for loop over video data

Hi guys,

I want to extract data from a video, which is saved as a 4d Array. My plan is to use a for loop to extract data from the first 30x30 pixels of 50 frames and process these. After that I want to do the same with the next 30x30 pixels.

Right now I have this, but it is not working as I thought it would

for i = 1:50 
    for r = 1:29:size(data,2)
        for c = 1:29:size(data,3)
            if c+29 <= 960 && r+29 <= 540
            red = mean(data(i,r:r+29,c:c+29,1),"all");
            red(red==0) = [];
            green = mean(data(i,r:r+29,c:c+29,2), 'all');
            green(red==0) = [];
            blue = mean(data(i,r:r+29,c:c+29,3), 'all');
            blue(red==0) = [];
            %%% Processing
            end
        end
    end
end

Thank you in advance :)

[Voss]

What is the size of your 4d Array?

[Milena]

It is an 199x540x960x3 array, but i only want the first 50 of 199 frames.

Answer 1

[Voss]

Does this work as expected?

[n_frames,n_rows,n_columns,n_color_channels] = size(data);
block_size = 30;
n_frames_to_process = 50;

for i = 1:n_frames_to_process
    for r = 1:block_size:n_rows-block_size+1
        for c = 1:block_size:n_columns-block_size+1
            red = mean(data(i, r:r+block_size-1, c:c+block_size-1, 1),"all");
            red(red==0) = [];
            green = mean(data(i, r:r+block_size-1, c:c+block_size-1, 2), 'all');
            green(green==0) = [];
            blue = mean(data(i, r:r+block_size-1, c:c+block_size-1, 3), 'all');
            blue(blue==0) = [];
            %%% Processing
        end
    end
end

Changes made:

• Changed the increment of the 2 innermost for loops from 29 to 30. Note that r = 1:29:size(data,2) gives r = [1 30 59 88 ...], but it sounds like you want r = [1 31 61 91 ...], which is r = 1:30:size(data,2). Similarly for c.
• Subtracted 29 from the end point of the 2 innermost for loops, to avoid having the if inside the loops.
• Changed green(red==0) = []; to green(green==0) = []; (and the same for blue), which seems to make more sense, since red is already not 0.
• Made relevant constants into variables (n_frames, n_rows, n_columns, n_color_channels, block_size, n_frames_to_process) to avoid hard-coding relevant values in the code.

Note that after running this line

red = mean(data(i, r:r+block_size-1, c:c+block_size-1, 1),"all");

red is a scalar, so that this

red(red==0) = [];

is the same as

if red == 0
    red = [];
end

I point that out in case you are expecting red not to be a scalar at that point.