04-S05-ImageProcessing

04-S05-ImageProcessing - Image Processing Lecture 4 CPSC...

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1 Image Processing Image Processing Lecture 4 CPSC 478/578 Spring 2005 Overview ± Image representation What is an image? ± Halftoning and dithering Trade spatial resolution for intensity resolution Reduce visual artifacts due to quantization ± Sampling and reconstruction Key steps in image processing Avoid visual artifacts due to aliasing ± Compositing What is an Image? ± An image is a 2D rectilinear array of pixels Continuous image Digital image What is a pixel? ± A pixel is not … a box a circle a teeny, tiny little light ± A pixel is a point that . .. has no dimension occupies no area cannot be seen can have a coordinate What is an Image? ± An image is a 2D rectilinear array of pixels Continuous image Digital image What is an Image? ± An image is a 2D rectilinear array of pixels Continuous image Digital image A pixel is more than just a point, it is a sample!
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2 More on Samples ± We think of most things in the real world as continuous, yet everything in a computer is discrete ± The process of mapping a continuous function to a discrete one is called sampling ± The process of mapping a continuous variable to a discrete one is called quantization ± When we represent or render an image using a computer we must both sample and quantize Image Acquisition ± Pixels are samples from continuous functions Photoreceptors in eye CCD cells in digital camera Rays in virtual camera Image Display ± Re-create continuous image from samples Example: cathode ray tube Image is reconstructed by displaying pixels with finite area (Gaussian) Image Resolution ± Intensity resolution Each pixel has only “depth” bits for colors/intensities ± Spatial resolution Image has only “width” x “height” pixels ± Temporal resolution Monitor refreshes images at only “rate” Hz Width x Height Depth Rate NTSC 640 x 480 8 30 Workstation 1280 x 1024 24 75 Film 3000 x 2000 12 24 Laser Printer 6600 x 5100 1 - Typical Resolutions Sources of Error ± Intensity quantization Not enough intensity resolution ± Spatial aliasing Not enough spatial resolution ± Temporal aliasing Not enough temporal resolution Overview ± Image representation What is an image? ± Halftoning and dithering Trade spatial resolution for intensity resolution Reduce visual artifacts due to quantization ± Sampling and reconstruction Key steps in image processing Avoid visual artifacts due to aliasing ± Compositing
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3 Quantization ± Artifacts due to limited intensity resolution Frame buffers have limited number of bits per pixel Physical devices have limited dynamic range Quantization and Thresholding ± The process of representing a continuous function with discrete values is called quantization ± The input values that cause the quantization levels to change output values are called thresholds ± When the spacing between the thresholds and the quantization levels is constant the process is called uniform quantization
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This note was uploaded on 11/21/2009 for the course CPSC 478 taught by Professor Hollyrushmeier during the Spring '05 term at Yale.

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04-S05-ImageProcessing - Image Processing Lecture 4 CPSC...

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