I want to partition a PGM image into overlapping blocks and do a DCT transformation to every block. Then I want to take the first column of each DCT matrix and put them all to a new matrix.
I have read the answer to the post: How to partition an image to 64 block in matlab, but I am not sure it's working properly. Can I use blockproc function to make overlapping block and if I can how am I supposed to use it?
I would prefer an answer with for loops.
I=im2double(rgb2gray(imread('yourimage.png'))); %select channels separation
%%%for non-overlapping blocks
fun = #(block_struct) dct2(block_struct.data);
imageY =(blockproc(I,[8 8],fun));
BY=im2col(imageY,[8 8],'distinct');
FinalOP=BY(1:8,:);
%dct-8x8 ,if you want u can choose [64 64].
%you need the first column of dct?.so ,i.e 8 values of dct in this case.
for overlapping blocks: use 'sliding' but you get memory problem.
then you have to use your own for loop.
Related
I created binarized images by using the Otsu methode in Matlab and cut out parts of the resulting image using a function. Now i want to take a look at these images with the VolumeViewer command. I know the x,y and z dimensions of the resulting imgages. I currently run this code doing it(excluding the volumeViewerwhich happens after the loop):
files= {'C3\C3_000mal_550_539_527.raw';...
};
for i=1:numel(files)
Image = fopen(files{i},'r');
ImageData{i} = fread(Image,Inf,'uint16=>uint16');
ImageData{i} = reshape(ImageData{i},550,539,[]);
fclose(openedCrystalImage);
end
Using this code runs into the following error using reshape:
Error using reshape
Product of known dimensions, 296450, not divisible into total number of elements, 78114575.
I did the maths and 550*539=296450 and 296450 * 527=156229150: If we divide the last number by the number of elements it equals 2 and thus is divisible into the total number of elements. In my opinion the reshape function is not able to find the size of the last dimension or defines it as 1.
Defining the size of z also results in an error suggesting using the brackets [], so the function can find it.
Error using reshape
Number of elements must not change. Use [] as one of the size inputs to automatically calculate the appropriate size
for that dimension.
Now to the weird part. This code works for another set of images, with diffrent sizes of the x,y and z ranges. So don´t know where the issue lies to be frank. So i would really appreciate and Answer to my question
I figured it out. The error lies here:
ImageData{i} = fread(Image,Inf,'uint16=>uint16');
Apparently by saving them as .raw before it converts the image to an 8 bit file rather than 16 bits it had before. Therefore, my dimension is double the size of the number of elements. With this alteration it works:
ImageData{i} = fread(Image,Inf,'uint8=>uint8');
The reason i was able to look at the other pictures was that the z range was divisble by 2.
So the reshape function was not the problem but size of the integer data while creating the array for the variable ImageData.
P.S. I just started out programming so the accuracy in the answer should be taken with a grain of salt
This is a cross-post from here:
Link to post in the Mathworks community
Currently I'm working with large data sets, I've saved those data set as matlab files with the two biggest files being 9.5GB and 5.9GB.
These files contain a cell array each of 1x8 (this is done for addressibility and to prevent mixing up data from each of the 8 cells and I specifically wanted to avoid eval).
Each cell then contains a 3D double matrix, for one it's 1001x2002x201 and the other it is 2003x1001x201 (when process it I chop of 1 row at the end to get it to 2002).
Now I'm already running my script and processing it on a server (64 cores and plenty of RAM, matlab crashed on my laptop, as I need more than 12GB ram on windows). Nonetheless it still takes several hours to finish running my script and I still need to do some extra operations on the data which is why I'm asking advice.
For some of the large cell arrays, I need to find the maximum value of the entire set of all 8 cells, normally I would run a for loop to get the maximum of each cel and store each value in a temporay numeric array and then use the function max again. This will work for sure I'm just wondering if there's a better more efficient way.
After I find the maximum I need to do a manipulation over all this data as well, normally I would do something like this for an array:
B=A./maxvaluefound;
A(B > a) = A(B > a)*constant;
Now I could put this in a for loop, adress each cell and run this, however I'm not sure how efficient that would be though. Do you think there's a better way then a for loop that's not extremely complicated/difficult to implement?
There's one more thing I need to do which is really important, each cell as I said before is a slice (consider it time), while inside each slide is the value for a 3D matrix/plot. Now I need to integrate the data so that I get more slices. The reason I need to do this that I need to create slices/frames/plots to create a movie/gif. I'm planning on plotting the 3d data using scatter3 where this data is represented by color. I plan on using alpha values to make it see through so that one can actually see the intensity in this 3d plot. However I understand how to use griddata but apparently it's quite slow. Some of the other methods where hard to understand. Thus what would be the best way to interpolate these (time) slices in an efficient way over the different cells in the cell array? Please explain it if you can, preferably with an example.
I've added a pic for the Linux server info I'm running it on below, note I can not update the matlab version unfortunately, it's R2016a:
I've also attached part of my code to give a better idea of what I'm doing:
if (or(L03==2,L04==2)) % check if this section needs to be executed based on parameters set at top of file
load('../loadfilewithpathnameonmypc.mat')
E_field_650nm_intAll=cell(1,8); %create empty cell array
parfor ee=1:8 %run for loop for cell array, changed this to a parfor to increase speed by approximately 8x
E_field_650nm_intAll{ee}=nan(szxit(1),szxit(2),xres); %create nan-filled matrix in cell 1-8
for qq=1:2:xres
tt=(qq+1)/2; %consecutive number instead of spacing 2
T1=griddata(Xsall{ee},Ysall{ee},EfieldsAll{ee}(:,:,qq)',XIT,ZIT,'natural'); %change data on non-uniform grid to uniform gridded data
E_field_650nm_intAll{ee}(:,:,tt)=T1; %fill up each cell with uniform data
end
end
clear T1
clear qq tt
clear ee
save('../savelargefile.mat', 'E_field_650nm_intAll', '-v7.3')
end
if (L05==2) % check if this section needs to be executed based on parameters set at top of file
if ~exist('E_field_650nm_intAll','var') % if variable not in workspace load it
load('../loadanotherfilewithpathnameonmypc.mat');
end
parfor tt=1:8 %run for loop for cell array, changed this to a parfor to increase speed by approximately 8x
CFxLight{tt}=nan(szxit(1),szxit(2),xres); %create nan-filled matrix in cells 1 to 8
for qq=1:xres
CFs=Cafluo3D{tt}(1:lxq2,:,qq)'; %get matrix slice and tranpose matrix for point-wise multiplication
CFxLight{tt}(:,:,qq)=CFs.*E_field_650nm_intAll{tt}(:,:,qq); %point-wise multiple the two large matrices for each cell and put in new cell array
end
end
clear CFs
clear qq tt
save('../saveanotherlargefile.mat', 'CFxLight', '-v7.3')
end
i am doing BPCS STEGANOGRAPHY.. i have separate secret blocks (4096- 8*8 blocks) and conjugation block( 64 - 8*8 blocks) ready for embedding. each can be accessed separately by calling
secret_block(:,:,1),
secret_block(:,:,2),
secret_block(:,:,3)
etc and
conju_block(:,:,1),
conju_block(:,:,2),
conju_block(:,:,3)
etc
During bpcs embedding cover image is bitplane sliced and stored as c1( lsb plane ),c2,c3,c4 c5,c6,c7,c8 (msbplane). I need to select lsb bit plane to msb plane and dividing each bitplane into 8*8 block, selected_block(:,:,i)) and among them complex blocks are replaced with initially secret_block (:,:,i) and later on conju_block(:,:,i) etc until all the 4096+64 blocks reached..
Please anyone tell me how replace on particular block of selected_block(:,:,i) with another block (conju_block(:,:,i) or secret_block(:,:,i)) ? Is it possible to use replace_block() for the same ,then what does 'sys' parameter means ????
I would start by putting all bit planes in one cell,
C{1} = c1;
C{2} = c2;
etc
so you can go through the bit planes in an automated manner.
for i = 1:8
curr_plane = C{i};
end
There are many ways to split a 2D array into blocks. A convenient one is mat2cell. The web documentation gives you a visual example of how it works.
Note that since you want 8x8 blocks, both the rows and columns must be exactly divisible by 8. You have 3 options:
only allow images with number of rows and columns divisible by 8,
automatically rescale images to such a size, or
take the biggest slice of the image matrix with such a size, do your steganography and put that modified image back into your slice of the original image, from where it came from.
We continue with the assumption that you have an image with number of rows and columns divisible by 8. Let's call your initial cover image cover.
row = size(cover, 1)/8;
col = size(cover, 2)/8;
blocks = mat2cell(curr_plane, ones(1, row)*8, ones(1, col)*8);
blocks is now a cell which holds all the 8x8 blocks of your bit plane, curr_plane. For example, if your bit plane had a size of 16x48, you now have 2x6 blocks. You can iterate through them simply with
for bi = 1:size(blocks,1)
for bj = 1:size(blocks,2)
my_8x8_block = blocks{bi,bj};
end
end
You're almost there. To sum up what we've done, we go from bit plane to bit plane and split each one in 8x8 blocks. Now, for every my_8x8_block, you can feed it into a function which calculates whether it's complex enough or not. If it is, you can simply overwrite it with the kth secret or conjugate block.
blocks{bi,bj} = secret_block(:,:,k);
Finally, once you have gone through a whole bit plane, you can put back the blocks to construct the modified bit plane
C{i} = cell2mat(blocks);
I'm attempting to run this simple diffusion case (I understand that it isn't ideal generally), and I'm doing fine with getting the inside of the solid, but need some help with the outer edges.
global M
size=100
M=zeros(size,size);
M(25,25)=50;
for diffusive_steps=1:500
oldM=M;
newM=zeros(size,size);
for i=2:size-1;
for j=2:size-1;
%we're considering the ij-th pixel
pixel_conc=oldM(i,j);
newM(i,j+1)=newM(i,j+1)+pixel_conc/4;
newM(i,j-1)=newM(i,j-1)+pixel_conc/4;
newM(i+1,j)=newM(i+1,j)+pixel_conc/4;
newM(i-1,j)=newM(i-1,j)+pixel_conc/4;
end
end
M=newM;
end
It's a pretty simple piece of code, and I know that. I'm not very good at using Octave yet (chemist by trade), so I'd appreciate any help!
If you have concerns about the border of your simulation you could pad your matrix with NaN values, and then remove the border after the simulation has completed. NaN stands for not a number and is often used to denote blank data. There are many MATLAB functions work in a useful way with these values.
e.g. finding the mean of an array which has blanks:
nanmean([0 nan 5 nan 10])
ans =
5
In your case, I would start by adding a border of NaNs to your M matrix. I'm using 'n' instead of 'size', since size is an important function in MATLAB, and using it as a variable can lead to confusing errors.
n=100;
blankM=zeros(n+2,n+2);
blankM([1,end],:) = nan;
blankM(:, [1,end]) = nan;
Now we can define 'M'. N.B that the first column and row will be NaNs so we need to add an offset (25+1):
M = blankM;
M(26,26)=50;
Run the simulation through,
m = size(blankM, 1);
n = size(blankM, 2);
for diffusive_steps=1:500
oldM = M;
newM = blankM;
for i=2:m-1;
for j=2:n-1;
pixel_conc=oldM(i,j);
newM(i,j+1)=newM(i,j+1)+pixel_conc/4;
newM(i,j-1)=newM(i,j-1)+pixel_conc/4;
newM(i+1,j)=newM(i+1,j)+pixel_conc/4;
newM(i-1,j)=newM(i-1,j)+pixel_conc/4;
end
end
M=newM;
end
and then extract the area of interest
finalResult = M(2:end-1, 2:end-1);
One simple change you might make is to add a boundary of ghost cells, or halo, around the domain of interest. Rather than mis-use the name size I've used a variable called sz. Replace:
M=zeros(sz,sz)
with
M=zeros(sz+2,sz+2)
and then compute your diffusion over the interior of this augmented matrix, ie over cells (2:sz+1,2:sz+1). When it comes to considering the results, discard or just ignore the halo.
Even simpler would be to simply take what you already have and ignore the cells in your existing matrix which are on the N,S,E,W edges.
This technique is widely used in problems such as, and similar to, yours and avoids the need to write code which deals with the computations on cells which don't have a full complement of neighbours. Setting the appropriate value for the contents of the halo cells is a problem-dependent matter, 0 isn't always the right value.
In Matlab, is it possible to measure local variation of a signal across an entire signal without using for loops? I.e., can I implement the following:
window_length = <something>
for n = 1:(length_of_signal - window_length/2)
global_variance(n) = var(my_signal(1:window_length))
end
in a vectorized format?
If you have the image processing toolbox, you can use STDFILT:
global_std = stdfilt(my_signal(:),ones(window_length,1));
% square to get the variance
global_variance = global_std.^2;
You could create a 2D array where each row is shifted one w.r.t. to the row above, and with the number of rows equal to the window width; then computing the variance is trivial. This doesn't require any toolboxes. Not sure if it's much faster than the for loop though:
longSignal = repmat(mySignal(:), [1 window_length+1]);
longSignal = reshape(longSignal(1:((length_of_signal+1)*window_length)), [length_of_signal+1, window_length])';
global_variance = sum(longSignal.*longSignal, 2);
global_variance = global_variance(1:length_of_signal-window_length));
Note that the second column is shifted down by one relative to the one above - this means that when we have the blocks of data on which we want to operate in rows, so I take the transpose. After that, the sum operator will sum over the first dimension, which gives you a row vector with the results you want. However, there is a bit of wrapping of data going on, so we have to limit to the number of "good" values.
I don't have matlab handy right now (I'm at home), so I was unable to test the above - but I think the general idea should work. It's vectorized - I can't guarantee it's fast...
Check the "moving window standard deviation" function at Matlab Central. Your code would be:
movingstd(my_signal, window_length, 'forward').^2
There's also moving variance code, but it seems to be broken.
The idea is to use filter function.