelines

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Guidelines

• Original digital photos and SEMs
• Save original highest-resolution copy
• Manipulate (contrast-enhance, etc.) first, then
  scale or rotate
• Use JPG at quality 8 / High for most purposes

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Guidelines

• Scanned photos and SEMs
• scan at fairly high resolution (6001200 DPI)
• use Photoshop to blur just until print-dots
  disappear
• save that original as PNG for archival
  purposes
• save as JPG (quality 8/High) for most usage

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Guidelines

• Bitmap sizes
• monitors and laptops are about this size small
  1024 wide x 768 high large 1680 wide x 1050
  high
• so to cover 1/3 to 1/2 screen width make
  bitmaps about 300600 pixels wide
• printers are 3001200 DPI and 8.5 inches wide
• print software will rescale images
• for good resolution, use bitmaps 1000 pixels
  wide

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• Questions problems comments answers

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