Chapter 2 Computer Imaging Systems

Content Computer Imaging Systems

Stockman MSU/CSE Fall 2009 Critical Issues Sensing: how do sensors obtain images of the world? Information/features: how do we obtain color, texture, shape, motion, etc.? Representations: what representations should/does a computer [or brain] use? Algorithms: what algorithms process image information and construct scene descriptions?

Imaging Systems Overview 2 primary component – hardware & software Hardware – image acquisition subsystem, computer, display devices Software - allows us to manipulate the image and perform any desired processing on the image data and also use software to control the image acquisition and storage process.

Digital camera interfaced with USB, FireWire, Camera Link or Gigabit Ethernet The computer system may be a general- purpose computer with a frame grabber, or image digitizer, board in it. The frame grabber is a special-purpose piece of hardware that accepts a standard video signal and outputs an image in the form that a computer can understand. This form is called a digital image. Imaging Systems Overview

The process of transforming a standard video signal into a digital image is called digitization. This transformation is necessary because the standard video signal is in analog (continuous) form, and the computer requires a digitized or sampled version of that continuous signal. A typical video signal contains frames of video information, where each frame corresponds to a full screen of visual information. Imaging Systems Overview

Each frame may then be broken down into fields, and each field consists of lines of video information In Figure 2.1-2a, the typical image on a display device, where the solid lines represent one field of information and the dotted lines represent the other field. These two fields make up one frame of visual information. This two-fields-per-frame model is referred to as interlaced video. Some types of video signals, called noninterlaced video, have only one field per frame - used in computer monitors. Imaging Systems Overview

Tell the display hw to start new line

In Figure 2.1-2b we see the electrical signal that corresponds to one line of video information. Note the horizontal synch pulse between each line of information; this synchronization pulse tells the display hardware to start a new line. After one frame has been displayed, a longer synchronization pulse called the vertical synch pulse, tells the display hardware to start a new field or frame. Imaging Systems Overview

The analog video signal is turned into a digital image by sampling the continuous signal at a fixed rate. In Figure we see one line of a video signal being sampled (digitized) by instantaneously measuring the voltage of the signal at fixed intervals in time. The value of the voltage at each instant is converted into a number that is stored, corresponding 0 the brightness of the image at that point. Note that the image brightness at a point depends on both the intrinsic properties of the object and the lighting conditions in the scene. When this process has been completed for an entire frame of video information, we have "grabbed" a frame, and the computer can store it and process it as a digital image. Imaging Systems Overview

The image can now be accessed as a two- dimensional array of data, where each data point is referred to as a pixel (picture element). For digital images we will use the following notation: I (r,c) = the brightness ofthe image at the point (r,c) where r = row and c = column Imaging Systems Overview

When we have the data in digital form, we can use the software to process the data. This processing can be illustrated in a hierarchical manner, as seen in Figure – At the very lowest level we deal with the individual pixels, where we may perform some low-level preprocessing. – The next level up is the neighborhood, which typically consists of a single pixel and the surrounding pixels and we may continue to perform some preprocessing operations at this level. – As we continue to go up the pyramid, we get higher and higher levels of image representations and, consequently, a reduction in the amount of data. – All the types of operations and image representations in Figure will be explored in the following chapters. Imaging Systems Overview