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Image Basics Hao Jiang Computer Science Department Sept. 4, 2014 1 Image Formulation The most common way to obtain an image is from a camera 2 A “Simple” Camera Let’s hold a sensor (a film) in front of the object. Hopefully we will have an image… 3 A “Simple” Camera Unfortunately, at the same image point, light may come from different source points on an object. 4 The Pinhole Camera 5 Camera with Lens 6 The Imaging Model lighting Camera pose, Optical properties Surface property: material, geometry. 7 Images as Surfaces Image can be treated as a 2D function z = f(x, y). Image Profile 9 Sampling To “digitize” the continuous image, we need to sample the image first. Sampling on a grid Sampling problem The image of Barbara Aliasing due to sampling 1 0.8 0.6 Amplitude 0.4 0.2 0 fs = 2.5f -0.2 -0.4 -0.6 -0.8 -1 0 10 20 30 40 50 t 60 70 80 90 100 1 0.8 A new component is added 0.6 Amplitude 0.4 0.2 0 fs = 1.67f -0.2 -0.4 This is denoted as aliasing. Original signal -0.6 -0.8 -1 0 10 20 30 40 50 t 60 70 80 90 100 Image Resolution Sensor: size of the real world scene into a single image pixel. Image: number of Pixels. 14 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 log2N bits. So, what determines the number of bits we need for an image? Image as Matrices 174 167 184 207 213 227 16 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. Binary Image A binary image has only two values (0 or 1). Binary image is quite important in image analysis and object detection applications. Gay Scale Image as a Stack of Binary Images [ b7 b6 b5 b4 b3 b2 b1 b0] MSB LSB Each bit plane is a binary image. Dithering A technique to represent a grayscale image with a binary one. Convert image to 4 levels: I’ = floor(I/64) 0 1 2 3 Color Image r g b 24 bit image Color Table Image with 256 colors b g It is possible to use much less colors to represent a color image r without much degradation. Clusters of colors Gamma Correction Display device’s brightness is not linearly related to the input. I’ = Ig To compensate for the nonlinear distortion we need to raise it to a power again (I’)1/g = I g for CRT is about 2.2. Gamma Correction Linearly increasing intensity without gamma correction Linearly increasing intensity with gamma correction 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) An image contains a header and a bunch of (integer) numbers. 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 Digital Video time Frame N-1 Frame 0 Digital video is digitized version of a 3D function f(x,y,t)