Displaying shapes in random locations?

Hi everyone!

I am coding an experiment that requires to click the shapes on the screen with mouse clicks. However, I could not find a way to make shapes randomly located on the screen for each trial.

I use 'FillRect' or 'FillOval':

Screen('FillOval',windowPtr,[150 150 150],[? ? ? ?])

I would be very happy if you can help me out with that.

Thank you in advance.

[Walter Roberson]

This appears to relate to the third-party Psychtoolbox, which defines Screen()

[Eren Serefoglu]

Exactly. I can draw a shape on the screen by this function. Just need to randomize the location for each trial.

Answer 1

[Walter Roberson]

http://psychtoolbox.org/docs/Screen-FillOval

Instead of filling one oval, you can also specify a list of multiple ovals to be
filled - this is much faster when you need to draw many ovals per frame. To fill
n ovals, provide “rect” as a 4 rows by n columns matrix, each column specifying
one oval, e.g., rect(1,5)=left border of 5th oval, rect(2,5)=top border of 5th
oval, rect(3,5)=right border of 5th oval, rect(4,5)=bottom border of 5th oval

We can deduce from this that a rect is defined by [left, top, right, bottom] . So choose random positions such that the left border + desired width does not exceed the right boundary, and top border + desired height does not exceed the bottom boundary. randi() can help for that.

See also the tutorials at https://peterscarfe.com/ptbtutorials.html which include a few that use randomization.

But one question I would have is whether the random shapes need to avoid the locations of existing objects in the frame? Packing randomly positioned objects in a non-overlapping way into a constrained area can get difficult.

Answer 2

[Image Analyst]

See my attached demo code that makes a bunch or randomly oriented shapes.