Basics of MATLAB and Beyond

ans(:,:,2) =999And to sum over “pages”:>> sum(a,3)ans =4 5 67 8 910 11 12Note that sum(a) is equal to sum(a,1). The sum over “pages” gives a3 × 3 matrix, which is the same as a 3 × 3 × 1 matrix.The sum function and other functions that operate on vectors, likemean, diff, max, and so on, work as you might expect them to for multidimensionalarrays. By default they usually operate on the first nonsingletondimension of the array. Many functions that operate on twodimensionalmatrices do not have such straightforward multidimensionalextensions. For example, if we try to take the transpose of our matrix:>> a’??? Error using = = > ’Transpose on ND array is not defined.The transpose operation (exchanging rows and columns) makes no sensehere because it is insufficiently specified. (If you want to rearrange amultidimensional array’s dimensional ordering, use the permute function;in our example, try permute(a,[2 1 3])). Another example isthe eigenvalue operator eig, which has no mathematical meaning formultidimensional arrays. In fact, none of the functions that appear ifyou type help matfun has a reasonable meaning for multidimensionalmatrices. Nor do the matrix operators *, ^, \ or /.29.3 RGB ImagesIntroduction to RGB ImagesRGB images in matlab are M ×N ×3 matrices consisting of red, green,and blue intensity maps. When such a three-dimensional matrix is usedas an input to the image command, matlab adds the red, green, andblue intensities to give the right colours on the screen. To illustrate theidea, our first example reproduces three overlapped discs of red, green,and blue light to give yellow, cyan, magenta, and white overlaps. Wegenerate matrices of (x, y) points covering the plane from −2 to2:[x,y] = meshgrid(linspace(-2,2,200));c○ 2000 by CRC Press LLC