I’m trying to make a clock for a Libet task – a cognitive task used by psychologists. By convention these clocks take 2560 ms to complete a revolution. Mine seems to be running quite a lot slower and I can figure out why.
I have made an example of the issue here: https://jsfiddle.net/Sumoza/re4v7Lcj/8/
On each iteration of a setInterval with a 1ms delay, I increase the angle of rotation of the hand by (Math.PI*2)/2560, i.e. 1/2560th of a circle in radians. So, incrementing one of these each ms should complete the circle in that number of ms. As you can see from the clock, this runs a fair bit slower. Can anyone shed any light as to why. Interestingly, *10 seems to look a lot closer to what I want – but that doesn’t make much sense to me so I’m wary of the fix. Relevant JS from the fiddle below. Thanks for any help.
var time = 0;
var rad_tick = (Math.PI*2)/2560; //radians per tick: last number is ms per revolution
// Draw Clock
const clock = document.getElementById('clock')
const clock_ctx = clock.getContext('2d')
clock_ctx.translate(clock.width/2, clock.height/2);
radius = (clock.height/2) * 0.7;
function drawClock() {
clock_ctx.clearRect(-clock.width/2, -clock.height/2, clock.width, clock.height);
clock.style.backgroundColor = 'transparent';
clock_ctx.beginPath();
clock_ctx.arc(0, 0, radius, 0 , 2 * Math.PI);
clock_ctx.stroke();
}
drawClock();
// Draw Hand
const hand = document.getElementById('hand')
const hand_ctx = hand.getContext('2d')
hand_ctx.translate(hand.width/2, hand.height/2);
function drawHand(time){
hand_ctx.clearRect(-hand.width/2, -hand.height/2, hand.width, hand.height);
hand_ctx.beginPath();
hand_ctx.moveTo(0,0);
hand_ctx.rotate(time);
hand_ctx.lineTo(0, -radius*0.9);
hand_ctx.stroke();
hand_ctx.rotate(-time);
}
function drawTime(){
time+=rad_tick;//*10
drawHand(time)
}
var intervalId = setInterval(drawTime, 1);