Module 4 (Robot Vision) Image Processing and Analysis with Vision Systems INTRODUCTION In this chapter, we will study and discuss some fundamental techniques for image processing and image analysis, with a few examples of routines developed for certain purposes. IMAGE PROCESSING VERSUS IMAGE ANALYSIS Imaggpe processing : The collection of routines and techniques that improve, simplify, enhance, or otherwise alter an image. Image analysis: The collection of processes in which a captured image that is prepared by image processing is analyzed in order to extract information about the image and to identify objects or facts about the object or its environment. Image Processing and Analysis with Vision Systems TWO-AND THREE-DIMENTIONAL IMAGE • All real scenes are three dimensional, images can either be two or three dimensional. • Three-dimensional image processing deals with operations that require motion detection, depth measurement, remote sensing, relative positioning, and navigation. • All three-dimensional systems share the problem of coping with many- to-one mappings of scenes to image. WHAT IS AN IMAGE? • An image is a representation of a real scene, either in black and white or in color , and either in print form or in a digital form . • An printed image, television and digital images are divided into small sections called picture cells, or pixels(volume cells or voxels), where the size of all pixels are the same, while the intensity of light in each pixel is varied to create the gray images. Image Processing and Analysis with Vision Systems ACQUISITION OF IMAGES • There are two types of vision cameras: analog and digital. Analog Camera are not very common any more, but are still around; they usedtd to b e st and ard at tt te lev is ion st a tion. Digital Camera are much more common and are mostly similar to each other. Vidicon Camera • Vidicon camera is an analog camera that transforms an image into an analog electrical signal. The signal, a variable voltage (or current) versus time, can be stored, digitized, broadcast, or reconstructed into image. Schematic of a vidicon camera. A raster scan depiction of a vidicon camera. Image Processing and Analysis with Vision Systems Digital Camera • Digital Camera is based on solid-state technology. The main part of the camera is a solid-state silicon wafer image area that has hundreds of thousands of extremely small photosensitive areas called photosites printed on it . Image acquisition with a digital camera involves the development, at each pixel location, of a charge proportional to the light at the pixel. The image is then read by moving the charges to optically isolated shift registers and reading them at a known rate. Image data collection model. Image Processing and Analysis with Vision Systems DIGITAL IMAGES • Sampled voltages are first digitized through an analog-to-digital converter (A DC) and then either stored in the computer storage unit in an image format suchTIFFJPGBitth as TIFF, JPG, Bitmap, etc. • An image that has different gray levels at each pixel location is called a gray image. The gray values are digitized by a digitizer, yielding strings of 0’s and 1’s that are subsequently displayed and stored . FREQUENCY DOMAIN VS. SPATIAL DOMAIN • Many processes that are used in image processing and analysis are based on the frequency domain or the spatial domain. • In frequency domain processing, the frequency spectrum of the image is used to alter , analyze , or process the image. • In spatial-domain processing, the process is applied to the individual pixel of the image. Image Processing and Analysis with Vision Systems FOURIER TRANSFORM AND FREQUENCY CONTENT OF A SIGNAL • Any periodic signal may be decomposed into a number of sines and cosines of different amplitudes and frequencies. ∞ ∞ a0 f (t) = + ∑ an cos nϖt + ∑ bn sin nϖt 2 n=1 n=1 • Allhthough th e signal is in th e ampl itud e-time dhfdomain, the frequency spectrum is in the amplitude-frequency domain. Time-domain and frequency-domain plots of a simple sine function. Image Processing and Analysis with Vision Systems Sine functions in the time and frequency domain for a successive set of frequencies. As the number of frequencies increases, the resulting signal becomes closer to a square function. Image Processing and Analysis with Vision Systems FREQUENCY CONTENT OF AN IMAGE: NOISE , EDGES • Consider sequentially plotting the gray values of the pixels of an image against time or pixel location as the image is scanned. • The result will be a discrete time plot of varying amplitudes showing the intensity of light at each pixel. • Both noises and edges are instances in which one pixel value is very different from the neighboring ones. Noise and edge information in an intensity diagram of an image. The pixels with intensities that are much different from the intensities of neighboring pixels can be considered to be edges or noise. Image Processing and Analysis with Vision Systems FREQUENCY CONTENT OF AN IMAGE: NOISE , EDGES • If a high-frequency signal is passed through a low-pass filter, the filter will reduce the influence of all high frequencies, including the noises and edges. • A high pass filter will increase the apparent effect of higher frequencies by severely attenuating the low-frequencies amplitudes. (a) Signal. (b) Discrete signal reconstructed from the Fourier transform of the signal in (a), using only four of the first frequencies in the spectrum. Image Processing and Analysis with Vision Systems SPATIAL-DOMAIN OPERATIONS: CONVOLUTION MASK • Spatial-domain processes access and operate on the information contained in a individual pixel. • Technique in Spatial-domain processes is the convolution mask, which can be adapted to many different tasks, from filtering to edge finding, to ppgpy,hotography, and many more. A convolution mask superimposed on an image can change the image pixel by pixel. Each step consists of superimposing the cells in the mask onto the corresponding pixels, multiplying the values in themask’scells bythepixel values, adding the numbers, and normalizing the result, which is substituted for the pixel in the center of the area of interest. The mask is moved over pixel in the center of the area of interest. The mask is moved over pixel by pp,ixel, and the operation is repeated until the image is completely processed. Image Processing and Analysis with Vision Systems n n M i, j × I n+1 n+1 ∑∑ [(x−( )+i),( y−( )+ j)] (I ) = i=1 j=1 2 2 x,y new S n n S = ∑∑ M i, j i==11j • The normalizing of scaling factor S is arbitrary and is used to prevent saturation of the image. As a result, The user can always adjust tthi this num ber to ge ttht the bes tit image w ithout sa tura tion. Image Processing and Analysis with Vision Systems SAMPLING AND QUANTIZATION • Sampling means as the light intensities are sampled at equally spaced intervals. A larggpger sampling rate will create a larg er number of p ixel data and thus better resolution. • The voltage or charge read at each pixel value is an analog value and must also be digitized. Digitization of the light intensity values at each pixel location is called Quantization. Effect of different sampling rates on an image at (a) 432×576 (b) 108×144 (c) 54×72 (d) 27×36 pixels. As the resolution decreases, the clarity of the image diminishes accordingly . An image quantized at 2, 4, 8, and 44 gray levels, As the quantization resolution increases, the image becomes smoother. Image Processing and Analysis with Vision Systems SAMPLING THEOREM • In order to prevent aliasing, according to what is referred to as the sampling theorem, the samppgling freq uency must be at least twice as larg e as the largest frequency present in the signal. Reconstruction of signals from sampled data. More than one signal may be reconstructed from the same sampled data. The image of Figure 8.15, presented at higher resolutions of (a) (a) Sinusoidal signal with a frequency of f. (b) Amplitudes sampled at the rate of f . 32×32 (b) 64× 64 (c) 256× 256. s Image Processing and Analysis with Vision Systems IMAGE-PROCESSING TECHNIQUES • Image processing techniques are used to enhance, improve, or otherwise alter an image and to prepare it for image analysis . • Image processing is divided into many subprocesses, including, histogram analysis, thresholding, masking, edge detection, segmentation, region growing, and modeling, among others. Image Processing and Analysis with Vision Systems HISTOGRAM OF IMAGES • A histogram is a representation of the total number of pixels of an image at each gray level. Histogram information can help in determining a cutoff point whiitbtfditbilhen an image is to be transformed into binary values. Effect of histogram equalization in improving an image. Image Processing and Analysis with Vision Systems THRESHOLDING • Thresholding is the process of dividing an image into different portions (or levels) by picking a certain grayness level as a threshold, comparing each pixel value with the threshold, and then assigning the pixel to the different portions or levels, depending on whether the pixel’s grayness level is below the threshold (off or zero, or not belonging) or above the threshold (on or 1, or belonging). Thresholding an imagggy()()e with 256 gray levels at values of (b) 100 and (c) 150 Image Processing and Analysis with Vision Systems CONNECTIVITY • Connectivity establishs whether they have the same properties, such as being of the same region, coming from the same object, having a similar texture, etc. • +4-connectivity applies when a pixel p’s relationship is analyzed only with respect to the four pixels immediately above, below, to the left, and to the right of p (b, g, d, e).