auto_trim.m

function [] = auto_trim(filename, output)

I = imread(filename, 1);

rows = size(I, 1);
cols = size(I, 2);

sensitive = false;
if rows > 1000 || cols > 1000
    sensitive = true;
end

decrementer = ones(rows, cols);

info = imfinfo(filename);

num_frames = length(info);
num_frames = num_frames - 1;
num_frames = num_frames/2;

bg_brightness = mean(mean(I));
totals = [num_frames, 1];

for i = 1:num_frames
    I = imread(filename, i);
    total = sum(sum(I));
    totals(i, 1) = total;
end

sigma = std(totals, 0, 1);

dim = false;
if (sigma(1)/bg_brightness) < 3.000e+03
    dim = true;
end

gradients = [num_frames, 1];
start = 0;
if ~dim
    for i = 1:num_frames
        I = imread(filename, i);
        I = imgaussfilt(I, 5);
        cutoff = 1.1;
        if sensitive
            cutoff = 1.5;
        end
        seg_I = imquantize(I, cutoff * bg_brightness);
        seg_I = seg_I - decrementer;
        total = sum(sum(seg_I));
        gradients(i, 1) = total;
        threshold = 1;
        if sensitive
            threshold = 20;
        end
        if total > threshold
            if start == 0
                start = i;
            end
        end
    end
else
    for i = 1:num_frames
        I = imread(filename, i);
        I = imgaussfilt(I, 5);
        cutoff = 1.07;
        seg_I = imquantize(I, cutoff * bg_brightness);
        seg_I = seg_I - decrementer;
        total = sum(sum(seg_I));
        gradients(i, 1) = total;
        threshold = 90;
        if total > threshold
            if start == 0
                start = i;
            end
        end
    end
end

start = start - 2;

if sensitive
    start = start - 4;
end

finish = 0;
if sensitive
    finish = num_frames;
else
    for j = 1:(num_frames - 5)
        dx = gradient(gradients(j:j+5, 1));
        if dx < 0
            if finish == 0 || j < 151 - 15
                if j < 151 - 15
                    finish = j+15;
                else
                    finish = j+5;
                end
            end
        end
    end
end

if start < 1
    disp('reset');
    start = 1;
end

if finish > num_frames
    finish = num_frames;
end

if finish == 0
    finish = num_frames;
end

disp(start);
disp(finish);

for k = start:finish
    I = imread(filename, k);
    imwrite(I(:, :), output, 'WriteMode', 'append',  'Compression','none');
end
return;