CS 450: Level operations - Part 1 includes point processing; adding a constant; amplification, linear level operators; negative; thresholding; quantization; logarithm; exponential; contrast enhancement;...
CS 450 Level Operations - Part Point Processing Simplest kind of enhancement: point operations Process each point independently of the others All you can is remap the sample’s value: b = f (b) where • b is the value at the point in question • b is the new value in the processed result • f is a level operation CS 450 Level Operations - Part Adding a Constant Simplest level operation: f (b) = b + c for some constant c c>0 Brighter Image c1 amplifies the signal (louder/more constrast) a T f (b) = otherwise for some intensity threshold T CS 450 Level Operations - Part Quantization Quantization is choosing different finite values to represent each value of a (possibly analog) input signal Quantization is usually monotonic: b1 ≤ b2 implies f (b1 ) ≤ f (b2 ) Quantization can be thought of as multi-level thresholding: q1 if bmin ≤ b < T1 q if T1 ≤ b < T2 q3 if T2 ≤ b < T3 f (b) = qn if Tn−1 ≤ b ≤ bmax CS 450 Level Operations - Part Logarithm Used to consider relative changes b1 /b2 instead of absolute ones b1 − b2 f (b) = log b Useful when the dynamic range is large Examples: • Apparent Brightness • Reichter Scale • Human Vision CS 450 Level Operations - Part Exponential Can be used to “undo” logarithmic processing: f (b) = eb CS 450 Level Operations - Part Contrast Enhancement Any time we use level operations to make one level more distinguishable from another we call it contrast enhancement If number of levels stays fixed, contrast enhancement means trading off decreased contrast in one part of our signal range for increased contrast in a range we’re interested in If we plot our level operation as a function, • all sections where the slope is greater than one increase the contrast in that intensity range • all sections where the slope is less than one diminish the contrast in that intensity range 10 CS 450 Level Operations - Part Windowing Windowing is contrast enhancement of one part of the signal range Example: mapping [0,4095] input to [0,255] display The simplest mapping is f (b) = 256 b = b/64 4096 Supposed that we’re interested mainly in the range [500,2000] Better mapping: if b < 500 f (b) = 256(b − 500)/(2000 − 500) if 500 ≤ b < 2000 255 if b ≥ 2000 Windowing is often continuous piecewise-linear mapping 11 ... Human Vision CS 450 Level Operations - Part Exponential Can be used to “undo” logarithmic processing: f (b) = eb CS 450 Level Operations - Part Contrast Enhancement Any time we use level operations. . .CS 450 Level Operations - Part Adding a Constant Simplest level operation: f (b) = b + c for some constant c c>0 Brighter Image c