Detecting corners location in subpixeles

Demo code for detecting corners using Shi-Tomasi method and refining corner locations.

In this sample you will learn how to use the OpenCV functions:

Sources:

function varargout = corner_subpixels_demo_gui(im)
    % load source image
    if nargin < 1
        src = imread(fullfile(mexopencv.root(),'test','blox.jpg'));
    elseif ischar(im)
        src = imread(im);
    else
        src = im;
    end

    % create the UI
    h = buildGUI(src);
    if nargout > 0, varargout{1} = h; end
end

function onChange(~,~,h)
    %ONCHANGE  Event handler for UI controls

    % retrieve current values from UI controls
    maxCorners = round(get(h.slid, 'Value'));
    % maxCorners=0 means all corners found are kept
    %if maxCorners == 0, maxCorners = 500; end
    set(h.txt, 'String',sprintf('Max Corners: %2d',maxCorners));

    % apply Shi-Tomasi corner detector
    corners = cv.goodFeaturesToTrack(h.srcgray, 'MaxCorners',maxCorners, ...
        'QualityLevel',0.01, 'MinDistance',10, ...
        'BlockSize',3, 'UseHarrisDetector',false, 'K',0.04);
    fprintf('Number of corners detected: %d\n', numel(corners));

    % draw corners detected
    out = h.src;
    for i=1:numel(corners)
        out = cv.circle(out, corners{i}, 5, 'Color',[255 0 0], 'Thickness',2);
    end

    % show result
    set(h.img, 'CData',out);
    drawnow;

    % calculate the refined corner locations
    corners2 = cv.cornerSubPix(h.srcgray, corners, 'WinSize',[5 5], ...
        'Criteria',struct('type','Count+EPS', 'maxCount',40, 'epsilon',0.001));
    for i=1:numel(corners)
        fprintf(' -- Refined Corner [%2d] (%3d,%3d) -> (%6.2f,%6.2f)\n', ...
            i, corners{i}, corners2{i});
    end
end

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

    % parameters
    maxCorners = 10;
    max_maxCorners = 50;
    sz = size(img);
    sz(2) = max(sz(2), 300);  % minimum figure width

    % build the user interface (no resizing to keep it simple)
    h = struct();
    h.src = img;
    if size(img,3) > 1
        h.srcgray = cv.cvtColor(img, 'RGB2GRAY');
    else
        h.srcgray = img;
    end
    h.fig = figure('Name','Image Corners Demo', ...
        'NumberTitle','off', 'Menubar','none', 'Resize','off', ...
        'Position',[200 200 sz(2) sz(1)+29]);
    if ~mexopencv.isOctave()
        %HACK: not implemented in Octave
        movegui(h.fig, 'center');
    end
    h.ax = axes('Parent',h.fig, 'Units','pixels', 'Position',[1 30 sz(2) sz(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
    h.txt = uicontrol('Parent',h.fig, 'Style','text', 'FontSize',11, ...
        'Position',[5 5 130 20], 'String',sprintf('Max Corners: %2d',maxCorners));
    h.slid = uicontrol('Parent',h.fig, 'Style','slider', 'Value',maxCorners, ...
        'Min',0, 'Max',max_maxCorners, 'SliderStep',[1 10]./(max_maxCorners-0), ...
        'Position',[135 5 sz(2)-135-5 20]);

    % hook event handlers, and trigger default start
    set(h.slid, 'Callback',{@onChange,h}, ...
        'Interruptible','off', 'BusyAction','cancel');
    onChange([],[],h);
end

Number of corners detected: 10
 -- Refined Corner [ 1] (138,162) -> (136.15,161.56)
 -- Refined Corner [ 2] (230, 98) -> (230.77, 96.75)
 -- Refined Corner [ 3] (135,184) -> (135.42,182.43)
 -- Refined Corner [ 4] ( 47,165) -> ( 48.30,163.26)
 -- Refined Corner [ 5] (168,159) -> (169.76,158.30)
 -- Refined Corner [ 6] ( 26,105) -> ( 24.52,103.50)
 -- Refined Corner [ 7] (238,203) -> (239.23,201.65)
 -- Refined Corner [ 8] ( 85,146) -> ( 85.07,147.57)
 -- Refined Corner [ 9] ( 61,229) -> ( 63.54,228.51)
 -- Refined Corner [10] (169,182) -> (169.13,181.32)