Checking Pixels in Matlab - matlab

Question

I have an image with 3 colors in matlab which value 0, 128, and 255. for example:
255 255 255 255 128 255 0 255 255 128 0 255
255 0 255 255 128 255 255 0 255 128 255 255
255 0 255 0 128 255 255 255 255 128 255 0
255 255 0 255 128 255 255 0 255 128 255 255
255 0 0 255 128 0 255 255 0 128 255 0
First, I want to check the pixels of the index (1,1) to (1,5).
If there is pixel value 0 (black), then the pixels of the index (1,1) to (1,5) is changed to 128 (gray), if none, then the pixels are changed to 0 (white).
Second, I want to do these steps again, checking of the index (2,1) to (2,5), (3,1) to (3,5), through to the bottom, then continue to the next, to the index (1,6) to (1,10), (2,6) to (2,10), through to the bottom, then went to the index (1,11) to (1,end), (2,11) to (2,end).

Answer 1

Do you absolutely need to do this sequentially? It sounds like you need to do this for each group of the form (n, (5*m : 5*m +1)). If so, you can do all of the tests simultaneously by reshaping the matrix into a 3d matrix of blocks which are 5 elements wide. Also I am assuming that you meant "if none, then the pixels are changed to 255 (white)", not 0.
Suppose your image is called myImage, then
numBlocks = numel(myImage)/(5*size(myImage,1));
% Generate a 3D matrix each of which is 5 elements long in dimension 2. Note reshape will throw an error if numblocks is fractional
foo = reshape(myImage,size(myImage,1),5,numBlocks);
blackTest = any(foo==0,2);
result = blackTest * 128 + ~blackTest*255; % result for each block
output = reshape(repmat(result,[1 5 1]),size(myImage));
This reorganises your image into a 3d matrix, where each submatrix corresponding to each "layer" of the 3d matrix is 5 elements wide. For the whole 3d matrix it checks whether any of the elements in dimension 2 are zero, leaving a logical matrix foo of length 1 in dimension 2. foo consists of logical ones and zeros, which in MATLAB can also be treated as numerical ones and zeros. So it multiplies foo by 128 (for grey output value) and adds the logical inverse of foo multiplied by 255, to get your white output values. Finally it repeats the matrix back to 5-element-wide blocks and restores it to its original dimensions.
Edit: Note that as mentioned in the code comment, this code won't work if your original image isn't a multiple of 5 pixels wide. To fix this you'd have to create a special case, or use a loop to step through each 5-element-wide block. In fact that might be a better approach all round:
index = 1;
output = zeros(size(myImage));
while index < size(myImage,2)
blockEnd = min(index+4,size(myImage,2));
blackTest = any(myImage(:,index:blockEnd)==0,2);
blackTest = blackTest(:,ones(1,5));
output(1:end,index:blockEnd) = blackTest * 128 + ~blackTest*255;
index = index+5;
end

Answer 2

% generate matrix
rand_data = randi(10,10);
I = zeros(10);
I(rand_data < 6 & rand_data > 3) = 128;
I(rand_data >= 6) = 255;
% here's the code
I = reshape(I',5,[])';
mask = repmat(any(I == 0,2),5,1);
I(mask) = 128;
I(~mask) = 255;
I = reshape(I',10,[])';