How to create a lineStim that can rotate around the centre of the screen?

Builder
1

A very simple question:

I need to create a lineStim that can rotate around the centre of the screen.
But rather than rotating around its centre, it needs to rotate around its end point – like a clock hand.

When I create a lineStim using the Polygon component, builder then creates a line that rotates around its centre point

So how can I create a line that rotates around one of its endpoints?

2

In principle you need to rotate the line by an angle and then also offset it from the centre of the screen by half its length at the same angle.

xpos = cos(x)*length/2
ypos = sin(x)*length/2

Best wishes,

Wakefield

3

HI @wakecarter,
thanks for this – Yup. Makes sense.
But, alas, that is not working. For some reason, the line is jumping around like crazy.
I want the line to rotate once per 2560ms. This equates to a change in angle of 140.625 degrees per second. Defining the angle (ori) in terms of trial time t thus gives t*140.625
I define my stims in height units and go for a length of .3

So I have ori = t140.625
pos = (.15cos(t140.625),.15 * sin(t140.625))

and that leads to the crazy jumping.

I then tried to bug catch and simply wanted to the move the x co-ordinate of a polygon in a sine-wave fashion, i.e. set ori = 0, and simply have pos = (.15cos(t140.625), 0)
This shows that the stimulus is jumping along the x axis, rather than moving smoothly.

What am I doing wrong?

4

I think you need to use radians not degrees

5

Switch to the Coder view and go to the “Demos” menu. There you will find a shape stim demo that draws a clock face, continually updating the second, minute, and hour hands.

6

The ori is certainly specified in degrees…

7

The sin and cos functions use radians

8

silly me! Duh!