Error in Matlab Code

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marie lasz
marie lasz 2020-12-14
评论： Image Analyst 2020-12-18
I am getting error in this code at line 13 '', 'Index in position 2 exceeds array bounds (must not exceed 512). Looking for the solution.Thanks in advance
clear all;
M=512;
N=512;
K=8;
A=imread('lena2.jpg');
A=rgb2gray(A);
A=imresize(A,[512,512]);
b=imread('Watermarked_Image.jpg');
for p=1:N
for q=1:N
x=(p-1)*K+1;
y=(q-1)*K+1;
BLOCK1=A(x:x+K-1,y:y+K-1);
BLOCK2=b(x:x+K-1,y:y+K-1);
BLOCK1=idct2(BLOCK1);
BLOCK2=idct2(BLOCK2);
if BLOCK1(1,1)~=0
a=(BLOCK2(1,1)/BLOCK1(1,1))-1;
if a<0
W(p,q)=0;
else
W(p,q)=1;
end
end
end
end
subplot(2,2,1);
imshow(W);
title('the extracted watermark image');
imwrite(W,'w1.jpg','jpg');
'

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采纳的回答

Image Analyst
Image Analyst 2020-12-14
Matrices are indexed (row, column), which is (y, x), NOT (x,y) as you have it.
Also check the limits of your for loop to make sure you don't step outside the image.
Attach your 2 images if you still need help so we can run your code.
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marie lasz
marie lasz 2020-12-18
well thanks for your help. I changed thre code and also changed the criteria of insertion from multiplicative to additive formula. But in the end I don't know why am I getting flipped image in extraction . I am thinking to apply flip function but really don't know why it is actually flipping? Plus if I am putting threshold or you can say alpha value less than and greater to 1 then it is not working properly but if put lets say 30 then it is working fine. I cannot put the code online here due to some reasons. But any kind suggestions will be helpful to me.
Image Analyst
Image Analyst 2020-12-18
Not sure without delving into it much more than I have time for. I notice that your're using x and y and a lot of novices confuse x and y with row and column. They do NOT correspond like that.
Matrixes are indexed M(row, column) which is M(y, x), NOT M(x,y) so make sure all that is correct. I saw for W you're using p as the first index which is derived from x, so that's not right in general but would be okay for a perfectly square matrix. And you're using x as the first index for A instead of y.

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更多回答（1 个）

Cris LaPierre
Cris LaPierre 2020-12-14
Error is in this line of code:
BLOCK1=A(x:x+K-1,y:y+K-1);
"Index in position 2 exceeds array bounds (must not exceed 512)"
Position 2 corresponds to your second index: y:y+K-1
Apparently A is a mx512 array. You define y as y=(q-1)*K+1; At its largest value (q=512, K=8), y is 511*8+1, which is >> 512.
You likely need to rethink your approach.
  1 个评论
marie lasz
marie lasz 2020-12-14
thanks for your answer, yes it is going outside the array.

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