SCSC 585 Imaging and Image Representation

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SCSC 585 Imaging and Image Representation

• Sebastian van Delden
• USC Upstate
• svandelden_at_uscupstate.edu

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Sensing Light

• Receptors in human eye sensitive to radiation
  (light) with wave lengths ranging from about 400
  nanometers (violet) to 800 nanometers (red).

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Simple Model of Common Photography

• Light reaches
• surfaces in 3D
• Surfaces reflect
• Sensor element
• receives light energy
• Intensity counts
• Angles count
• Material counts

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Imaging Devices

• CCD Camera
• Charge-coupled Device (CCD)
• Instead of chemical film that reacts to light
  (like 35mm film), tiny solid cells convert light
  energy into electrical charge.

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CCD Cameras

• Camera inputs
• to frame buffer
• a high speed
• image store
• which can
• store several
• images

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Image Formation

• Each point of 3D scene is projected through the
  center of projection or lens center onto the
  image plane.
• Pin-Hole camera model
• Lenses
• Light Collector light reaches image point from
  entire cone of rays
• Circle of Confusion spot on image plane is
  blurred.
• Each discrete CCD sensor receives light from
  several neighboring 3D points.

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CCD Arrays

• Usually about 1 cm x 1 cm 640 x 480 pixels or
  512 x 512 pixels actual width 0.001 inch.
• Other ways to position sensor cells

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Video Cameras

• Sequences of images at 30 frames per second
• 60 half frames per second are used all odd rows
  followed by all even rows
• MPEG (Motion Picture Expert Group) compression
  scheme (based on spatial and temporal redundancy)
  to reduce the amount of data that would need to
  be transmitted
• Consider a 640x480 CCD array where each sensor
  stores 1 byte. 30 frames per seconds equals
  9216000 bytes almost 1 GB per second.

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The Human Eye

• 100M sensing elts in retina
• Rods sense intensity
• Cones sense color
• Fovea has tightly packed elts, more cones
• Periphery has more rods
• Focal length is about 20mm
• Pupil/iris controls light entry

• Eye scans, or saccades to image details on fovea
• 100M sensing cells funnel to 1M optic nerve
  connections to the brain

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Problem with Digital Images

• Blooming

• Difficult to insulate adjacent sensing elements.
• Charge often leaks from hot cells to neighbors,
  making bright regions larger.

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Problem with Digital Images

• Clipping or Wraparound

• Dark grid intersections at left were actually
  brightest of scene.
• In A/D conversion the bright values were clipped
  to lower values.

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Problem with Digital Images

• Lens distortion distorts image

• Barrel distortion of rectangular grid is common
  for cheap lenses (50)
• Precision lenses can cost 1000 or more.
• Zoom lenses often show severe distortion.

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Problem with Digital Images

• CCD Variations
• CCD sensors imperfections can cause different
  reading for the same light intensity.
• Chromatic Distortion
• Different light wavelength bent differently.
• Quantization Effects
• Mixing and Rounding problems.

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Resolution

• Nominal Resolution of a CCD sensor is the size of
  the scene that images to a pixel.
• Resolution
• Pixels per unit length (nominal resolution)
• Another definition number of pixels captured.
• Field of View (FOV) size of scene that can be
  captured often denoted with angles.

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Resolution cont

• Resolution decreases by one half in cases at left
• Human faces can be recognized at 64 x 64 pixels
  per face

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Spatial Quantization Problems

• Both pixel size relative position make a
  difference.
• Mixed pixel represents a mixture of intensity
  values in a real scene.
• Small features can be lost or blended.

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Digital Image Formats

• Portable Gray Map (PGM/PBM/PPM) older form
• GIF was early commercial version
• JPEG (JPG) is modern version
• Many others exist
• Do they handle color?
• Do they provide for compression?
• Need to have size parameters pixels

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Portable Bit Map Image (PGM)

• P2 means
• ASCII gray
• Comments
• W16 H8
• 192 is max
• intensity
• Can be made
• with editor

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Compression

• Lossless if a decompression method exists to
  precisely recover the original image.
• Lossy information is lost during compression
  and cannot to recovered during decompression
  (JPG, GIF)
• GIF (Graphics Interchange Format) only 8 bits
  used for color can contain transparency and
  animation.
• JPG (Joint Photographic Experts Group) for high
  quality images considers human vision systems
  and uses discrete cosine transform and Huffman
  coding.

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Run-Length Encoding

• Label consecutive runs of like value
• Consider a binary image row
• Image Row 00000000111110001111000000111
• Run Code A 8(0)5(1)3(0)4(1)6(0)3(1)
• Run Code B (8,12) (16,19) (26,28)

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3D Structure from 2D Images

• Interposition if object A is in front of object
  B then object is closer to the camera than object
  B
• Vanishing point - Where parallel lines meet in a
  image.
• Foreshortening closer objects appear larger
  than farther away ones.
• Texture Gradient texture of an object changes
  with distance.

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Five frames of reference
W world O object (P, B) C camera F real
image I pixel image
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Pixel Coordinate Frame (I)

• In the pixel array, each point has integer pixel
  coordinates.
• Pixel coordinate frame can be useful in obtaining
  information about the object such as texture,
  color, intensity etc.
• Pixel coordinates however cannot to be used to
  obtain size of objects and depth information.

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Object coordinate frame (O)

• An object coordinate frame is used to model ideal
  objects for computer graphics and computer
  vision.
• The coordinates of 3D corner point B relative to
  the object frame are xb,0,zb.
• The object coordinate frame is needed to inspect
  an object, for example, to check if a particular
  hole relatively in the correct place.

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Camera coordinate frame (C)

• The camera coordinate frame C is often needed for
  egocentric view (cameracentric).
• Helps represent whether the object is in front of
  the camera or moving etc.
• Computer graphics systems allow the user to
  select different camera views of the 3D scene
  being viewed.

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Real image coordinate frame (F)

• 3D points project to the real image plane at
  coordinates xf,yf,f, where f is the focal
  length. xf,yf are not subscripts of the pixels in
  the image array but relate to the pixel size and
  pixel position along the optical axis. xf,yf are
  based on the world coordinate system.
• Frame F contains the picture function that is
  digitized to form the digital image I.

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World coordinate frame (W)

• The world coordinate frame is needed to relate
  objects in 3D.
• Humans robots etc operate based on the world
  coordinate system.