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COMP 9517 Computer Vision

Explore the fundamentals of digital images, from construction to resolution, including color, projection, devices, and functions. Learn about sampling, quantization, spatial resolution, and coordinate systems. Gain insight into how computers interpret images and add graphics. Understand the crucial components that define the quality and perception of digital images.

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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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