Contents

Lucas-Kanade Optical Flow

A demo of Lukas-Kanade optical flow. It uses camera by default, but you can provide a path to video as an argument.

Sources:

function varargout = lk_demo_gui(varargin)

    % setup video capture
    cap = createVideoCapture([], 'chess');
    assert(cap.isOpened());
    frame = cap.read();
    assert(~isempty(frame) && size(frame,3)==3);
    prev = cv.cvtColor(frame, 'RGB2GRAY');

    % create the UI, and hook event handlers
    h = buildGUI(frame);
    set(h.fig, 'WindowKeyPressFcn',@onType, ...
        'WindowButtonDownFcn',@onMouseDown, ...
        'Interruptible','off', 'BusyAction','cancel');
    if nargout > 0, varargout{1} = h; end

    % initialize state
    pts = [];
    initCorners = false;
    nightMode = false;

    % main loop
    while ishghandle(h.fig)
        % get next frame
        frame = cap.read();
        if isempty(frame), break; end
        next = cv.cvtColor(frame, 'RGB2GRAY');

        % black background in night mode
        if nightMode
            frame(:) = 0;
        end

        if initCorners
            % automatic points initialization
            pts = detectPoints(next);
            initCorners = false;
        elseif ~isempty(pts)
            % track points
            pts = trackPoints(pts, prev, next);
            % draw points
            frame = cv.circle(frame, pts, 3, ...
                'Color',[0 255 0], 'Thickness','Filled');
        end

        % show output
        set(h.img, 'CData',frame);
        drawnow limitrate;

        % next iteration
        prev = next;
    end
    cap.release();  % release video source

Callback functions

    function onMouseDown(~,~)
        %ONMOUSEDOWN  Event handler for mouse down on figure

        % current mouse location
        p = get(h.ax, 'CurrentPoint');
        p = p(1,1:2) - 1;

        % check if user clicked close to an existing point
        if ~isempty(pts)
            [d,idx] = cv.batchDistance(p, pts, 'K',1);
            if d <= 5
                % remove point
                pts(idx,:) = [];
                return;
            end
        end

        % check if we still have room for more points
        if size(pts,1) <= 300
            % add point
            pts(end+1,:) = p;
        end
    end

    function onType(~,e)
        %ONTYPE  Event handler for key press on figure

        switch e.Key
            case 'h'
                helpdlg({
                    'To add/remove points, click with the mouse.'
                    'Hot keys:'
                    'h - this help dialog'
                    'q - quit the program'
                    'n - switch "night" mode on/off'
                    'c - clear all the points'
                    'r - initialize tracking by auto-detecting corners'
                });
            case {'q', 'escape'}
                close(h.fig);
            case 'n'
                nightMode = ~nightMode;
            case 'c'
                pts = [];
            case 'r'
                initCorners = true;
        end
    end

end

Helper functions

function pts = detectPoints(gray)
    %DETECTPOINTS  Find and refine corners in image

    % detect corners
    pts = cv.goodFeaturesToTrack(gray, ...
        'MaxCorners',100, 'QualityLevel',0.01, 'MinDistance',10);
    if isempty(pts)
        pts = [];
        return;
    end

    % refine corners
    pts = cv.cornerSubPix(gray, pts, 'WinSize',[10 10], ...
        'Criteria',struct('type','Count+EPS', 'maxCount',20, 'epsilon',0.03));

    % return Nx2 matrix of points
    pts = cat(1, pts{:});
end

function pts = trackPoints(pts, prev, next)
    %TRACKPOINTS  Calculate flow of sparse points

    % calculate sparse optical flow
    [pts, status] = cv.calcOpticalFlowPyrLK(prev, next, pts, ...
        'WinSize',[31 31], 'MinEigThreshold',0.001);

    % drop points that are not found
    pts = cat(1, pts{status == 1});
end

function h = buildGUI(img)
    %BUILDGUI  Creates the UI

    % build the user interface (no resizing to keep it simple)
    sz = size(img);
    h = struct();
    h.fig = figure('Name','LK Demo', 'NumberTitle','off', 'Menubar','none', ...
        'Pointer','cross', 'Resize','off', 'Position',[200 200 sz(2) sz(1)]);
    if ~mexopencv.isOctave()
        %HACK: not implemented in Octave
        movegui(h.fig, 'center');
    end
    h.ax = axes('Parent',h.fig, 'Units','normalized', 'Position',[0 0 1 1]);
    if ~mexopencv.isOctave()
        h.img = imshow(img, 'Parent',h.ax);
    else
        %HACK: https://savannah.gnu.org/bugs/index.php?45473
        axes(h.ax);
        h.img = imshow(img);
    end
end