Image Representation Last update st March Heejune Ahn, SeoulTech

1. Image Digital Image  Pixels, 2-Dimensional function of sensor value  Origin: top-left, not bottom-left  m (row index, vertical, i.e., y), n (col. Index, horizontal, ie. x) 3-D image  voxels  E.g. medical, 3D scanning I(m, n) at (m, n), m = [1, M], n = [1, N] I(0,n) I(m,0) I(m,n)

Analog vs Digital  Analog Continuous value at continuous locations Used in differential-integral math  Digital Continuous/discrete value at discrete locations Used in linear algebra math I(x, y) at (x, y) I(m, n) at (m, n), m = [1, M], n = [1, N]

2. Image values # of values/channels :1 to many Single value  gray scale  Color map Gray map : [0, max] to [black to white] False color map : [0. max] to “many colors”  Medical, astronomical application  Better recognition (HVS: limited only 40 contrast levels)  E.g.) Jet-color map 3 channels  (R,G,B), (H,S,V) [0, max] Color-map Color display (R, G, B)

2. Resolution Accuracy of data Spatial resolution: # of pixels, e.g. MxN in 2D Temporal resolution: fps (frame/sec) Bit resolution (dynamic range): 1 bit, 8 bits, 24 bits, floating points.

Real image resolution  Representation vs real resolution  How to determine image’s resolution  Useful for real-time implementation & performance

3. Image file format Image file  File header + image values  Types file type, resolution, compression, etc

Image data types  Binary image: value = {0, 1} 0: black, background, 1: white, foreground Often mapped into [0, 255] E.g. Fax, resultant image  Gray-scale (intensity): [0,255]  R,G,B true color: I[m,n, channel] channel=1,2,3  Floating point types scientific & medical image, e.g. TIFF, medical DICOM

4. Color spaces RGB  [min, max] to [0, 1]  RGB to gray-scale I GRAY (n,m) =  I R (n,m) +  I G (n,m) +  I B (n,m) I GRAY (n,m)=0.2989I R (n,m) I G (n,m) I B (n,m) Gray to RGB is impossible (irreversible) HSV perceived luminancephysical power

HSV (perceptual color space)  H (hue) : dominant wavelength  S (saturation) : the purity of color  V (value) : brightness/luminance  Less sensitive to lighting condition than RGB  In Matlab : (0, 1)

5. Image in Matlab “im” prefix  image toolbox function starts with it. Read, write, & Query image files  iminfo, imwrite, imread Basic display  imshow: image data (value range [0,255])  imagesc: auto scaling/color map(any matrix input) accessing pixel value  imview Converting types  rgb2gray, rgb2hsv, hsv2rgb