use shading to demonstrate zero area functions

k = 2;
T = 1;
t = linspace(-1,1,201)*T/2;
ys = cos(2*pi*k*t);
plot(t,ys,[-T T]/2,[0 0],'k');
grid

x = linspace(-1,1,512)*T/2;
y = linspace(1,-1,256);
tmp = ones(size(y));
tmp = x'*tmp;
xi = tmp';
tmp = ones(size(x));
yi = y'*tmp;
subplot(1,2,1);
imshow(xi/T+0.5);
subplot(1,2,2);
imshow(0.5*yi+0.5);

subplot(1,2,1);
z = cos(2*pi*k*xi);
msk1 = (yi>0) & (z>yi);
imshow(msk1);
subplot(1,2,2);
msk2 = (yi<=0) & (z<yi);
imshow(msk2);
msk3 = ones(size(msk1));

rgb = im2double(cat(3,~msk1,~(msk1|msk2),~msk2));
subplot(1,1,1);
imshow(rgb);

subplot(2,1,1);
plot(t,ys,[-T T]/2,[0 0],'k');
grid
subplot(2,1,2);
imshow(rgb);