Q: Help processing large arrays?

*To*: mathgroup at smc.vnet.net*Subject*: [mg2563] Q: Help processing large arrays?*From*: robert fuentes <robert at mps.lfwc.lockheed.com>*Date*: Mon, 20 Nov 1995 01:50:21 -0500*Organization*: Lockheed Martin Tactical Aircraft Systems

I'm doing several things in the area of image processing. Generally, I have rather large arrays (e.g., 256 by 256 and larger) that I need to apply some type of operation on. My programming technique is not very mature and I am seeking suggestions. Here is a typical example of my approach. In this case, I have a 381 by 289 image that I need to run a Sobel operator over (i.e., convolve with a 3 by 3 Sobel operator). This involves taking a 3 by 3 array and moving it over the image pixel by pixel, applying the multipliers of the Sobel operator to the corresponding 3 by 3 area of the image and replacing the value of the image with the sum of the operation. Here is my code that I use, it seems to be very slow (680+ seconds on a P90 with 32Mb ram): where image is the input image rows is the number of rows in the image cols is the number of columns in the image kernel is the 3 by 3 Sobel operator ki & kj are the dimensions of the kernel basically, I calculate an adjustment for the edges around the image where the operator will extend beyond the image boundaries, then for each pixel in the image I construct a 3 by 3 array and multiply it by the input kernel and then sum these up and replace the point in the image with the result... BeginPackage["`RoadProc`"] Begin["`Private`"] RPConvolve[image_,rows_,cols_,kernel_,ki_,kj_] := Module[ {iadjust,jadjust,mistart,mistop,mjstart,mjstop, divider,nuimage,i,j,maskarea,istart,jstart,m,n}, iadjust = (ki-1)/2; jadjust = (kj-1)/2; mistart = iadjust+1; mistop = rows - iadjust; mjstart=jadjust+1; mjstop = cols - jadjust; nuimage = image; For[i=mistart,i<=mistop,i++, For[j=mjstart,j<=mjstop,j++, maskarea = Table[image[[m,n]], {m,i-iadjust,i+iadjust}, {n,j-jadjust,j+jadjust}]; nuimage[[i,j]] = Apply[Plus,Flatten[maskarea*kernel]]; ]; ]; nuimage ] thanks for any help, Robert Fuentes robert at mps.lfwc.lockheed.com