Add modulating plaid stimuli

[k1o0]

Users have had issues with the order of textures in Signals. An example that uses inverted contrasts will hopefully make things clearer. Below is an example of a modulating plaid that takes into account the layer order:

%%% Create a modulating plaid
% Creates some sinewave gratings then modulates their contrasts, accounting
% for layer opacity.  
%%%

% Setup playground
PsychDebugWindowConfiguration % Make our screen transparant
[t, setgraphic] = sig.test.playgroundPTB; 
vs = StructRef; % This mimicks the fourth input arg of your expDef

% Create our stimuli

% This is a vertical sinewave grating
vertical = vis.grating(t, 'sine', 'none');
vertical.orientation = 0;
vertical.contrast = (sin(t) * 0.5) + 0.5;

% This is a horizontal sinewave grating
horizontal = vis.grating(t, 'sine', 'none');
horizontal.orientation = 90;
horizontal.contrast = sin(t);

[horizontal.show, vertical.show] = deal(true); % show both

vs.horizontal = horizontal;
vs.vertical = vertical;

% Render stimulus 
setgraphic(vs)
% Programmatically press the play button
startstop = get(findobj(gcf, 'String', 'Play'), 'Callback');
startstop() % Play button callback

[k1o0]

Modulating plaids turns out to be more difficult than expected. I've implemented a new stimulus function that generates the plaid as a single array, instead of overlaying two sines: plaid = grating1*newelem.contrast(1) + grating2'*newelem.contrast(2);. This works better although the text must be re-loaded each time there's a change, which is less efficient.

plaid.txt