Error with matrix dimensions

Hi, I'm making a code for scene cut detection, but I always get an error for line 14: matrix dimensions must agree... How do I fix this? Thank you.

THRESH = 0.5;

video = mmreader('rhinos.avi');

numFrames = video.NumberOfFrames;

prevHist = zeros(256*3,1);

diffHist= zeros(numFrames,1);

cutInd = 1;

for i = 1 : numFrames

currFrame = read(video, i);

currHistR = imhist(currFrame(:, :, 1));

currHistG = imhist(currFrame(:, :, 2));

currHistB = imhist(currFrame(:, :, 3));

currHist = [currHistR currHistG currHistB];

whos prevHist currHist;

diffHist(i) = sqrt(sum((currHist - prevHist).^2));

if (diffHist(i) > THRESH)

cutFrames(:, :, :, cutInd) = currFrame(:, :, :);

cutInd = cutInd + 1;

end

prevHist = currHist;

end

stem(diffHist);

figure;

montage(cutFrames);

[Andrew Reibold]

Is the following line 14?:

diffHist(i) = sqrt(sum((currHist - prevHist).^2));

If you stop the code at line 14, what are the results of

size(currHist)

and

size(prevHist)

[Srdjan]

Yes it is. Size(currHist) is 256x3, an prevHist is 768x1. But, when i make prevHist to be 256x3, then I get an error for the same line that says: In an assignment A(I) = B, the number of elements in B and I must be the same... And I don't know what to do with that... As you can see, I am the beginner. :)

[Andrew Reibold]

You will need to make them both the same size to use the subtraction operation in line 14 on them.

What code are you using to make the 768x1 into a 256x3? (What code are you using giving you the A(I)=B error. That should be easy to troubleshoot hopefully) I assume you are just trying to split it into 3 parts?

There are more efficient ways to make a new matrix, but have you tried something like this?

prevHist2(:,1) = prevHist(1:256)
prevHist2(:,2) = prevHist(257:512)
prevHist2(:,3) = prevHist(513:768)

and then using the new version of prevHist that is now rearranged?

[Andrew Reibold]

Nevermind, I see you have a solution! Best wishes

Answer 1

[MA]

you have two mistake:

prevHist = zeros(256*3,1) 
and 
diffHist(i) = sqrt(sum((currHist - prevHist).^2))

your correct code should be:

clear all
close all
clc;

THRESH = 0.5;

video = mmreader('rhinos.avi');

numFrames = video.NumberOfFrames;

prevHist = zeros(256,3);

diffHist= zeros(numFrames,1);

cutInd = 1;

for i = 1 : numFrames

currFrame = read(video, i);

currHistR = imhist(currFrame(:, :, 1));

currHistG = imhist(currFrame(:, :, 2));

currHistB = imhist(currFrame(:, :, 3));

currHist = [currHistR currHistG currHistB];

whos prevHist currHist;

diffHist(i) = sqrt(sum((currHist(i) - prevHist(i)).^2));

if (diffHist(i) > THRESH)

cutFrames(:, :, :, cutInd) = currFrame(:, :, :);

cutInd = cutInd + 1;

end

prevHist = currHist;

end

stem(diffHist);

figure;

montage(cutFrames);

[Srdjan]

Thx mate, it works fine. :)

Answer 2

[MA]

you have two mistake:

prevHist = zeros(256*3,1) 
and 
diffHist(i) = sqrt(sum((currHist - prevHist).^2))

your correct code should be:

clear all
close all
clc;

THRESH = 0.5;

video = mmreader('rhinos.avi');

numFrames = video.NumberOfFrames;

prevHist = zeros(256,3);

diffHist= zeros(numFrames,1);

cutInd = 1;

for i = 1 : numFrames

currFrame = read(video, i);

currHistR = imhist(currFrame(:, :, 1));

currHistG = imhist(currFrame(:, :, 2));

currHistB = imhist(currFrame(:, :, 3));

currHist = [currHistR currHistG currHistB];

whos prevHist currHist;

diffHist(i) = sqrt(sum((currHist(i) - prevHist(i)).^2));

if (diffHist(i) > THRESH)

cutFrames(:, :, :, cutInd) = currFrame(:, :, :);

cutInd = cutInd + 1;

end

prevHist = currHist;

end

stem(diffHist);

figure;

montage(cutFrames);

Answer 3

[Kevin Claytor]

Let's take a look at the definition of;

diffHist= zeros(numFrames,1);

This is a [numFrames x 1] size array. Into this you're trying to put;

sqrt(sum((currHist - prevHist).^2));

So, first in regards to the sizes, you can only subtract them if they're the same size;

>> size(currHist)
>> 256x3
>> size(prevHist)
>> 768x1

Which you seem to have resolved in your comments. But now you're trying to put a 256x3 array (sqrt(...)) into a 1x1 slot (diffHist(i)). Either you want the entire array;

diffHist = zeros(256, 3, numFrames);
diffHist(:, :, i) = sqrt(...)

Or you just want some representative value of the difference histogram;

   temp_dist_hist = sqrt(...);
   representative_value = sum(temp_dist_hist(:)); % Just an example, depends on what you're trying to do
   diffHist(i) = representative_value;

Hope, this helps.

[Srdjan]

Your example helped a lot, thank you.