Hi
I am trying to write a code that will show an acceleration wheel. as long as the user press 'a' the wheel should accelerate counterclock wize. the thing is that it turn in the right direction but it doesn't accelerate. This is the code I am using . PTB-3 windows XP.

img=imread('c:\images.jpg');
[yimg,ximg,z]=size(img);
rot_spd = 1;
larrow = KbName('a'); % modify this for Windows
rarrow = KbName('b');
[w,rect]=Screen('OpenWindow',0,[0 0 0]);
sx = 400; % desired x-size of image (pixels)
sy = yimg*sx/ximg; % desired y-size--keep proportional
t = Screen('MakeTexture',w,img);
bdown=0;
th = 0; % initial rotation angle (degrees)
HideCursor
while(~any(bdown)) % exit loop if mouse button is pressed
[x,y,bdown]=GetMouse;
[keyisdown,secs,keycode] = KbCheck;
if(keycode(larrow))
th = th - rot_spd-1; % accelerate counterclockwise
th
end
if(keycode(rarrow))
th = th + rot_spd+1; % accelerate clockwise
th
end
destrect=[x-sx/2,y-sy/2,x+sx/2,y+sy/2];
Screen('DrawTexture',w,t,[],destrect,th);
Screen('Flip',w);
end
Screen('Close',w)
ShowCursor

If anyone has an odea why it doesn't accelerate I'd appriciate it very much.

Ariel