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

Image types: Realworld Continuous resolution Infinitely zoomable No limits on rotation

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

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

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

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

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

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

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?

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

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?

Bitmaps: Scaling example

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

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

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

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…

Bitmaps: RGB colors Usually 24-bit color –3 numbers (one each for red, green, blue) –each between (8 bits) Representations –red,green,blue as decimals: 255,0,0 0,0, ,0, ,127,0 100,50,0 –#rrggbb as hexadecimals: #ff0000 #0000ff # #ff8000 #643200

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

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)

Bitmaps: Storage and compression Example of JPG over-compression

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

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…

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

Bitmaps: Scanned photos & SEMs

Naïve rescaling leads to trouble: Moiré distortionoriginal very close up

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

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

Bitmaps: Scanned photos & SEMs Naïve rescalingBetter rescaling

Bitmaps: Original digital art Created in Photoshop or similar program Like original photos, observe scaling issues

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

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

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

Vector images: Lines and shapes

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

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

Vector images: Storage Always lossless Numbers and shape descriptions Very compact, vs. bitmaps.8 setgray clippath fill translate 1.1 dup scale 0 g 0 G 0 i 0 J 0 j w 10 M []0 d k m v C V b m v C V b

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”

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.

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

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

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

Questions problems comments answers…