Issue 16

F. R. Renzetti et alii, Frattura ed Integrità Strutturale, 16 (2011) 43-51; DOI: 10.3221/IGF-ESIS.16.05 44 A NALYSIS METHOD he spatial relationship of gray levels is expressed by the co-occurrence matrix suggested by Haralick [3, 4] in the 70’s. GLCM method assumes that the information on the texture image is adequately provided by a particular matrix. The technique is based on the manipulation of the gray levels of an image. If we consider a digital color image (Fig.1) the first step is to convert it in gray tones (Fig.2) using any image viewer. Figure 1 : Digital color image. Figure 2: Gray tones image (256 levels). Digital images are made up of pixels that are stored in memory using 8 bit because it is assumed that 2 8 = 256 gray levels are sufficient to describe the nuances noticeable to the human eye. The gray levels are distinguished with numbers ranging from 0 (black) to 255 (white). Each gray level is stored as a binary number or as a sequence of bits. Each number corresponds to different gray level or different shades of gray level. A particular shade of gray is then associated to each pixel component. The transformation in 256 gray levels is common with any Image Analysis program. Number of gray levels is an important factor in GLCM computation. More levels would mean more accurate textural information, but also an increase in computational costs. A procedure of segmentation reduces the shades of gray. The pictures below show the difference between an image in 64 shades of gray (Fig.3) and one in 8 (Fig.4): Figure 3 : Gray tones image (64 levels) Figure 4 : Gray tones image (8 levels) In this article has been implemented a Matlab® program that carried out the conversion of the used images in shades of gray, and subsequently reduced the levels of them. In particular we will compare a performance at 8 and 64 levels of gray. The goal is to be able to recognize the "objects" that make up the image, i.e. phases, grain boundaries, inclusions, defects, etc. Below the construction of the GLCM [5] matrix is shown with a simple example.