Digital Cameras CCD (Monochrome) RGB Color Filter Array

V( ) 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 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

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

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

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

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.Wandell and Silverstein