function GazeContingentDemo(mode, ms, myimgfile, usehmd)
%
% ___________________________________________________________________
%
% Demo implementation of a generic gaze-contingent display.
% We take one input image and create - via image processing - two images
% out of it: An image to show at the screen location were the subject
% fixates (According to the eye-tracker). A second image to show in the
% peripery of the subjects field of view. These two images are blended into
% each other via a gaussian weight mask (an aperture). The mask is centered
% at the center of gaze and allows for a smooth transition between the two
% images.
%
% This illustrates an application of OpenGL Alpha blending by compositing
% two images based on a spatial gaussian weight mask. Compositing is done
% by the graphics hardware.
%
% If you set the optional 'usehmd' parameter to 1 then the demo will display on
% a VR HMD, and if that HMD has a suported eyetracker, it will be used to move
% the foveated area gaze contingent with the users tracked gaze.
%
% See also: PsychDemos, MovieDemo, DriftDemo

% HISTORY
%
% mm/dd/yy
%
%  7/23/05    mk      Derived it from Frans Cornelissens AlphaImageDemoOSX.
% 11/19/06    dhb     Remove OSX from name.

% Set hurryup = 1 for benchmarking - Syncing to retrace is disabled
% in that case so we'll get the maximum refresh rate.
hurryup=0;

% Setup default mode to color vs. gray.
if nargin < 1 || isempty(mode)
    mode = 1;
end

% Setup default aperture size to 2*200 x 2*200 pixels.
if nargin < 2 || isempty(ms)
    ms=200;
end

basepath = [ PsychtoolboxRoot 'PsychDemos' filesep ];

% Use default demo images, if no special image was provided.
if nargin < 3 || isempty(myimgfile)
    myimgfile= [basepath 'konijntjes1024x768.jpg'];
end

myblurimgfile= [basepath 'konijntjes1024x768blur.jpg'];
mygrayimgfile= [basepath 'konijntjes1024x768gray.jpg'];

if nargin < 4 || isempty(usehmd)
    usehmd = 0;
end

% No gaze tracking by default:
gazetracking = 0;

try
    fprintf('GazeContingentDemo (%s)\n', datestr(now)); %#ok<TNOW1,DATST>
    fprintf('Press a key or click on mouse to stop demo.\n');

    PsychDefaultSetup(1);

    % Get the list of screens and choose the one with the highest screen number.
    % Screen 0 is, by definition, the display with the menu bar. Often when
    % two monitors are connected the one without the menu bar is used as
    % the stimulus display.  Chosing the display with the highest display number is
    % a best guess about where you want the stimulus displayed.
    screenNumber=max(Screen('Screens'));
    backgroundcolor = GrayIndex(screenNumber);

    % Open a double buffered fullscreen window.
    PsychImaging('PrepareConfiguration');
    if usehmd
        hmd = PsychVRHMD('AutoSetupHMD', 'Monoscopic', 'NoTimingSupport NoTimestampingSupport DebugDisplay Eyetracking');
        if isempty(hmd)
            fprintf('User wants to use HMD, but no HMD detected. Game over!\n');
            return;
        end
    end

    [w, wRect] = PsychImaging('OpenWindow', screenNumber, backgroundcolor);

    % Enable gaze tracking driven rendering if HMD with eye tracking is available:
    if usehmd
        hmdinfo = PsychVRHMD('GetInfo', hmd);
        if hmdinfo.eyeTrackingSupported
            gazetracking = 1;
        end
    end

    % Load image file:
    fprintf('Using image ''%s''\n', myimgfile);
    imdata=imread(myimgfile);
    imdatablur=imread(myblurimgfile);
    imdatagray=imread(mygrayimgfile);

    % Crop image if it is larger then screen size. There's no image scaling
    % in maketexture
    [iy, ix,~]=size(imdata);
    [wW, wH]=WindowSize(w);

    if ix>wW || iy>wH
        disp('Image size exceeds screen size');
        disp('Image will be cropped');
    end

    if ix>wW
        cl=round((ix-wW)/2);
        cr=(ix-wW)-cl;
    else
        cl=0;
        cr=0;
    end

    if iy>wH
        ct=round((iy-wH)/2);
        cb=(iy-wH)-ct;
    else
        ct=0;
        cb=0;
    end

    % imdata is the cropped version of the image.
    imdata=imdata(1+ct:iy-cb, 1+cl:ix-cr,:);
    imdatablur=imdatablur(1+ct:iy-cb, 1+cl:ix-cr,:);
    imdatagray=imdatagray(1+ct:iy-cb, 1+cl:ix-cr,:);

    % Compute image for foveated region and periphery:
    switch (mode)
        case 1
            % Mode 1:
            % Fovea contains original image data:
            foveaimdata = imdata;
            % Periphery contains grayscale-version:
            peripheryimdata = imdatagray;
        case 2
            % Fovea contains original image data:
            foveaimdata = imdata;
            % Periphery contains blurred-version:
            peripheryimdata = imdatablur;
        case 3
            % Fovea contains color-inverted image data:
            foveaimdata(:,:,:) = 255 - imdata(:,:,:);
            % Periphery contains original data:
            peripheryimdata = imdata;
        case 4
            % Test-case: One shouldn't see any foveated region on the
            % screen - this is a basic correctness test for blending.
            foveaimdata = imdata;
            peripheryimdata = imdata;
        otherwise
            % Unknown mode! We force abortion:
            fprintf('Invalid mode provided!');
            abortthisbeast
    end

    % Build texture for foveated region:
    foveatex=Screen('MakeTexture', w, foveaimdata);
    tRect=Screen('Rect', foveatex);

    % Build texture for peripheral (non-foveated) region:
    nonfoveatex=Screen('MakeTexture', w, peripheryimdata);
    [ctRect, dx, dy]=CenterRect(tRect, wRect);

    % We create a Luminance+Alpha matrix for use as transparency mask:
    % Layer 1 (Luminance) is filled with 'backgroundcolor'.
    transLayer=2;
    [x,y]=meshgrid(-ms:ms, -ms:ms);
    maskblob=ones(2*ms+1, 2*ms+1, transLayer) * backgroundcolor;

    % Layer 2 (Transparency aka Alpha) is filled with gaussian transparency
    % mask.
    xsd=ms/2.2;
    ysd=ms/2.2;
    maskblob(:,:,transLayer)=round(255 - exp(-((x/xsd).^2)-((y/ysd).^2))*255);

    % Build a single transparency mask texture:
    masktex=Screen('MakeTexture', w, maskblob);
    mRect=Screen('Rect', masktex);

    fprintf('Size image texture: %d x %d\n', RectWidth(tRect), RectHeight(tRect));
    fprintf('Size  mask texture: %d x %d\n', RectWidth(mRect), RectHeight(mRect));

    % Do initial flip to show blank screen:
    Screen('Flip', w);

    % The mouse-cursor position will define gaze-position (center of
    % fixation) to simulate (x,y) input from an eyetracker. Set cursor
    % initially to center of screen:
    [a,b]=RectCenter(wRect);
    SetMouse(a,b,screenNumber); % set cursor and wait for it to take effect

    HideCursor;
    buttons=0;

    priorityLevel=MaxPriority(w);
    Priority(priorityLevel);

    % Wait until all keys on keyboard are released:
    KbReleaseWait;

    mxold = 0;
    myold = 0;
    mx = mxold;
    my = myold;

    tavg = 0;
    ncount = 0;

    oldvbl = Screen('Flip', w);

    % Infinite display loop: Whenever "gaze position" changes, we update
    % the display accordingly. Loop aborts on keyboard press or mouse
    % click or after 10000 frames...
    while (ncount < 10000)
        % Query current mouse cursor position (our "pseudo-eyetracker") -
        % (mx,my) is our gaze position.
        if hurryup == 0
            if gazetracking
                state = PsychVRHMD('PrepareRender', hmd, [], 4);
                if state.gazeStatus(1) >= 3
                    mx = state.gazePos{1}(1);
                    my = state.gazePos{1}(2);
                end
            else
                [mx, my, buttons] = GetMouse(w);
            end
        else
            % In benchmark mode, we just do a quick sinusoidal motion
            % without query of the mouse:
            mx=500 + 500*sin(ncount/10); my=300;
        end

        % We only redraw if gazepos. has changed:
        if (mx~=mxold || my~=myold)
            % Compute position and size of source- and destinationrect and
            % clip it, if necessary...
            myrect=[mx-ms my-ms mx+ms+1 my+ms+1]; % center dRect on current mouseposition
            dRect = ClipRect(myrect,ctRect);
            sRect=OffsetRect(dRect, -dx, -dy);

            % Valid destination rectangle?
            if ~IsEmptyRect(dRect)
                % Yes! Draw image for current frame:

                % Step 1: Draw the alpha-mask into the backbuffer. It
                % defines the aperture for foveation: The center of gaze
                % has zero alpha value. Alpha values increase with distance from
                % center of gaze according to a gaussian function and
                % approach 255 at the border of the aperture...
                Screen('BlendFunction', w, GL_ONE, GL_ZERO);
                Screen('DrawTexture', w, masktex, [], myrect);

                % Step 2: Draw peripheral image. It is only drawn where
                % the alpha-value in the backbuffer is 255 or high, leaving
                % the foveated area (low or zero alpha values) alone:
                % This is done by weighting each color value of each pixel
                % with the corresponding alpha-value in the backbuffer
                % (GL_DST_ALPHA).
                Screen('BlendFunction', w, GL_DST_ALPHA, GL_ZERO);
                Screen('DrawTexture', w, nonfoveatex, [], ctRect);

                % Step 3: Draw foveated image, but only where the
                % alpha-value in the backbuffer is zero or low: This is
                % done by weighting each color value with one minus the
                % corresponding alpha-value in the backbuffer
                % (GL_ONE_MINUS_DST_ALPHA).
                Screen('BlendFunction', w, GL_ONE_MINUS_DST_ALPHA, GL_ONE);
                Screen('DrawTexture', w, foveatex, sRect, dRect);

                % Show final result on screen. This also clears the drawing
                % surface back to black background color and a zero alpha
                % value.
                % Actually... We use clearmode=2: This doesn't clear the
                % backbuffer, but we don't need to clear it for this kind
                % of stimulus and it gives us 2 msecs extra headroom for
                % higher refresh rates! For benchmark purpose, we disable
                % syncing to retrace if hurryup is == 1.
                vbl = Screen('Flip', w, 0, 2, 2*hurryup);
                if hurryup
                    vbl = GetSecs; %#ok<UNRCH>
                end
                tavg = tavg + (vbl-oldvbl);
                oldvbl=vbl;
                ncount = ncount + 1;
            end
        end

        % Keep track of last gaze position:
        mxold=mx;
        myold=my;

        % We wait 1 ms each loop-iteration so that we
        % don't overload the system in realtime-priority:
        WaitSecs(0.001);

        % Abort demo on keypress our mouse-click:
        if KbCheck || any(buttons) % break out of loop
            break;
        end
    end

    % Display full image a last time, just for fun...
    Screen('BlendFunction', w, GL_ONE, GL_ZERO);
    Screen('DrawTexture', w, foveatex);
    Screen('Flip', w);

    % The same command which closes onscreen and offscreen windows also
    % closes textures.
    sca;
    ShowCursor;
    Priority(0);
    tavg = tavg / ncount * 1000;
    fprintf('End of GazeContingentDemo. Avg. redraw time is %f ms = %f Hz.\n\n', tavg, 1000 / tavg);
    return;
catch
    %this "catch" section executes in case of an error in the "try" section
    %above.  Importantly, it closes the onscreen window if its open.
    sca;
    ShowCursor;
    Priority(0);
    psychrethrow(psychlasterror);
end %try..catch..