怎么用psychotoolbox写一个贪吃蛇？

%matlab psychtoolbox3
function TBX_snake    
    AssertOpenGL;

    % 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 dislay number is
    % a best guess about where you want the stimulus displayed.
    screens=Screen('Screens');
    screenNumber=max(screens);

      % Find the color values which correspond to white and black: Usually
    % black is always 0 and white 255, but this rule is not true if one of
    % the high precision framebuffer modes is enabled via the
    % PsychImaging() commmand, so we query the true values via the
    % functions WhiteIndex and BlackIndex:
    white=WhiteIndex(screenNumber);
    black=BlackIndex(screenNumber);

    % Round gray to integral number, to avoid roundoff artifacts with some
    % graphics cards:
    gray=round((white+black)/2);
    ScreenRct = [100 100 700 700];
    M = zeros( ScreenRct(3) - ScreenRct(1) ,ScreenRct(4) - ScreenRct(2) );
    S = size(M,1) ;
    % This makes sure that on floating point framebuffers we still get a
    % well defined gray. It isn't strictly neccessary in this demo:
    if gray == white
        gray=white / 2;
    end
    KbName('UnifyKeyNames');
    escapeKey = KbName('ESCAPE');
    % Contrast 'inc'rement range for given white and gray values:
    inc=white-gray;

    % Open a double buffered fullscreen window and select a gray background
    % color:
    
    % Use realtime priority for better timing precision:

    w=Screen('OpenWindow',screenNumber, gray , ScreenRct);
    priorityLevel=MaxPriority(w);
    Priority(priorityLevel);
    
    Stage = zeros(50); 
    Snak.loc = size(Stage)/2  ;
    Snak.direct= 'l';
    Snak.predirect = Snak.direct;
    Snak.lenth = 5;

    Snak.body = [[Snak.loc(1):Snak.loc(1)+Snak.lenth]'  repmat( Snak.loc(2), Snak.lenth+1 ,1)   ];
    Snak.food = [];%randperm(length(Stage),1);
    Snak.crash = false;
    while true
%     for i=1:movieDurationFrames
%         M(randperm(N,100))=255;

        [Stage, Snak] =  snak_food(Stage, Snak);
        [Stage, Snak ]=updateSnake(Stage, Snak );
        if Snak.crash 

            break
        end
        M = magnify( Stage ,S);
        
%         T = Screen('MakeTexture', w, M );       
%         Screen('DrawTexture', w, T);     
        Screen('Putimage',w,M)

        [secs, keyCode, deltaSecs] = KbWait([], [],GetSecs+0.08);
%         [ keyIsDown, ~, keyCode ] = KbCheck;
         keyCode = find(keyCode, 1); % 37:40
          
        if ~isempty(keyCode)
            if keyCode == escapeKey
                disp escape!
                break;
            else
                switch keyCode
                    case 37
                        Snak.direct= 'l';
                    case 38
                        Snak.direct= 'u';
                    case 39
                        Snak.direct= 'r';
                    case 40
                        Snak.direct= 'd';
                end
            end
        end
       
        Screen('Flip', w ,[],1);
        
    end    
    
    
    
if ~Snak.crash 
    sca;
else
    msgbox 'crashed!'
end
    Priority(0);
    
    
end

function [Stage, Snak] =  snak_food(Stage, Snak)

if ~isempty(Snak.food)
    return
end
    freeStage = Stage==0;
    idx = randperm(sum(freeStage,'all'),1)  ;
    [x,y] = find(freeStage) ;
    
    Stage(x(idx) , y(idx)) = 0.5;
    
    Snak.food = [x(idx) , y(idx)];
    
%     Snak.food = [x(idx),y(idx)];

 
end

function [Stage, Snak ]=updateSnake(Stage, Snak )
% Snak.loc = [0 0 ] ;
% Snak.direct= 'l';
loc = Snak.loc;


fprintf('pre=%s ; now=%s \n',Snak.predirect,Snak.direct);

linearIndold = sub2ind([size(Stage)], Snak.body(:,1)  ,Snak.body(:,2) );
d={'u' 'd';'l' 'r'};

nd = intersect( [{Snak.direct} {Snak.predirect}] , d(1,:)) ;
if length(nd)>1
    Snak.direct = Snak.predirect ;
end
nd = intersect( [{Snak.direct} {Snak.predirect}] , d(2,:)) ;
if length(nd)>1
    Snak.direct = Snak.predirect ;
end 
Snak.predirect = Snak.direct ;

switch Snak.direct
    case 'u'
        loc(1) = loc(1)-1;        
    case 'd'
        loc(1) = loc(1)+1;        
    case 'l'
        loc(2) = loc(2)-1;        
    case 'r'
        loc(2) = loc(2)+1;        
end


Snak.body =   [loc ; Snak.body    ] ;
    
Snak.loc = loc  ;
if ~isequal(Snak.loc, Snak.food)
    Snak.body(end,:) = [] ;
    try
    if Stage(loc(1),loc(2))==1 
        Snak.crash = true;         
    end
    catch
        Snak.crash = true;         
    end
    
else

    Snak.food=[];
    
end

 
if Snak.crash 
    return
end

linearInd = sub2ind([size(Stage)], Snak.body(:,1)  ,Snak.body(:,2) );
 
Stage(linearIndold) = 0;    
Stage(linearInd) = 1;    
 

end

function M = magnify( Stage, S )
N = floor( S/size(Stage,1));


M = resizem(Stage,N);


M = M*255;

% color
M = repmat(M,1,1,3);
M(:,:,3) = ones(size(M,1));


end