Pre-render frames before displaying moving grating - matlab

Question

I am using DemoDrift2 from Psychtool box to create a moving grating. The grating moves perfectly and works great. However, i need to use it with some other devices connected and working with other softwares. In these situations the grating is displayed slower and jiggles. How can i pre-render the frames before the grating is displayed?
function DriftDemo2(angle, cyclespersecond, f, drawmask, gratingsize)
% function DriftDemo2([angle=30][, cyclespersecond=1][, f=0.05][, drawmask=1],[gratingsize=400])
if nargin < 5
gratingsize = [];
end
if isempty(gratingsize)
% By default the visible grating is 400 pixels by 400 pixels in size:
gratingsize = 400;
end
if nargin < 4
drawmask = [];
end
if isempty(drawmask)
% By default, we mask the grating by a gaussian transparency mask:
drawmask=1;
end;
if nargin < 3
f = [];
end
if isempty(f)
% Grating cycles/pixel: By default 0.05 cycles per pixel.
f=0.05;
end;
if nargin < 2
cyclespersecond = [];
end
if isempty(cyclespersecond)
% Speed of grating in cycles per second: 1 cycle per second by default.
cyclespersecond=1;
end;
if nargin < 1
angle = [];
end
if isempty(angle)
% Angle of the grating: We default to 30 degrees.
angle=30;
end;
movieDurationSecs=20; % Abort demo after 20 seconds.
% Define Half-Size of the grating image.
texsize=gratingsize / 2;
% Screen('Preference', 'SkipSyncTests', 1);
try
% This script calls Psychtoolbox commands available only in OpenGL-based
% versions of the Psychtoolbox.
AssertOpenGL;
% Get the list of screens and choose the one with the highest screen number.
screens=Screen('Screens');
screenNumber=max(screens);
% Find the color values which correspond to white and black:
% 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);
% This makes sure that on floating point framebuffers we still get a
% well defined gray:
if gray == white
gray=white / 2;
end
% Contrast 'inc'rement range for given white and gray values:
inc=white-gray;
% Open a double buffered fullscreen window and set default background
% color to gray:
[w screenRect]=Screen('OpenWindow',screenNumber, gray);
if drawmask
% Enable alpha blending for proper combination of the gaussian aperture
% with the drifting sine grating:
Screen('BlendFunction', w, GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
end
% Calculate parameters of the grating:
p=ceil(1/f);
% frequency in radians:
fr=f*2*pi;
% This is the visible size of the grating. It is twice the half-width
% of the texture plus one pixel to make sure it has an odd number of
% pixels and is therefore symmetric around the center of the texture:
visiblesize=2*texsize+1;
% Create one single static grating image:
%
% We only need a texture with a single row of pixels(i.e. 1 pixel in height) to
% define the whole grating! If the 'srcRect' in the 'Drawtexture' call
% below is "higher" than that (i.e. visibleSize >> 1), the GPU will
% automatically replicate pixel rows. This 1 pixel height saves memory
% and memory bandwith, ie. it is potentially faster on some GPUs.
%
% However it does need 2 * texsize + p columns, i.e. the visible size
% of the grating extended by the length of 1 period (repetition) of the
% sine-wave in pixels 'p':
x = meshgrid(-texsize:texsize + p, 1);
% Compute actual cosine grating:
grating=gray + inc*cos(fr*x);
% Store 1-D single row grating in texture:
gratingtex=Screen('MakeTexture', w, grating);
% Create a single gaussian transparency mask and store it to a texture:
% The mask must have the same size as the visible size of the grating
% to fully cover it. Here we must define it in 2 dimensions and can't
% get easily away with one single row of pixels.
%
% We create a two-layer texture: One unused luminance channel which we
% just fill with the same color as the background color of the screen
% 'gray'. The transparency (aka alpha) channel is filled with a
% gaussian (exp()) aperture mask:
mask=ones(2*texsize+1, 2*texsize+1, 2) * gray;
[x,y]=meshgrid(-1*texsize:1*texsize,-1*texsize:1*texsize);
mask(:, :, 2)=white * (1 - exp(-((x/90).^2)-((y/90).^2)));
masktex=Screen('MakeTexture', w, mask);
% Query maximum useable priorityLevel on this system:
priorityLevel=MaxPriority(w); %#ok<NASGU>
% We don't use Priority() in order to not accidentally overload older
% machines that can't handle a redraw every 40 ms. If your machine is
% fast enough, uncomment this to get more accurate timing.
%Priority(priorityLevel);
% Definition of the drawn rectangle on the screen:
% Compute it to be the visible size of the grating, centered on the
% screen:
dstRect=[0 0 visiblesize visiblesize];
dstRect=CenterRect(dstRect, screenRect);
% Query duration of one monitor refresh interval:
ifi=Screen('GetFlipInterval', w);
% Translate that into the amount of seconds to wait between screen
% redraws/updates:
% waitframes = 1 means: Redraw every monitor refresh. If your GPU is
% not fast enough to do this, you can increment this to only redraw
% every n'th refresh. All animation paramters will adapt to still
% provide the proper grating. However, if you have a fine grating
% drifting at a high speed, the refresh rate must exceed that
% "effective" grating speed to avoid aliasing artifacts in time, i.e.,
% to make sure to satisfy the constraints of the sampling theorem
% (See Wikipedia: "Nyquist?Shannon sampling theorem" for a starter, if
% you don't know what this means):
waitframes = 1;
% Translate frames into seconds for screen update interval:
waitduration = waitframes * ifi;
% Recompute p, this time without the ceil() operation from above.
% Otherwise we will get wrong drift speed due to rounding errors!
p=1/f; % pixels/cycle
% Translate requested speed of the grating (in cycles per second) into
% a shift value in "pixels per frame", for given waitduration: This is
% the amount of pixels to shift our srcRect "aperture" in horizontal
% directionat each redraw:
shiftperframe= cyclespersecond * p * waitduration;
% Perform initial Flip to sync us to the VBL and for getting an initial
% VBL-Timestamp as timing baseline for our redraw loop:
vbl=Screen('Flip', w);
% We run at most 'movieDurationSecs' seconds if user doesn't abort via keypress.
vblendtime = vbl + movieDurationSecs;
i=0;
% Animationloop:
while(vbl < vblendtime)
% Shift the grating by "shiftperframe" pixels per frame:
% the mod'ulo operation makes sure that our "aperture" will snap
% back to the beginning of the grating, once the border is reached.
% Fractional values of 'xoffset' are fine here. The GPU will
% perform proper interpolation of color values in the grating
% texture image to draw a grating that corresponds as closely as
% technical possible to that fractional 'xoffset'. GPU's use
% bilinear interpolation whose accuracy depends on the GPU at hand.
% Consumer ATI hardware usually resolves 1/64 of a pixel, whereas
% consumer NVidia hardware usually resolves 1/256 of a pixel. You
% can run the script "DriftTexturePrecisionTest" to test your
% hardware...
xoffset = mod(i*shiftperframe,p);
i=i+1;
% Define shifted srcRect that cuts out the properly shifted rectangular
% area from the texture: We cut out the range 0 to visiblesize in
% the vertical direction although the texture is only 1 pixel in
% height! This works because the hardware will automatically
% replicate pixels in one dimension if we exceed the real borders
% of the stored texture. This allows us to save storage space here,
% as our 2-D grating is essentially only defined in 1-D:
srcRect=[xoffset 0 xoffset + visiblesize visiblesize];
% Draw grating texture, rotated by "angle":
Screen('DrawTexture', w, gratingtex, srcRect, dstRect, angle);
if drawmask==1
% Draw gaussian mask over grating:
Screen('DrawTexture', w, masktex, [0 0 visiblesize visiblesize], dstRect, angle);
end;
% Flip 'waitframes' monitor refresh intervals after last redraw.
% Providing this 'when' timestamp allows for optimal timing
% precision in stimulus onset, a stable animation framerate and at
% the same time allows the built-in "skipped frames" detector to
% work optimally and report skipped frames due to hardware
% overload:
vbl = Screen('Flip', w, vbl + (waitframes - 0.5) * ifi);
% Abort demo if any key is pressed:
if KbCheck
break;
end;
end;
% Restore normal priority scheduling in case something else was set
% before:
Priority(0);
%The same commands wich close onscreen and offscreen windows also close
%textures.
Screen('CloseAll');
catch
%this "catch" section executes in case of an error in the "try" section
%above. Importantly, it closes the onscreen window if its open.
Screen('CloseAll');
Priority(0);
psychrethrow(psychlasterror);
end %try..catch..enter code here
Any help is appreciated!

Answer 1

Pre-rendering is done by calling owpnt = Screen('OpenOffscreenWindow',...) and then issuing your regular drawing commands, but passing owpnt as the destination. These are then written to a texture, which you can display with Screen('DrawTexture').
You'd need one offscreen window per frame. This will quickly exhaust your video card's memory. So:
Make sure to keep the dimensions of the offscreen window as small as possible: make it just fit your grating
Screen('Close') unneeded offscreen windows asap