Set pixels on screen efficiently - matlab

Question

I am using WindowAPI (http://www.mathworks.com/matlabcentral/fileexchange/31437) to show a black full screen in matlab.
When drawing on screen, turns out drawing using line() and rectangle() functions is extremely slow.
How can I set values of pixels without going through matlab's mechanism? Getting the window's canvas for example would be great.

Answer 1

One way to imitate a "canvas" is by using a MATLAB image. The idea is to manually change its pixels and update the 'CData' of the plotted image.
Note that you can use an image with the same dimensions as your screen size (image pixels will correspond to screen pixels one-to-one), but updating it would be slower. The trick is to keep it small and let MATLAB map it to the entire fullscreen. That way the image can be thought of as having "fat" pixels. Of course the resolution of the image is going to affect the size of the marker you draw.
To illustrate, consider the following implementation:
function draw_buffer()
%# paramters (image width/height and the indexed colormap)
IMG_W = 50; %# preferably same aspect ratio as your screen resolution
IMG_H = 32;
CMAP = [0 0 0 ; lines(7)]; %# first color is black background
%# create buffer (image of super-pixels)
%# bigger matrix gives better resolution, but slower to update
%# indexed image is faster to update than truecolor
img = ones(IMG_H,IMG_W);
%# create fullscreen figure
hFig = figure('Menu','none', 'Pointer','crosshair', 'DoubleBuffer','on');
WindowAPI(hFig, 'Position','full');
%# setup axis, and set the colormap
hAx = axes('Color','k', 'XLim',[0 IMG_W]+0.5, 'YLim',[0 IMG_H]+0.5, ...
'Units','normalized', 'Position',[0 0 1 1]);
colormap(hAx, CMAP)
%# display image (pixels are centered around xdata/ydata)
hImg = image('XData',1:IMG_W, 'YData',1:IMG_H, ...
'CData',img, 'CDataMapping','direct');
%# hook-up mouse button-down event
set(hFig, 'WindowButtonDownFcn',#mouseDown)
function mouseDown(o,e)
%# convert point from axes coordinates to image pixel coordinates
p = get(hAx,'CurrentPoint');
x = round(p(1,1)); y = round(p(1,2));
%# random index in colormap
clr = randi([2 size(CMAP,1)]); %# skip first color (black)
%# mark point inside buffer with specified color
img(y,x) = clr;
%# update image
set(hImg, 'CData',img)
drawnow
end
end