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Introduction to PsychoJL

PsychoJL is a module for writing psychology and psychophysics experiments. The general framework and style is inspired by PsychoPy, but there is no collaboration with the authors of PsychoPy.

Matt Peterson, 2023-2024

Overview

PsychoJL is a module for writing psychology experiments. Typically, before a trial begins, stimuli are drawn offscreen into the video buffer. When it is time to present the stimuli, the flip() function is called and the offscreen image is displayed.

Differences between PsychoPy and PsychoJL

The main difference between the two involves how objects are called in Julia. For example, to make and draw a TextStim, you would write something like this in PsychoPy:

stim = visual.TextStim(win, 
                    	'Hello World!',
                    	pos=(0.0, 0.0),		# center of the screen
                    	color=(1, 0, 0), 
                    	colorSpace='rgb')
TextStim.draw()

In Julia, it would look like this:

stim = TextStim(win, 
                "Hello World!",
                [300, 100], 		# position
                color=(255, 0, 0))
draw(stim)

Notice that Julia does not use the Object.method() syntax of Python. Instead, the stimulus is passed to the draw() function.

In addition, Julia objects are really structures (data) with a constructor function of the same name. For example, I can make a new TextStim using the TextStim() constructor function, and latter change one of its properties using dot notation.

stim = TextStim(win, 
                "Hello World!",
                [300, 100], 		# position
                color=(255, 0, 0))
stim.textMessage = "Goodbye, world!"

Usage Rules

  1. The function InitPsychoJL() just be called before any PsychoJL functions are called.
  2. The Window() constructor for the main window should be called before using any PsychoJL functions, other than GUI calls.
  3. GUI dialog windows should be called before the main Window has been made.
  4. GUI dialog windows can be callled after the main Window has been closed.
  5. Do not taunt Happy Fun Ball.

Example

The function 

using PsychoJL

function DemoWindow()
	InitPsychoJL()
	myWin = Window( [1000,1000], false)	# dimensions, fullscreen = false

	newRect = Rect(myWin, 
			100,			# width
			100, 			# height
			[200,200],		# position
			lineColor = [255,0,0], 
			fillColor = [255,128,128] 
			)
	draw(newRect) 		# in PsychoPy this would have been newRect.draw()

	myText = TextStim(myWin,  		# window
			"Using a textStim", 	# text
			[300, 100], 		# position
			color = [255, 255, 128]
			)
	draw(myText) 		# in PsychoPy this would have been myText.draw()
	flip(myWin)
	wait(2000)		# core.wait in Psychopy.  Default timeScale (see Window) is in milliseconds.
end
#------
DemoWindow()

Known issues

Color

Currently, color is r,g,b, alpha, with values from 0-255. Planned color spaces include:

• 0.0 ... 1.0: (Float64)

• -1.0 ... +1.0: (Float64) PsychoPy style color

• Strings: "red", "brown", "gray", etc.

Coordinate system

Currently, the origin is in the top-left and measurements are in pixels. Planned coordinate systems include:

• Percentage of height: origin is in the top left, and x and y coordinates are a percentage of screen height. On a 2560 x 1440, the bottom right coordinate would be ( 1.78, 1.0)

• Psychopy "height": origin is in the center of the screen. Negative y-values are below the origin, and positive are above the origin. On a 2560 x 1440, the top left coordinate would be (-0.89,+0.50), and the bottom right coordinate would be (+0.89,-0.50)

Coordinate system

The default timescale is milliseconds, but seconds is also an option. The timescale used for your experiment is set by passing milliseconds or seconds as one of the optional parameters when creating a main window.

Technology

All graphics and input are handled by SDL.jl. I translated parts of SDL2_gfxPrimitives from C to Julia, with some code replaced with more efficient algorithms.

Manual Outline