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 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. To rescale, new pixels have to by synthesized by interpolation from the original pixels Minor rescale – 29 pixels wide (from 30 pixels original)

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

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 0-255 10 (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, 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

29. Naïve rescaling leads to trouble: Moiré distortionoriginal 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 you’re seeing the full resolution on the screen Set up Gaussian Blur Not enough blurToo much blurJust about right

32. Bitmaps: Scanned photos & SEMs Naïve rescalingBetter 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. Vector images: Storage 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

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. 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. 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 high large: 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…