Canvas - Fill a specific area of an arc/circle

Question

Starting from a W3School example I've created a canvas timer clock.

I would like to fill with another color (Yellow), the area where the hand is passed.

The clock is available here: https://jsfiddle.net/nzexyd6j/1/

And here is the code:

<canvas id="canvas" width="400" height="400" style="background-color:#fff">
</canvas>
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var radius = canvas.height /2;
var minuti = 3;
var tempoInit = minuti * 60;
var tempo = tempoInit;
//var tempo = 180;
var lancetta;
ctx.translate(radius, radius);
radius = radius * 0.90
setInterval(drawClock, 1000);

function drawClock() {
  drawFace(ctx, radius);
 // drawNumbers(ctx, radius);
  drawTime(ctx, radius);
}
//f2634a
function drawFace(ctx, radius) {
  var grad;
  ctx.beginPath();
  ctx.arc(0, 0, radius, 0, 2*Math.PI);
  ctx.fillStyle = '#fff';
  ctx.fill();
  grad = ctx.createRadialGradient(0,0,radius*0.95, 0,0,radius*1.05);
  grad.addColorStop(0, '#f2634a');
  grad.addColorStop(1, '#f2634a');
  ctx.strokeStyle = grad;
  ctx.lineWidth = radius*0.1;
  ctx.stroke();
  ctx.beginPath();
  ctx.arc(0, 0, radius*0.1, 0, 2*Math.PI);
  ctx.fillStyle = '#f2634a';
 // ctx.fill();
}

function drawNumbers(ctx, radius) {
  var ang;
  var num;
  ctx.font = radius*0.15 + "px arial";
  ctx.textBaseline="middle";
  ctx.textAlign="center";
  for(num = 1; num < (minuti + 1); num++){
    ang = (num * Math.PI / minuti ) * 2;
    ctx.rotate(ang);
    ctx.translate(0, -radius*0.85);
    ctx.rotate(-ang);
    ctx.fillText(num.toString(), 0, 0);
    ctx.rotate(ang);
    ctx.translate(0, radius*0.85);
    ctx.rotate(-ang);
  }
}

function drawTime(ctx, radius){
    tempo--;
    lancetta = (tempo*Math.PI/tempoInit) * 2;
    drawHand(ctx, lancetta, radius*1, radius*0.07);
    drawHand(ctx, (180*Math.PI/tempoInit) * 2, radius*1, radius*0.07);
}

function drawHand(ctx, pos, length, width) {
    ctx.beginPath();
    ctx.lineWidth = width;
    ctx.lineCap = "round";
    ctx.moveTo(0,0);
    ctx.rotate(pos);
    ctx.lineTo(0, -length);
    ctx.stroke();
    ctx.rotate(-pos);
}

I've tried using lineWidth, but without success.

Do you have any suggestions?

Answer 1

Your clock seems to be running backwards?!

Anyway, you want to fill a wedge based on how many clock minutes have elapsed.

So given a time in clock minutes, this function returns the angle at that clock time.

// given minutes (a minute-hand on a clock)
// return the associated angle in radians
function minutesToAngle(minutes){
    var twelveOClock=-Math.PI/2;
    var fullCircle=Math.PI*2;
    return(twelveOClock+fullCircle*(minutes/60));
}

The returned angle is in radians, which is the unit-of-measure used to draw an arc on the canvas.

To draw a wedge, you simply fill an arc from the arc's centerpoint like this:

function fillWedge(cx,cy,radius,startAngle,endAngle,fillcolor){
    ctx.beginPath();
    ctx.moveTo(cx,cy);
    ctx.arc(cx,cy,radius,startAngle,endAngle);
    ctx.closePath();
    ctx.fillStyle=fillcolor;
    ctx.fill();
}

Putting the 2 functions together, you can fill your elapsed time like this:

// fill 5 elapsed minutes
var cx=canvas.width/2;  // or your clock's centerX
var cy=canvas.height/2; // or your clock's centerY
var radius=Math.min(canvas.width,canvas.height)*.90; // or your clock's radius
var startMinutes=0; // start at 12 o'clock
var endMinutes=5;   // end at 5 minutes past 12
var startAngle=minutesToAngle(0); // the angle at 12 o'clock
var endAngle=minutesToAngle(5);   // the angle at 5 minutes past 12

// fill the wedge for 5 elapsed minutes
fillWedge(cx,cy,radius,startAngle,endAngle,'gold');

By adding these 2 functions to your existing code, you can:

1. Erase the canvas,
2. Fill the background of the elapsed time with yellow,
3. Stroke the clock face and clock hands.

Example code and a Demo:

var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var cw=canvas.width;
var ch=canvas.height;

var cx=cw/2;
var cy=ch/2;
var radius=135;
var minutes=0;
var minutesIncrement=0.334;

animate();

function animate(time){
    ctx.clearRect(0,0,cw,ch);
    ctx.beginPath();
    ctx.arc(cx,cy,radius,0,Math.PI*2);
    ctx.strokeStyle='indianred';
    ctx.lineWidth=10;
    ctx.lineJoin='round';
    ctx.stroke();
    fillWedge(cx,cy,radius,minutesToAngle(0),minutesToAngle(minutes),'gold');
    ctx.stroke();
    minutes+=minutesIncrement;
    if(minutes>60){minutes=0;}
    requestAnimationFrame(animate);
}

    var cx=canvas.width/2;
    var cy=canvas.height/2;
    var radius=Math.min(canvas.width,canvas.height)*.90/2;
    var startMinutes=0; // start at 12 o'clock
    var endMinutes=5;   // end at 5 minutes past 12
    var startAngle=minutesToAngle(0); // the angle at 12 o'clock
    var endAngle=minutesToAngle(5);   // the angle at 5 minutes past 12

    // fill the wedge for 5 elapsed minutes
    fillWedge(cx,cy,radius,startAngle,endAngle,'gold');

    function fillWedge(cx,cy,radius,startAngle,endAngle,fillcolor){
        ctx.beginPath();
        ctx.moveTo(cx,cy);
        ctx.arc(cx,cy,radius,startAngle,endAngle);
        ctx.closePath();
        ctx.fillStyle=fillcolor;
        ctx.fill();
    }

    function minutesToAngle(minutes){
        var twelveOClock=-Math.PI/2;
        var fullCircle=Math.PI*2;
        return(twelveOClock+fullCircle*(minutes/60));
    }

#canvas{border:1px solid red; margin:0 auto; }

<canvas id="canvas" width=300 height=300></canvas>