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COMP 9517 Computer Vision

COMP 9517 Computer Vision. Digital Images. Overview of Digital Images. Humans derive a great deal of information about the world through their visual sense – eyes. Three components for construction of images: A scene of objects Illumination of the objects Sensing the illumination.

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COMP 9517 Computer Vision

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  1. COMP 9517 Computer Vision Digital Images COMP 9517 S2, 2009

  2. Overview of Digital Images • Humans derive a great deal of information about the world through their visual sense – eyes. • Three components for construction of images: • A scene of objects • Illumination of the objects • Sensing the illumination COMP 9517 S2, 2009

  3. Overview of Digital Image • 2D digital images is an array of intensity samples reflected from or transmitted through objects • Digital images contain fixed number of rows and columns of Pixels • Pixels (picture elements) are little tiles holding quantised values (0-255) represent the brightness at the points of the image • Colour images have three values for each pixel (for example, RGB) COMP 9517 S2, 2009

  4. Overview of Digital Image 159 3 band for colour image 159 159 COMP 9517 S2, 2009

  5. Digital Images - 2D Projection of 3D • 3D world has color, texture, surfaces, volumes, light sources, objects, motion, connections, etc. • 2D image is a projection of a scene from a specific viewpoint; many 3D features are captured, but some missed. COMP 9517 S2, 2009

  6. Image Receives Reflections • Light reaches surfaces of objects • Surfaces reflect • Camera receives light energy COMP 9517 S2, 2009

  7. Radiation • Different types of electromagnetic radiation, such as X-ray, infra-red • Different wavelengths of radiation have different properties • Different devices to detect different radiation COMP 9517 S2, 2009

  8. Image Devices • CCD (charge-coupled device) cameras • Lens collects light rays • Cells (array of small fixed elements) convert light energy into electrical charge • Through frame grabber or IEEE 1394 to PC COMP 9517 S2, 2009

  9. Computer Vision System • Camera inputs to frame buffer • Program can interpret data • Program can add graphics • Program can add imagery COMP 9517 S2, 2009

  10. Image Formation • The geometry of image formation: the projection of each point of the 3D scene through the centre of projection (or lens centre) onto the image plane • Pinhole Camera • Perspective projection • Affine projection COMP 9517 S2, 2009

  11. Perspective Projection • The apparent size of object depends on their distance: far object appear smaller • By similar triangles • Ignore the third coordinate, and get COMP 9517 S2, 2009

  12. Affine Project • Scene depth is small relative to the average distance from the camera • Let magnification to be positive constant, since is negative, i.e. treat all points in scene being at constant distance from camera • Leads to weak perspective projection COMP 9517 S2, 2009

  13. Affine Project • The camera always remains at a roughly constant distance from the scene • Orthographic projection when normalise m to be -1 COMP 9517 S2, 2009

  14. Picture function • A picture function is a mathematical representation f(x,y) of a picture as a function of two spatial variables x and y. • x and y: real values defining points of the picture • f(x,y): real value defining the intensity of point (x,y) COMP 9517 S2, 2009

  15. Picture Function and Digital Images • Analog image: a 2D image F(x,y) has infinite precision in both spatial parameters x, y and intensity at each spatial point (x,y) • Digital image: a 2D image I[r,c] by a discrete 2D array of intensity samples with limited precision • Can be stored in a 2D computer memory structure • 2D array of discrete values. In C, char I[512][512] • Intensity as an 8-bit number allows values of 0-255 • 3 such values for colour image. COMP 9517 S2, 2009

  16. Sampling and Quantisation • Digitisation: convert analog image to digital image • Sampling: digitising the coordinate • spatial discretisation of a picture function f (x,y) • use a grid of sampling points, normally rectangular: image sampled at points x = j Δx, y = k Δy, j = 1...M, k = 1...N. • Δx, Δy called the sampling interval. COMP 9517 S2, 2009

  17. Spatial Resolution • Spatial Resolution: pixels per unit of length • Resolution decreases by one half • Human faces can be recognized at 64 x 64 pixels per face • Appropriate resolution is essential: • too little resolution, poor recognition • too much resolution,slow and wastes memory COMP 9517 S2, 2009

  18. Sampling and Quantisation • Quantisation: digitising the amplitude values • called intensity or gray level quantisation • Gray-level resolution: • usually has 16, 32, 64, ...., 128, 256 levels • number of levels should be high enough for human perception of shading details - human visual system requires about 100 levels for a realistic image. COMP 9517 S2, 2009

  19. Image Coordinate System • Raster oriented: down-leftward (a) • Cartesian coordinate: up-leftward (b, c) • Relationship btn pixel centre point to I[i,j] COMP 9517 S2, 2009

  20. Type of images • Gray-scale image: a monochrome digital image I[r,c] with one intensity value per pixel • Multispectral image: a 2D image M[x,y] has a vector of values at each pixel, colour image (r,g,b) • Binary image: a digital image with all pixel values 0 or 1 • Labelled image: a digital image L[r,c] with pixel values as symbols denoting the decisions made for that pixel COMP 9517 S2, 2009

  21. Digital Image Format • Image file header: non image information for labelling and decoding data • Image data • Data Compression • Lossless: can be recovered exactly • Lossy: may lose quality COMP 9517 S2, 2009

  22. Common Image Format • Run-Coded Binary Image: an efficient coding scheme for binary or labelled images COMP 9517 S2, 2009

  23. Common Image Format • PGM(PBM/PGM, PPM): Portable gray map One of the simplest file formats COMP 9517 S2, 2009

  24. Common Image Format • Gif(GIF): Graphics Interchange Format, WWW, 8 bits – 256 colour levels, may be lossless • Tiff(TIFF/TIF): Tag Image File Format, 1-24 bits, lossy or lossless • Jpeg(JFIF/JFI/JPG): Joint Photographic Experts Group, up to 24 bits, recent standard, independent of colour system, lossy or lossless • PostScript(PDF/PDL/EPS): encoded by ASCII • Mpeg(MPG/MPEG/MPEG-2): Motion Picture Expert Group, stream-oriented encoding of video COMP 9517 S2, 2009

  25. References • Driscoll, W. and Vaughan, W., eds (1978), Handbook of Optics, McGrtaw-Hill. • Boyle, W. and Smith, G. (1970), Charge coupled semiconductor devices, Bell Syst. Tech. J. 49, 587-593. • Huang, T.S.(1965), PCM Picture Transmission. IEEE Spectrum, vol. 2, no.12, pp.57-63. COMP 9517 S2, 2009

  26. Acknowledgement • Some material, including images and tables, were drawn from the textbook and Stockman’s online resources. COMP 9517 S2, 2008

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