rriley
commented
7 months ago Here is a simple program to test drift:
from cmu_graphics import *
import datetime
# Patched drift: 0.3 seconds in 60 seconds
# Unpatched drift: 14.5 seconds in 60 seconds
def onAppStart(app):
app.stepsPerSecond = 30
app.startTime = None
app.count = 0
def redrawAll(app):
now = datetime.datetime.now()
if app.startTime != None:
delta = now - app.startTime
drift = delta.total_seconds() - (app.count / app.stepsPerSecond)
drawLabel(
f"Internal timer: {app.count/app.stepsPerSecond:.2f} seconds",
app.width // 2,
app.height // 3,
size=app.width // 20,
)
drawLabel(
f"Real time: {delta.total_seconds():.2f} seconds",
app.width // 2,
app.height // 2,
size=app.width // 20,
)
drawLabel(
f"Drift: {drift:.2f} seconds",
app.width // 2,
2 * app.height // 3,
size=app.width // 20,
)
def onStep(app):
# This seems like something that should be done in onAppStart, but it is
# better to do here so that we make sure both timers start at as close
# to the same time as possible.
if app.startTime == None:
app.count = 0
app.startTime = datetime.datetime.now()
else:
app.count += 1
# This is how you run the program
runApp(width=800, height=800)
dkosbie
schmave
This change fixes how often
onStepis called by using a tighter tolerance for how often to call it based on the milli-seconds passed since the last tick. This is important because any programs that count ticks to estimate time elapsed were previously very inaccurate. For astepsPerSecondof 30, the old code would drift by 14.5 seconds every minute for a simple stopwatch, which is excessive and noticeable. This modified version only drifts 0.3 seconds.If a student needs more accuracy, they should probably hook the
onMainLoopEventto get the exact milli-seconds passed each loop and do the math themselves.