sampling_aliasing_image - Recall: a pixel is a point...

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1 Aliasing, Image Sampling and Reconstruction Many of the slides are taken from Thomas Funkhouser course slides and the rest from various sources over the web… Recall: a pixel is a point… • It is NOT a box, disc or teeny wee light • It has no dimension • It occupies no area • It can have a coordinate More than a point, it is a SAMPLE Image Sampling • An image is a 2D rectilinear array of samples o Quantization due to limited intensity resolution o Sampling due to limited spatial and temporal resolution Pixels are infinitely small point samples Imaging devices area sample. • In video camera the CCD array is an area integral over a pixel. • The eye: photoreceptors Intensity, I J. Liang, D. R. Williams and D. Miller, "Supernormal vision and high- resolution retinal imaging through adaptive optics," J. Opt. Soc. Am. A 14, 2884- 2892 (1997) Sampling and Reconstruction Figure 19.9 FvDFH Slide © Rosalee Nerheim-Wolfe Reconstruction artefact
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2 Sampling and Reconstruction Sampling Reconstruction Sources of Error • Intensity quantization o Not enough intensity resolution • Spatial aliasing o Not enough spatial resolution • Temporal aliasing o Not enough temporal resolution ( ) = ) , ( 2 2 ) , ( ) , ( y x y x P y x I E Aliasing (in general) • In general: o Artifacts due to under-sampling or poor reconstruction • Specifically, in graphics: o Spatial aliasing o Temporal aliasing Figure 14.17 FvDFH Under-sampling Sampling & Aliasing • Real world is continuous • The computer world is discrete • Mapping a continuous function to a discrete one is called sampling • Mapping a continuous variable to a discrete one is called quantizaion • To represent or render an image using a computer, we must both sample and quantize Spatial Aliasing • Artifacts due to limited spatial resolution Slide © Rosalee Nerheim-Wolfe Can be a serious problem…
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3 Spatial Aliasing • Artifacts due to limited spatial resolution “Jaggies” Temporal Aliasing • Artifacts due to limited temporal resolution o Strobing o Flickering Temporal Aliasing • Artifacts due to limited temporal resolution o Strobing o Flickering Temporal Aliasing • Artifacts due to limited temporal resolution o Strobing o Flickering Temporal Aliasing • Artifacts due to limited temporal resolution o Strobing o Flickering The raster aliasing effect – removal is called antialiasing Images by Don Mitchell Staircasing or Jaggies
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4 Nearest neighbor sampling Filtered Texture: Blurring doesn’t work well. Removed the jaggies , but also all the detail ! Reduction in resolution Unweighted Area Sampling Weighted Area Sampling …with Overlap
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5 Sampling and Reconstruction Figure 19.9 FvDFH Antialiasing • Sample at higher rate o Not always possible o Doesn’t always solve problem • Pre-filter to form bandlimited signal o Form bandlimited function (low-pass filter) o Trades aliasing for blurring Must consider sampling theory!
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This note was uploaded on 12/04/2011 for the course TRADE 1056596 taught by Professor Ayladinç during the Spring '11 term at Istanbul Bilgi University.

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sampling_aliasing_image - Recall: a pixel is a point...

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