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Digital Cameras

RGB Color Filter Array. Digital Cameras. CCD (Monochrome). V( l ). XYZ=[0,1,0] L*a*b*=[9,-39,15] RGB=[0,38,0]. Thus suitable green filter can be an approximation to luminance channel. Color Filter Arrays. RGB Color Filter Array Green is perceptually reasonable achromatic channel

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Digital Cameras

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  1. RGB Color Filter Array Digital Cameras CCD (Monochrome)

  2. V(l) • XYZ=[0,1,0] • L*a*b*=[9,-39,15] • RGB=[0,38,0] • Thus suitable green filter can be an approximation to luminance channel

  3. Color Filter Arrays • RGB Color Filter Array • Green is perceptually reasonable achromatic channel • Hence need more spatial resolution in green, so twice as many green samples as red or blue. • But each sample has implied R,G,B. Calculate what’s not sampled

  4. Color Filter Arrays • RGB Color Filter Array • Green is perceptually reasonable achromatic channel • Demosaic by averaging at intersections or by • interpolation at centers • or by other methods

  5. CFA Demosaic Techniques:Luminance channel • First find appropriate luminance (i.e. green for an RGB CFA) at pixels not sampled by filter. • Linear filtering • simple average of all adjacent green values • Gaussian or other weighted average (See Photoshop) • All blur edges. Instead use edge detection algorithms and average along edges instead of across edges. Requires more computation

  6. CFA Demosaic Techniques:Chrominance channels • Need two chrominance channels at each pixel (or at intersections) • CR = R-G, CB=B-G • At blue and red pixels, already computed a green value in luminance computations, so CR, CB easy CR G CRG CB GCR G CR For green pixels, average adjacent horizontal chrominances to get CR, adjacent vertical to get CB

  7. Digital Cameras:Other issues • Aliasing due to undersampling • White balance to correct for illuminant • Characterization (XYZ of primaries) • Calibration (tables of correction to known color patches, suitable for correction of all colors with linear methods) • Poor demosaic algorithms. See Wandell and Silverstein p. 14.

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