Dean Pentcheff NHMLAC MBPC/Crustacea 17 April 2006

transcript

Dean PentcheffNHMLAC MBPC/Crustacea

17 April 2006

Digital Imaging

• Image types (realworld, bitmap, vector)

• Bitmap specifics• Vector specifics• Guidelines

Digital Imaging

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

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

• 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?

• 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 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

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

• Naïve rescaling leads to trouble:

Moiré distortionoriginal very close up

• 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

Photoshop start

Make sure

you’re

seeing

the full

resolutio

n on the

screen

Set up

Gaussian

Not enough blur Too much blur Just about right

Naïve rescaling Better 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

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 setgrayclippath fill-110 -300 translate1.1 dup scale

0 g0 G0 i0 J0 j0.172 w10 M[]0 d0 0 0 0 k

177.696 715.715 m177.797 713.821 176.973 713.84 v176.149 713.859 159.695 761.934

139.167 759.691 C156.95 767.044 177.696 715.715 Vb181.226 718.738 m180.677 716.922 179.908 717.221 v179.14 717.519 180.023 768.325

159.957 773.199 C179.18 774.063 181.226 718.738 Vb

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

– sizenumber of pixels = size x DPI

– color or B&W

• Hence rendering may differ for printed paper, screen, PDF, etc.

Digital Imaging

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 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

Questions problems comments answers…

Dean Pentcheff NHMLAC MBPC/Crustacea 17 April 2006

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