PsychoPy-Arduino Exercise #3
Using the calibrated Arduino light sensor to measure the LATENCY
of a screen stimulus onset (i.e., brief WHITE screen stimulus).

In Arduino Exercise #1, we built a PsychoPy project to calibrate the 
Black and White levels of the Arduino light sensor.  In Exercise #3,
we will extend this project with code to allow us to measure the
onset latency of a video screen stimulus.

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Intro Routine
-------------

Custom code|Begin Experiment Tab
-------------------------------

Add the following code to your tab that creates the 'arduino'
serial communication object:

	#synchronous screen-flip trigger routine
	#example usage: win.callOnFlip(send_trigger,'l')
	import time
	global trigger_time
	def send_trigger(char):
		global trigger_time
		trigger_time = globalClock.getTime()
		arduino.write(bytes(char,'utf-8'))

	#screen latency global
	usdLatency = 0.0

NEXT, CREATE/ADD 2 NEW ROUTINES TO YOUR PSYCHOPY PROJECT.
NAME THEM: 'BlackScreen' and 'WhiteScreen'

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BlackScreen Routine
-------------------
Add a Polygon/Rectangle component the fills the entire screen.
Set its fill color to Black and its time duration to 30 frames.
No custom code needed for this routine.

-------------------
WhiteScreen Routine
-------------------
This routine generates the stimulus whose onset latency will be measured.
Create a Polygon/Rectangle component.
The rectangle should fill the entire screen and its fill color = White.
Set its Start Time to Frame=0 and its duration to 60 frames.
Add the following custom code to the routine:

Custom code|Each Frame Tab
--------------------------
	#send synchronous screen trigger to Arduino
	if frameN == 0:
		win.callOnFlip(send_trigger,'l')  #request Latency mesurement ('l' = ell)

	#scan for Arduino LATENCY measurement
	if arduino.in_waiting > 0:
		serial_input = arduino.readline()
		if len(serial_input) > 0:
			serial_str = serial_input.strip().decode()
			serial_data = int(serial_str)
			usdLatency = serial_data
			print("LATENCY:",serial_str)
			continueRoutine = False  #end WhiteScreen routine
        
	#exit if no Arduino ressponse within 30 frames
	if frameN >= 30:
		usdLatency = 0.0
		print("WHITE screen timed-out")
		continueRoutine = False

Custom code|End Routine Tab
---------------------------
	#save trigger time
	thisExp.addData('trigger',trigger_time)
	thisExp.addData('latency',usdLatency)

Custom code|End Experiment Tab
------------------------------
	arduino.close()


Run the PsychoPy script and check the console and Excel output file
for the results of the latency measurement.