How to resolve the "Index exceeds matrix dimensions" error ?

Hi, I did some coding for lossless image compression and I want to calculate the entropy value. the error "Index exceeds matrix dimensions" keeps appearing at line _FreqPixInt(PixPos(x,y)+1)=FreqPixInt(PixPos(x,y)+1)+1. I don't know how to solve. Please help me. Below are my coding ;

clc
clear
x1= imread('barbara.bmp');
im=double(x1);
En1 = entropy(x1);

% Start from the Haar wavelet and get the
% corresponding lifting scheme.
lshaar = liftwave('haar');

% Add a primal ELS to the lifting scheme.
els = {'p',[-0.125 0.125],0};
lshaarInt = liftwave('haar','int2int');
lsnewInt = addlift(lshaarInt,els);

[cAint,cHint,cVint,cDint] = lwt2(im,lsnewInt);
x2=double(cAint);
[cAint2,cHint2,cVint2,cDint2] = lwt2(x2,lsnewInt);
test=[cAint2,cHint2;cVint2,cDint2];

xRecInt2 = ilwt2(cAint2,cHint2,cVint2,cDint2,lsnewInt);
errInt2 = max(max(abs(x2-xRecInt2)))
one(1:128,1:128)=int8(cAint2); 
one(129:256,1:128)=int8(cHint2);
one(1:128,129:256)=int8(cVint2);
one(129:256,129:256)=int8(cDint2);
level2=[cAint2,cHint2;cVint2,cDint2]; 
two=[level2,cHint;cVint,cDint];
test1=[level2,cHint;cVint,cDint];
figure(3);
imshow(int8(two));

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%entropy%%%%%%%%%%%%%%%%%%%%%%%%%%
%convert to positive integer - kalau tak convert tak boleh kira
for  y=1:512;
    for x=1:512;
        if test1(x,y)==0;
            PixPos(x,y)=1;
        else if test1(x,y)>0;
            PixPos(x,y)=(test1(x,y)*2)+1;
        else
            PixPos(x,y)=abs(test1(x,y))*2;
        end
    end
    end
end

%calculate frequency occurence of symbols
FreqPixInt(1:512)=0;
for y=1:512 %all pixel values in rows
    for x=1:512 %all pixel values in columns
        FreqPixInt(PixPos(x,y)+1)=FreqPixInt(PixPos(x,y)+1)+1;
    end
end
FreqPixInt;

%calculate entropy of symbols(positive integer)
En2=0;% initialize value entropy to 0
for i=1:512 % all pixels value in  first rows
    if FreqPixInt(i)==0
        En2=En2; %dummy
    else    
        En2=En2+(FreqPixInt(i)/262144)*log2(FreqPixInt(i)/262144); %512x512=262144 total pixels
    end
end
En2=En2*(-1)

[Bob Thompson]

FreqPixInt is a 1x512 array, but when you call PixPos(x,y)+1 at x = 512 or y = 512 you're trying to look at the 513 element of FreqPixInt.

[nur aqila]

so what should I change? this coding only works with certain image, some image gives the same error. I only used 512x512 image for this coding

Answer 1

[Cam Salzberger]

Hello Nur,

There's a lot to go through there, and a lot of loop iterations as well. I suggest that you run this command in the Command Window:

dbstop if error

then rerun the code. Now when the error occurs, it will stop in debug mode at that point. You can see what are the values of x, y, PixPos(x, y), test1(x, y), and so on. You should be able to see that the value is (probably) exceeding 512, and then you can work backwards to figure out why that is and whether something should be calculated or handled differently.

Also, it doesn't really matter since your matrices are square, but the first index is row, and the second index is column. You seem to have conflicting comments and possibly code.

Also it should be using "elseif" not "else if". That results in some misaligned indentation and an extra "end" statement, though it won't actually have an impact on the output in this case.

-Cam

[nur aqila]

Thanks for the suggestion. I already tried. my coding only works with the certain image. Some image gives the same error. I only used 512x512 image for this coding. Do you have any suggestion where should I change ?

[Cam Salzberger]

As per Bob's suggestion, it may help to determine if you are attempting to index into the 513th pixel at any point, and where. Then you should go through the logic you used to write the code and determine if the code is correct. If you do need to access the "i+1" pixel, then maybe the loop should be limited to n-1. Or maybe you are calculating the index incorrectly. It's not really for us to tell you how to code, since only you know what your code should be doing.

-Cam