hugs · November 5, 2016 15:52
diff --git a/arc-example.html b/arc-example.html
 <html>
  <head>
    <meta charset="utf-8">
    <style>
      body {
       margin: 30px;
      }
    </style>
  </head>
  <body>
    <canvas id="tutorial" width="800" height="800">
    </canvas>
    <script>
      // https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/arc
      // https://html.spec.whatwg.org/multipage/scripting.html#dom-context-2d-arc
  
      var width = 800;
      var height = 800;
      var canvas = document.getElementById('tutorial');
      var ctx = canvas.getContext('2d');
      ctx.scale(1,1);
      ctx.font = "14px sans-serif";

      var tau = Math.TAU = 2*Math.PI;

      function arcExample(xOffset, yOffset, radius, start, end, directionLabel) {
        var startAngle = eval(start);
        var endAngle = eval(end);
        var direction = directionLabel === 'clockwise' ? false : true;
        var directionArg = direction === false ? '' : ', true'
        var description;
    
        if (startAngle > endAngle) {
          description = 'startAngle > endAngle, '
        } else {
          description = 'startAngle < endAngle, '
        }
    
        // Draw UI
        drawAxes(xOffset,yOffset);
        drawText(description + directionLabel, xOffset, yOffset, true);
        drawText('arc(' + 
                  xOffset + ', ' + 
                  yOffset + ', ' + 
                  radius  + ', ' + 
                  start   + ', ' +
                  end     +
                  directionArg + ')', 
                  xOffset, yOffset+20);
        drawArcOutline(xOffset, yOffset, radius, startAngle, endAngle, direction);

        // Draw the arc
        arc(xOffset, yOffset, radius, startAngle, endAngle, direction );      
      }  

      function drawAxes(xOffset, yOffset) {
    
        lineLength = 140;
    
        ctx.beginPath();
        ctx.moveTo(xOffset, yOffset + lineLength/2);
        ctx.lineTo(xOffset, yOffset - lineLength/2);
        ctx.strokeStyle = 'LightGrey';
        ctx.stroke();

        ctx.beginPath();
        ctx.moveTo(xOffset - lineLength/2, yOffset);
        ctx.lineTo(xOffset + lineLength/2, yOffset);
        ctx.strokeStyle = 'LightGrey';
        ctx.stroke();
    
        ctx.font = "16px serif";    
        // Tau
        ctx.fillText('0, τ', xOffset+75, yOffset+5);
        ctx.fillText('τ/2', xOffset-90, yOffset+5);
        ctx.fillText('τ/4', xOffset-10, yOffset+85);
        ctx.fillText('3τ/4', xOffset-15, yOffset-80);
    
      }

      function drawText (text, xOffset, yOffset, bold) {
        if (typeof(bold) !== "undefined") {
          ctx.font = "bold 14px sans-serif";
        } else {
          ctx.font = "14px sans-serif";
        }
        ctx.fillText(text, xOffset-100, yOffset+120);
      }

      function drawArcOutline(centerX, centerY, radius, startAngle, endAngle, anticlockwise) {
        // Draw path outline
        ctx.beginPath();
        ctx.strokeStyle = 'LightGrey';
        ctx.arc(centerX, centerY, radius, startAngle, endAngle, anticlockwise);
        ctx.stroke();
        ctx.strokeStyle = 'Black';
      }

      function areValid(arguments) {
        // Check for correct number of arguments
        if (arguments.length < 5) { return false; }
        if (arguments.length > 6) { return false; }

        // If any of the arguments are infinite or NaN, then abort.
        for (var parameter in arguments) {
          if (!isFinite(arguments[parameter])) { return false; }
          if (isNaN(arguments[parameter])) { return false; }
        }
        return true;
      }
  
      function calcPoint(cX, cY, r, radian) {
        var pointX = cX + r * Math.cos(radian);
        var pointY = cY + r * Math.sin(radian);
        return {x: pointX, y: pointY}
      }

      function plotPoint(x, y, pointType) {
          var w = 3
          var h = 3;

          var xOffset = w/2;
          var yOffset = h/2;

          ctx.fillStyle = "Black";

          if (typeof(pointType) !== "undefined") {
            w = h = 12;
            xOffset = yOffset = w/2;

            if (pointType === "start") {
              ctx.fillStyle = "LightGreen";

              ctx.beginPath();
              ctx.arc(x,y,8,0,tau);
              ctx.fill();
              ctx.strokeStyle = "Black";
              ctx.stroke();
              ctx.fillStyle = "Black";
              return;

            } else if (pointType === "end") {
              ctx.fillStyle = "Red";
            }
          }
          ctx.fillRect(x-xOffset,y-yOffset,w,h);
          ctx.fillStyle = "Black";
      }
  

      function arc(centerX, centerY, radius, startAngle, endAngle, anticlockwise) {

        if (!areValid(arguments)) return;

        // Test if anticlockwise was given. If not, set to false.
        if (typeof(anticlockwise) === "undefined") {
          var anticlockwise = false;
        } else {
          // Abort if not boolean
          if (typeof(anticlockwise) !== "boolean") {
            return;
          }
        }

        // Modulo angles into range
        startAngle = startAngle % tau;
        endAngle = endAngle % tau;

        var clockwise = !anticlockwise;

        if (clockwise === true) {
          // If needed, offset startAngle by one full turn so the loop works
          if (startAngle > endAngle) {
            startAngle -= tau;
          }

          // Plot starting point
          var point = calcPoint(centerX, centerY, radius, startAngle);
          plotPoint(point.x,point.y, 'start');

          // Plot all path points
          var counter = 0;
          for (var radian = startAngle; radian <= endAngle; radian += .05) {
            var point = calcPoint(centerX, centerY, radius, radian);;
            //plotPoint(point.x,point.y);
            setTimeout(plotPoint, counter, point.x, point.y);
            counter += 20;
          }

          // Plot ending point
          plotPoint(point.x,point.y, 'end');

        } else {
          // If needed, offset startAngle by one full turn so the loop works
          if (startAngle < endAngle) {
            startAngle += tau;
          }

          // Plot starting point
          var point = calcPoint(centerX, centerY, radius, startAngle);
          plotPoint(point.x,point.y, 'start');

          // Plot all path points
          var counter = 0;
          for (var radian = startAngle; radian >= endAngle; radian -= .05) {
            var point = calcPoint(centerX, centerY, radius, radian);
            setTimeout(plotPoint, counter, point.x, point.y);
            counter += 20;
          }

          // Plot ending point
          plotPoint(point.x,point.y, 'end');
        }
      }


      //-----------------------------------------------------  
      // Example #1: startAngle > endAngle, clockwise
      arcExample(100, 100, 50, 'tau/2', 'tau/4', 'clockwise'); 

      // Example #2: startAngle > endAngle, anticlockwise
      arcExample(400, 100, 50, 'tau/2', 'tau/4', 'anticlockwise');

      // Example #3: startAngle < endAngle, clockwise
      arcExample(100, 400, 50, 'tau/4', 'tau/2', 'clockwise');  

      // Example #4: startAngle < endAngle, anticlockwise
      arcExample(400, 400, 50, 'tau/4', 'tau/2', 'anticlockwise');


    </script>
  </body>
 </html>
	<html>
	<head>
	<meta charset="utf-8">
	<style>
	body {
	margin: 30px;
	}
	</style>
	</head>
	<body>
	<canvas id="tutorial" width="800" height="800">
	</canvas>
	<script>
	// https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/arc
	// https://html.spec.whatwg.org/multipage/scripting.html#dom-context-2d-arc

	var width = 800;
	var height = 800;
	var canvas = document.getElementById('tutorial');
	var ctx = canvas.getContext('2d');
	ctx.scale(1,1);
	ctx.font = "14px sans-serif";

	var tau = Math.TAU = 2*Math.PI;

	function arcExample(xOffset, yOffset, radius, start, end, directionLabel) {
	var startAngle = eval(start);
	var endAngle = eval(end);
	var direction = directionLabel === 'clockwise' ? false : true;
	var directionArg = direction === false ? '' : ', true'
	var description;

	if (startAngle > endAngle) {
	description = 'startAngle > endAngle, '
	} else {
	description = 'startAngle < endAngle, '
	}

	// Draw UI
	drawAxes(xOffset,yOffset);
	drawText(description + directionLabel, xOffset, yOffset, true);
	drawText('arc(' +
	xOffset + ', ' +
	yOffset + ', ' +
	radius + ', ' +
	start + ', ' +
	end +
	directionArg + ')',
	xOffset, yOffset+20);
	drawArcOutline(xOffset, yOffset, radius, startAngle, endAngle, direction);

	// Draw the arc
	arc(xOffset, yOffset, radius, startAngle, endAngle, direction );
	}

	function drawAxes(xOffset, yOffset) {

	lineLength = 140;

	ctx.beginPath();
	ctx.moveTo(xOffset, yOffset + lineLength/2);
	ctx.lineTo(xOffset, yOffset - lineLength/2);
	ctx.strokeStyle = 'LightGrey';
	ctx.stroke();

	ctx.beginPath();
	ctx.moveTo(xOffset - lineLength/2, yOffset);
	ctx.lineTo(xOffset + lineLength/2, yOffset);
	ctx.strokeStyle = 'LightGrey';
	ctx.stroke();

	ctx.font = "16px serif";
	// Tau
	ctx.fillText('0, τ', xOffset+75, yOffset+5);
	ctx.fillText('τ/2', xOffset-90, yOffset+5);
	ctx.fillText('τ/4', xOffset-10, yOffset+85);
	ctx.fillText('3τ/4', xOffset-15, yOffset-80);

	}

	function drawText (text, xOffset, yOffset, bold) {
	if (typeof(bold) !== "undefined") {
	ctx.font = "bold 14px sans-serif";
	} else {
	ctx.font = "14px sans-serif";
	}
	ctx.fillText(text, xOffset-100, yOffset+120);
	}

	function drawArcOutline(centerX, centerY, radius, startAngle, endAngle, anticlockwise) {
	// Draw path outline
	ctx.beginPath();
	ctx.strokeStyle = 'LightGrey';
	ctx.arc(centerX, centerY, radius, startAngle, endAngle, anticlockwise);
	ctx.stroke();
	ctx.strokeStyle = 'Black';
	}

	function areValid(arguments) {
	// Check for correct number of arguments
	if (arguments.length < 5) { return false; }
	if (arguments.length > 6) { return false; }

	// If any of the arguments are infinite or NaN, then abort.
	for (var parameter in arguments) {
	if (!isFinite(arguments[parameter])) { return false; }
	if (isNaN(arguments[parameter])) { return false; }
	}
	return true;
	}

	function calcPoint(cX, cY, r, radian) {
	var pointX = cX + r * Math.cos(radian);
	var pointY = cY + r * Math.sin(radian);
	return {x: pointX, y: pointY}
	}

	function plotPoint(x, y, pointType) {
	var w = 3
	var h = 3;

	var xOffset = w/2;
	var yOffset = h/2;

	ctx.fillStyle = "Black";

	if (typeof(pointType) !== "undefined") {
	w = h = 12;
	xOffset = yOffset = w/2;

	if (pointType === "start") {
	ctx.fillStyle = "LightGreen";

	ctx.beginPath();
	ctx.arc(x,y,8,0,tau);
	ctx.fill();
	ctx.strokeStyle = "Black";
	ctx.stroke();
	ctx.fillStyle = "Black";
	return;

	} else if (pointType === "end") {
	ctx.fillStyle = "Red";
	}
	}
	ctx.fillRect(x-xOffset,y-yOffset,w,h);
	ctx.fillStyle = "Black";
	}


	function arc(centerX, centerY, radius, startAngle, endAngle, anticlockwise) {

	if (!areValid(arguments)) return;

	// Test if anticlockwise was given. If not, set to false.
	if (typeof(anticlockwise) === "undefined") {
	var anticlockwise = false;
	} else {
	// Abort if not boolean
	if (typeof(anticlockwise) !== "boolean") {
	return;
	}
	}

	// Modulo angles into range
	startAngle = startAngle % tau;
	endAngle = endAngle % tau;

	var clockwise = !anticlockwise;

	if (clockwise === true) {
	// If needed, offset startAngle by one full turn so the loop works
	if (startAngle > endAngle) {
	startAngle -= tau;
	}

	// Plot starting point
	var point = calcPoint(centerX, centerY, radius, startAngle);
	plotPoint(point.x,point.y, 'start');

	// Plot all path points
	var counter = 0;
	for (var radian = startAngle; radian <= endAngle; radian += .05) {
	var point = calcPoint(centerX, centerY, radius, radian);;
	//plotPoint(point.x,point.y);
	setTimeout(plotPoint, counter, point.x, point.y);
	counter += 20;
	}

	// Plot ending point
	plotPoint(point.x,point.y, 'end');

	} else {
	// If needed, offset startAngle by one full turn so the loop works
	if (startAngle < endAngle) {
	startAngle += tau;
	}

	// Plot starting point
	var point = calcPoint(centerX, centerY, radius, startAngle);
	plotPoint(point.x,point.y, 'start');

	// Plot all path points
	var counter = 0;
	for (var radian = startAngle; radian >= endAngle; radian -= .05) {
	var point = calcPoint(centerX, centerY, radius, radian);
	setTimeout(plotPoint, counter, point.x, point.y);
	counter += 20;
	}

	// Plot ending point
	plotPoint(point.x,point.y, 'end');
	}
	}


	//-----------------------------------------------------
	// Example #1: startAngle > endAngle, clockwise
	arcExample(100, 100, 50, 'tau/2', 'tau/4', 'clockwise');

	// Example #2: startAngle > endAngle, anticlockwise
	arcExample(400, 100, 50, 'tau/2', 'tau/4', 'anticlockwise');

	// Example #3: startAngle < endAngle, clockwise
	arcExample(100, 400, 50, 'tau/4', 'tau/2', 'clockwise');

	// Example #4: startAngle < endAngle, anticlockwise
	arcExample(400, 400, 50, 'tau/4', 'tau/2', 'anticlockwise');


	</script>
	</body>
	</html>