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1 Image Basics Hao Jiang Computer Science Department Sept. 4, 2014
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Image Formulation The most common way to obtain an image is from a camera 2
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A “Simple” Camera 3 Let’s hold a sensor (a film) in front of the object. Hopefully we will have an image…
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A “Simple” Camera 4 Unfortunately, at the same image point, light may come from different source points on an object.
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The Pinhole Camera 5
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Camera with Lens 6
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The Imaging Model 7 lighting Surface property: material, geometry. Camera pose, Optical properties
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Images as Surfaces Image can be treated as a 2D function z = f(x, y).
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Image Profile 9
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Sampling To “digitize” the continuous image, we need to sample the image first. Sampling on a grid Sampling problem
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The image of Barbara
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Aliasing due to sampling
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fs = 2.5f fs = 1.67f Original signal A new component is added This is denoted as aliasing.
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Image Resolution Sensor: size of the real world scene into a single image pixel. Image: number of Pixels. 14
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Digitization The samples are continuous and have infinite number of possible values. The digitization process approximates these values with a fixed number of numbers. To represent N numbers, we need log 2 N bits. So, what determines the number of bits we need for an image?
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Image as Matrices 16 174 167 184 207 213 227
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Types of Digital Images Grayscale image Usually we use 256 levels for each pixel. Thus we need 8bits to represent a pixel (2^8 == 256) Some images use more bits per pixel, for example MRI images could use 16bits / pixel. A 8bit grayscale Image.
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Binary Image A binary image has only two values (0 or 1). Binary image is quite important in image analysis and object detection applications.
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Gay Scale Image as a Stack of Binary Images [ b7 b6 b5 b4 b3 b2 b1 b0] MSBLSB Each bit plane is a binary image.
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Dithering A technique to represent a grayscale image with a binary one. 0 1 2 3 Convert image to 4 levels: I’ = floor(I/64)
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Color Image r g b 24 bit image
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Color Table Image with 256 colors r g b Clusters of colors It is possible to use much less colors to represent a color image without much degradation.
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Gamma Correction Display device’s brightness is not linearly related to the input. I’ = I To compensate for the nonlinear distortion we need to raise it to a power again (I’) 1/ = I for CRT is about 2.2.
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Gamma Correction Linearly increasing intensity without gamma correction Linearly increasing intensity with gamma correction
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Image File Formats An image in “ppm” format: P6: (this is a ppm image) Resolution: 512x512 Depth: 0-255 (8bits per pixel in each channel)
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An image contains a header and a bunch of (integer) numbers.
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Image Compression and Encoding Raw image takes a lot of space. Compute the file sizes of a raw image that has resolution 512x512 in true color. BMP, PPM, TXT Images can be “compressed” losslessly or lossly Lossy image format: JPEG Losslessly compressed image format: PNG Compression ratio and bit rate 27
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Digital Video Frame N-1 Frame 0 time Digital video is digitized version of a 3D function f(x,y,t)
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