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Dean Pentcheff NHMLAC MBPC/Crustacea 17 April 2006

Dean Pentcheff NHMLAC MBPC/Crustacea 17 April 2006. Digital Imaging. Image types (realworld, bitmap, vector) Bitmap specifics Vector specifics Guidelines. Digital Imaging. Image types (realworld, bitmap, vector) Bitmap specifics Vector specifics Guidelines. Image types.

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Dean Pentcheff NHMLAC MBPC/Crustacea 17 April 2006

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  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 pixelssize 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 there’s 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 0,0,255127,0,127 255,127,0100,50,0 • #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, there’s 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: original very close up Moiré distortion

  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 you’re 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 • Examples:Adobe 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 doesn’t 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. Always lossless Numbers and shape descriptions Very compact, vs. bitmaps .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 b 181.226 718.738 m 180.677 716.922 179.908 717.221 v 179.14 717.519 180.023 768.325 159.957 773.199 C 179.18 774.063 181.226 718.738 V b Vector images: Storage

  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 ≈ 300–1200 • size number of pixels = size x DPI • color or B&W • 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 (600–1200 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 highlarge: 1680 wide x 1050 high • so to cover 1/3 to 1/2 screen width: make bitmaps about 300–600 pixels wide • printers are 300–1200 DPI and 8.5 inches wide • print software will rescale images • for good resolution, use bitmaps ≥ 1000 pixels wide

  48. Questions problems comments answers…

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