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Dean Pentcheff

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Dean Pentcheff – PowerPoint PPT presentation

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Title: Dean Pentcheff


1
  • Dean Pentcheff
  • NHMLAC MBPC/Crustacea
  • 17 April 2006

2
Digital Imaging
  • Image types (realworld, bitmap, vector)
  • Bitmap specifics
  • Vector specifics
  • Guidelines

3
Digital Imaging
  • Image types (realworld, bitmap, vector)
  • Bitmap specifics
  • Vector specifics
  • Guidelines

4
Image types
  • Realworld what you see around you
  • Bitmap grid of pixels (GIF, JPG, PNG, )
  • Vector list of curves (AI, Postscript, PDF, )

5
Image types Realworld
  • Continuous resolution
  • Infinitely zoomable
  • No limits on rotation

6
Image types Bitmap
  • Grid of pixels
  • Scaling is limited
  • Examples
  • JPG/JPEG
  • GIF
  • PNG
  • BMP
  • TIF/TIFF

7
Image types Vector
  • List of curve and shape postions
  • Infinitely scalable
  • Examples
  • Adobe Illustrator
  • Postscript
  • PDF

8
Digital Imaging
  • Image types (realworld, bitmap, vector)
  • Bitmap specifics
  • Vector specifics
  • Guidelines

9
Bitmaps Grid of pixels
A bitmap is a list of colors, each corresponding
to a dot on a grid
10
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
11
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

12
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?

13
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

14
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?

15
Bitmaps Scaling example
16
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

17
Bitmaps Rotation example
  • Similarly small rotation 3 degrees
  • Same situation new pixels have to be synthesized
    by interpolation from the original pixels

18
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

19
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

20
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

21
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

22
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)

23
Bitmaps Storage and compression
  • Example of JPG over-compression

24
Bitmaps Image sources
  • Original (digital) photographs and SEMs
  • Photos and SEMs scanned from printed material
  • Original digital art (Photoshop-created)

25
Bitmaps Original photos SEMs
  • Recall the scaling/rotating issues
  • Since the target is (usually) a bitmap, theres
    not too much to be said here

26
Bitmaps Scanned photos SEMs
  • Printing does violence to images
  • When scanning, we need to recover the original
    qualities of a digital image

27
Bitmaps Scanned photos SEMs
28
Bitmaps Scanned photos SEMs
29
Bitmaps Scanned photos SEMs
  • Naïve rescaling leads to trouble

Moiré distortion
original very close up
30
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

31
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
32
Bitmaps Scanned photos SEMs
Naïve rescaling
Better rescaling
33
Bitmaps Original digital art
  • Created in Photoshop or similar program
  • Like original photos, observe scaling issues

34
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

35
Digital Imaging
  • Image types (realworld, bitmap, vector)
  • Bitmap specifics
  • Vector specifics
  • Guidelines

36
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

37
Vector images Lines and shapes
38
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

39
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

40
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

41
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

42
Vector images Rendering
43
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.

44
Digital Imaging
  • Image types (realworld, bitmap, vector)
  • Bitmap specifics
  • Vector specifics
  • Guidelines

45
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

46
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

47
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

48
  • Questions problems comments answers

49
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