I need to show the area of objects found using bwconncomp for successive frames

I have a series of frames, and I am counting the number of objects in these frames. However, I also need to know the area of the object(s) throughout the frames. The objects are being counted correctly, and using region props I am finding the area, however I am unsure of what the output is showing. When I just used 'stats = regionprops(cc,area);' the output was a 1x154 structure. There are 154 objects counted in the last frame, so I assume this is giving me the area of the different objects in the last frame. Therefore I also used 'areaOfObject(k) = stats.Area;' to attempt to get the area of the different objects in the different frames. This returned a 1x99 double (there are 99 frames). There is only one value for the area here for each frame, and so at first I thought this was giving me the total area of objects in each frame, however the values are considerably smaller than the values of areas from each frame added together. Can anyone suggest what the output given here is, and how I can get the different areas of objects in each frame? I will eventually need to plot the area pf objects vs the frames. I have attached the code below; the majority is dealing with the frames in order to be analysed so can be ignored, but I have left it in for relevance.

folder = fullfile('Folder1\Folder2\Frames');
filePattern = fullfile(folder, '*.jpg');
fileList = dir(filePattern);
numberOfFiles = numel(fileList);

subtractionFileName = 'subtractexample1.jpg';
if ~isfile(subtractionFileName)
    errorMessage = sprintf('Subtraction image not found:\n%s', subtractionFileName);
    uiwait(errordlg(errorMessage));
    return;
end
backgroundImage = imread(subtractionFileName);
for k = 1:numberOfFiles
    baseFileName = fileList(k).name;
    if contains(baseFileName, 'subtract', 'IgnoreCase', true)
        continue;
    end
    fullFileName = fullfile(fileList(k).folder, baseFileName);
    fprintf(1, 'Now reading %s\n', fullFileName);
    thisImage = imread(fullFileName);
    subtractionImage{k} = backgroundImage - thisImage;
    imshow(subtractionImage{k}, [])
    caption = sprintf('Image #%d of %d : %s', k, numberOfFiles, baseFileName);
    title(caption, 'Interpreter', 'none');
    drawnow;
end

for k = 1:numberOfFiles
    graySubtractedImage=rgb2gray(subtractionImage{k});
    binaryImage = graySubtractedImage>114;
    binaryImage = imclearborder(binaryImage);
    binaryimage = bwareaopen(binaryImage,75);
    cc = bwconncomp(binaryimage);
    blobCountsPerFrame(k) = cc.NumObjects;
    stats = regionprops(cc, "Area");
    areaOfObject(k) = stats.Area;
    imshow(binaryImage);
    drawnow;
end

Answer 1

[Walter Roberson]

areaOfObject{k} = [stats.Area];

You have multiple objects per frame, so stats is a nonscalar structure. stats.Area would then be structure expansion, giving one output for each element of stats . You need to collect them all together into a vector. Then because you have a vector, you cannot assign it to a scalar location areaOfObject(k) so you need to use a cell array for that.

I recommend that you also collect Centroid information, as the only way to follow objects from frame to frame is by looking to see which ones are where.

There is an object tracking demo in Computer Vision toolbox that tracks centroids, and I think also handles velocity estimation to predict where the object will be next (this helps distinguish multiple moving objects.)

[Andrew Palmer]

Thanks for your response. I understand now why it is not working. I changed the line of code suggested above, however an error appeared saying 'brace indexing is not supported for variables of this type'. Apologies since I'm sure that error is telling me what is wrong, however I don't really understand it. Do I need to be changing lines above that one too, saving the area data in a vector before that line? Since the will be stored in a cell once this is fixed, will it be easy enough to plot the total area of objects? I ask since I've had trouble plotting data from cell arrays before. I've attached the updated code below. Thanks again,

for k = 1:numberOfFiles

graySubtractedImage=rgb2gray(subtractionImage{k});

binaryImage = graySubtractedImage>114;

binaryImage = imclearborder(binaryImage);

binaryimage = bwareaopen(binaryImage,75);

cc = bwconncomp(binaryimage);

blobCountsPerFrame(k) = cc.NumObjects;

stats = regionprops(cc, "Area");

areaOfObject{k} = [stats.Area];

imshow(binaryImage);

drawnow;

[Walter Roberson]

You probably have an old variable areaOfObject in memory that needs to be cleared. Or before the for k loop put in

areaOfObject = cell(numberOfFiles,1);

I really recommend tracking centroids as well:

centroids = cell(numberOfFiles, 1);
totalArea = zeros(numberOfFiles, 1);
%...

stats = regionprops(cc, "Area", "Centroid");
areaOfObject{k} = vertcat(stats.Area);
totalArea(k) = sum(areaOfObject{k});
centroids{k} = vertcat(stats.Centroid);

[Andrew Palmer]

Thanks again. Everything there works and thats exactly what I need. And yes, I am now tracking the centroids too which is definitely going to be useful, so thank you for that suggestion too.

[Andrew Palmer]

And yes, I had not cleared the memory which was causing the inital problem