what do u mean by the error message "index exceeds matrix dimensions" ?

i have written a simple code to subtract two images and applying threshold after subtracting. when it is executed it giver error message "ndex exceeds matrix dimensions". what does it means ? how can i solve this problem? Program is given here.please help me. Thanking you.

img1=imread('seq1.jpg');%define your image. 
img1=rgb2gray(img1);

img2=imread('seq2.jpg');
img2=rgb2gray(img2);

diff1=abs(int16(img2)-int16(img1));

[r1,c1]=size(diff1);
for x1=1:r1
    for y1=1:c1
        if diff1(x1,y1)<56  %threshold value.Here is the error.
            diff1=0;
        end 
    end
end

Answer 1

[Paulo Silva]

diff1=0 causes that problem because it turns the array in just a double value (size = [1 1]), do you mean diff1(x1,y1)=0 instead?

[Pramod Bhat]

thanks dear. yes. you are right. thanks again.