Dean Pentcheff NHMLAC MBPC/Crustacea 17 April 2006
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Transcript of Dean Pentcheff NHMLAC MBPC/Crustacea 17 April 2006
Dean PentcheffNHMLAC 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
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
Bitmaps: Scanned photos & SEMs
Bitmaps: Scanned photos & SEMs
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
resolutio
n on the
screen
Set up
Gaussian
Blur
Not enough blur Too much blur Just about right
Bitmaps: Scanned photos & SEMs
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
• 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 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
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
• Image types (realworld, bitmap, vector)
• Bitmap specifics• Vector specifics• Guidelines
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…