Foveon Sensor • Bayer Filter Means Need to Interpolate Colours

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Foveon Sensor Bayer Filter means need to interpolate colours (demosaicing) This results in colour error, especially at edges Real resolution is ¼ of the pixel count As with film could you get one pixel to measure all 3 Foveon Sensor developed by Richard Mierrill in 1997 Concept is based on depth of wavelength absorption short wavelengths (blue) absorbed shallow depth ,1 um Long wavelengths deep depth (5 um) Create a vertical pixel Implemented by Sigma as the Foveon X3 sensor How Foveon Works Have 3 separate photodiodes Top Blue – read out blue (<1 um) Middle green Bottom Red (5 um) Each pixel has separate readout Foveon Accuracy/Problems Foveon implemented in Sigma X3 /SD15 Much better resolution for color Higher color accuracy/no edge effects Problem is fab is more difficult Makes chips yield/price higher Demosaicing Methods There is no exact way to interpolate the colors All methods use interpolation from neighboring area Great if uniform color But fails at edges/complex image 3 methods Simple interpolation Statistical Adaptive Each camera manufacture uses their own method Also methods affect the color balance method Bilinear Interpolation Simplest linear demosaicing method estimation of the missing color is based on the neighbouring pixels Use the same color channels G2 G4 G6 G8 Green: G5 4 R R R1 R3 1 7 R2 R4 2 ; 2 ; Red, Blue: R1 R3 R7 R9 R5 4 Bayer Color mask Green red and blue pixels Fast – low process cost it suffers poor image quality and moiré effect as well Bilinear Problems Bilinear spread effects Consider a single hot pixel on black background Spread color causes problem Made worse by JPEG compression (a) TIFF (b) JPEG 9 (c) JPEG 6 (d) JPEG 3 Bilinear demosaic image for red defect with IOffset = 0.8. Worse With 2 bright pixels 2 hot pixels close get overlap effects Tiff Tiff histogram JPEG 12 (low loss) JPEG 12 histogram JPEG 7 (medium loss) JPEG 7 histogram JPEG 4 (high loss) JPEG 4 histogram Figure 6:Two Red hot pixels in 5x5 square on the diagonal with uniform green background with bilinear demosaicing in(tiff), JPEG levels 12 (low), 7 (medium) and 4 (high loss). with histogram shows color spread .

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