Contents

Image Warping

A demo program shows how perspective transformation applied on an image. Based on a sample code from MareArts blog.

Sources:

function varargout = warp_perspective_demo_gui(im)

    % load source image and set initial ROI corners (4 points clockwise)
    roi = [];
    if nargin < 1
        if true
            im = fullfile(mexopencv.root(), 'test', 'books_right.jpg');
            roi = [360 109; 532 138; 460 417; 317 338];
        else
            im = fullfile(mexopencv.root(), 'test', 'box_in_scene.png');
            roi = [243 188; 328 151; 388 294; 319 351];
        end
        img = cv.imread(im, 'Color',true);
    elseif ischar(im)
        img = cv.imread(im, 'Color',true);
    else
        img = im;
    end
    if isempty(roi)
        roi = bsxfun(@rdivide, [size(img,2) size(img,1)], ...
            [1.7 4.2; 1.15 3.32; 1.33 1.1; 1.93 1.36]);
    end

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

Callback Functions

    function redraw()
        %REDRAW  Apply transformation using current ROI and display results

        % reapply transformation
        out1 = drawROI(img, roi);
        [out2, sz] = warpROI(img, roi);

        % show results and ajust second plot to fit image
        set(h.img(1), 'CData',out1);
        set(h.img(2), 'CData',out2, 'XData',[1 sz(1)], 'YData',[1 sz(2)]);
        set(h.ax(2), 'XLim',[0 sz(1)]+0.5, 'YLim',[0 sz(2)]+0.5);
        pos = get(h.fig(2), 'Position');
        set(h.fig(2), 'Position',[pos(1:2) sz]);
        drawnow;
    end

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

        switch e.Key
            case 'h'
                helpdlg({
                    'Use your mouse to select a point and move it'
                    'to see transformation changes.'
                    ''
                    'Hot keys:'
                    'h - this help dialog'
                    'q - quit the program'
                    'r - change order of points to rotate transformation'
                    'i - change order of points to invert transformation '
                });
            case {'q', 'escape'}
                close(h.fig);
            case 'r'
                roi = circshift(roi, -1);
                redraw();
            case 'i'
                roi = roi([2 1 4 3],:);
                redraw();
        end
    end

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

        % hit-test for closest ROI corner
        pt = getCurrentPoint(h.ax(1));
        d = sum(abs(bsxfun(@minus, roi, pt)), 2);
        [mn, pt_idx] = min(d);
        if mn < 20
            % attach event handlers, and change mouse pointer
            set(h.fig(1), 'Pointer','cross', ...
                'WindowButtonMotionFcn',{@onMouseMove, pt_idx}, ...
                'WindowButtonUpFcn',@onMouseUp);
        end
    end

    function onMouseMove(~,~,idx)
        %ONMOUSEMOVE  Event handler for mouse move on figure

        % move specified ROI corner
        pt = getCurrentPoint(h.ax(1));
        roi(idx,:) = pt;
        redraw();
    end

    function onMouseUp(~,~)
        %ONMOUSEUP  Event handler for mouse up on figure

        % detach event handlers, and restore mouse pointer
        set(h.fig(1), 'Pointer','arrow', ...
            'WindowButtonMotionFcn','', ...
            'WindowButtonUpFcn','');
    end

end

Helper Functions

function out = drawROI(img, pts)
    %DRAWROI  Show ROI corners with labels

    labels = {'TL', 'TR', 'BR', 'BL'};
    out = cv.polylines(img, pts, 'Closed',true, 'Color',[0 0 255], 'Thickness',2);
    out = cv.circle(out, pts, 5, 'Color',[0 255 0], 'Thickness',3);
    out = cv.putText(out, labels, pts, ...
        'FontScale',0.8, 'Color',[255 0 0], 'Thickness',2);
end

function [out, dsz] = warpROI(img, srcPts)
    %WARPROI  Compute warped image from ROI

    % map ROI corners to corresponding destination rectangle
    len = diff(srcPts([1:end 1],:), 1, 1);  % [TR-TL; BR-TR; BL-BR; TL-BL]
    len = cellfun(@norm, num2cell(len, 2)); % lengths of sides
    w = max(len(1), len(3));
    h = max(len(2), len(4));
    dstPts = [0 0; w 0; w h; 0 h];

    % compute homography between points and apply persepective transformation
    dsz = round([w h]);
    H = cv.findHomography(srcPts, dstPts);
    out = cv.warpPerspective(img, H, 'DSize',dsz);
end

function p = getCurrentPoint(ax)
    %GETCURRENTPOINT  Retrieve current mouse location

    p = get(ax, 'CurrentPoint');
    p = p(1,1:2) - 1;
end

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

    % apply initial perspective transformation
    out1 = drawROI(img, roi);
    out2 = warpROI(img, roi);
    sz1 = size(out1);
    sz2 = size(out2);

    % properties
    fprops = {'Menubar','none', 'Resize','on'};
    aprops = {'Units','normalized', 'Position',[0 0 1 1]};
    h = struct();

    % show input image + ROI corners
    h.fig(1) = figure('Name','Image', ...
        'Position',[100 200 sz1(2) sz1(1)], fprops{:});
    h.ax(1) = axes('Parent',h.fig(1), aprops{:});
    if mexopencv.isOctave()
        h.img(1) = imshow(out1);
    else
        h.img(1) = imshow(out1, 'Parent',h.ax(1));
    end

    % show warped image
    h.fig(2) = figure('Name','Warped', ...
        'Position',[200+sz1(2) 200 sz2(2) sz2(1)], fprops{:});
    h.ax(2) = axes('Parent',h.fig(2), aprops{:});
    if mexopencv.isOctave()
        h.img(2) = imshow(out2);
    else
        h.img(2) = imshow(out2, 'Parent',h.ax(2));
    end
end