- HTML5 Canvas Cookbook
- Eric Rowell
- 498字
- 2021-08-27 12:08:03
Drawing an arc
When drawing with the HTML5 canvas, it's sometimes necessary to draw perfect arcs. If you're interested in drawing happy rainbows, smiley faces, or diagrams, this recipe would be a good start for your endeavor.
How to do it...
Follow these steps to draw a simple arc:
- Define a 2D canvas context and set the arc style:
window.onload = function(){ var canvas = document.getElementById("myCanvas"); var context = canvas.getContext("2d"); context.lineWidth = 15; context.strokeStyle = "black"; // line color - Draw the arc:
context.arc(canvas.width / 2, canvas.height / 2 + 40, 80, 1.1 * Math.PI, 1.9 * Math.PI, false); context.stroke(); }; - Embed the canvas tag inside the body of the HTML document:
<canvas id="myCanvas" width="600" height="250" style="border:1px solid black;"> </canvas>
How it works...
We can create an HTML5 arc with the arc() method which is defined by a section of the circumference of an imaginary circle. Take a look at the following diagram:
The imaginary circle is defined by a center point and a radius. The circumference section is defined by a starting angle, an ending angle, and whether or not the arc is drawn counter-clockwise:
context.arc(centerX,centerY, radius, startingAngle,
endingAngle,counterclockwise);
Notice that the angles start with 0π at the right of the circle and move clockwise to 3π/2, π, π/2, and then back to 0. For this recipe, we've used 1.1π as the starting angle and 1.9π as the ending angle. This means that the starting angle is just slightly above center on the left side of the imaginary circle, and the ending angle is just slightly above center on the right side of the imaginary circle.
There's more...
The values for the starting angle and the ending angle do not necessarily have to lie within 0π and 2π. In fact, the starting angle and ending angle can be any real number because the angles can overlap themselves as they travel around the circle.
For example, let's say that we define our starting angle as 3π. This is equivalent to one full revolution around the circle (2π) and another half revolution around the circle (1π). In other words, 3π is equivalent to 1π. As another example, - 3π is also equivalent to 1π because the angle travels one and a half revolutions counter-clockwise around the circle, ending up at 1π.
Another method for creating arcs with the HTML5 canvas is to make use of the arcTo() method. The resulting arc from the arcTo() method is defined by the context point, a control point, an ending point, and a radius:
context.arcTo(controlPointX1, controlPointY1, endingPointX, endingPointY, radius);
Unlike the arc() method, which positions an arc by its center point, the arcTo() method is dependent on the context point, similar to the lineTo() method. The arcTo() method is most commonly used when creating rounded corners for paths or shapes.