Overlay visual stimuli on to a video

I'm currently using Psychtoolbox to attempt to write a visual task on top of a video background, and looking at EEG patterns and response times. I'm currently attempting to adapt the Demo function "DetectionRTInVideoDemo" to do this and not having much luck. Could anyone tell me if there is a way to either 1. create a transparent screen where stimuli can be placed while still viewing the video underneath, or 2. placing small pictures (e.g. small circles) over top of the texture at certain times in the video ?

Answer 1

[Mihaela Duta]

If you need an image with transparent background, you can create it in other packages, for example GIMP

In your script, you first need to read the image with the transparent background together with the information about the alpha channel:

[img_data ~ img_alpha] = imread(img_fname); img_data(:,:,4) = img_alpha(:,:);

When opening the psychtoolbox window, enable alpha blending

Screen('BlendFunction', theWindow, GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

Create the texture for the image

img_tex  = Screen('MakeTexture', win, double(img_data));

In the main loop of movie playback, overlay the image texture over every movie texture

Screen('DrawTexture', win, movietexture);
Screen('DrawTexture', win, img_tex);

[Joanna Scanlon]

Thanks this was really helpful for adding the images, however I'm having trouble with collecting the img_alpha.

When I run:

 [img_data ~ img_alpha] = imread(cross);

I get:

Error: An array for multiple LHS assignment cannot contain expressions.

But if I replace the '~' with a comma it runs, but does not have a transparent background and img_alpha is empty.

[Walter Roberson]

[imdata, imgmap, imgalpha] = imread(...)

You appear to be using R2008b or earlier

[Joanna Scanlon]

It was on 2012b, but yes this makes sense. Thanks!

Answer 2

[Walter Roberson]

If you were drawing in MATLAB instead of using the Psychtoolbox drawing routines, then you could place image() objects on top of the display (set the sorting order to be sure it is on top; see http://blogs.mathworks.com/graphics/2014/11/04/sortmethod/) . Set the AlphaData channel of the image objects to 0 in the places where what is behind is to show through and set it to 1 in the places where the overlay stimulus is to show.