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HTML5 Canvas by Jeff Fulton, Steve Fulton

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Advanced Path Methods

Let’s take a deeper look at some of the other methods we can use to draw paths on the canvas, including arcs and curves that can be combined to create complex images.


There are four functions we can use to draw arcs and curves onto the canvas. An arc can be a complete circle or any part of a circle


context.arc(x, y, radius, startAngle, endAngle, anticlockwise)

The x and y values define the center of our circle, and the radius will be the radius of the circle upon which our arc will be drawn. startAngle and endAngle are in radians, not degrees. anticlockwise is a true or false value that defines the direction of the arc.

For example, if we want to draw a circle with a center point at position 100,100 and with a radius of 20, as shown in Figure 2-4, we could use the code below for the contents of drawScreen():

context.arc(100, 100, 20, (Math.PI/180)*0, (Math.PI/180)*360, false);

Example 2-4 illustrates the code necessary to create a simple circle.

Example 2-4. A circle arc

function drawScreen() {

      context.strokeStyle = "black";
      context.lineWidth = 5;
      context.arc(100, 100, 20, (Math.PI/180)*0, (Math.PI/180)*360, false);

      //full circle

A basic circle arc

Figure 2-4. A basic circle arc

Notice that we have to convert our start angle (0) and our end angle (360) into radians by multiplying them by (Math.PI/180). By using 0 as the start angle and 360 as the end, we create a full circle.

We can also draw a segment of a circle by not specifying the entire 0 to 360 start and stop angles. This code for drawScreen() will create one-quarter of a circle drawn clockwise, as shown in Figure 2-5:

context.arc(100, 200, 20, (Math.PI/180)*0, (Math.PI/180)*90, false);
A one-quarter circle arc

Figure 2-5. A one-quarter circle arc

If we want to draw everything but the 0–90 angle, as shown in Figure 2-6, we can employ the anticlockwise argument and set it to true:

context.arc(100, 200, 20, (Math.PI/180)*0, (Math.PI/180)*90, true);
A three-fourths circle arc

Figure 2-6. A three-fourths circle arc


context.arcTo(x1, y1, x2, y2, radius)

The arcTo method has only been implemented in the latest browsers—perhaps because its capabilities can be replicated by the arc() function. It takes in a point (x1,y1) and draws a straight line from the current path position to this new position. Then it draws an arc from that point to the y1,y2 point using the given radius.

The context.arcTo method will work only if the current path has at least one subpath. So, let’s start with a line from position 0,0 to position 100,200. Then we will build our small arc. It will look a little like a bent wire coat hanger (for lack of a better description), as shown in Figure 2-7:

context.lineTo(100, 200);
An arcTo() example

Figure 2-7. An arcTo() example

Bezier Curves

Bezier curves, which are far more flexible than arcs, come in both the cubic and quadratic types:

The Bezier curve is defined in 2D space by a “start point,” an “end point,” and one or two “control” points, which determine how the curve will be constructed on the canvas. A normal cubic Bezier curve uses two points, while a quadric version uses a single point.

The quadratic version, shown in Figure 2-8, is the simplest, only needing the end point (last) and a single point in space to use as a control point (first):

A simple quadratic Bezier curve

Figure 2-8. A simple quadratic Bezier curve

This curve starts at 0,0 and ends at 0,50. The point in space we use to create our arc is 100,25. This point is roughly the center of the arc vertically. The 100 value for the single control point pulls the arc out to make an elongated curve.

The cubic Bezier curve offers more options because we have two control points to work with. The result is that curves—such as the classic “S” curve shown in Figure 2-9—are easier to make:

A Bezier curve with two control points

Figure 2-9. A Bezier curve with two control points

The Canvas Clipping Region

Combining the save() and restore() functions with the Canvas clip region limits the drawing area for a path and its subpaths. We do this by first setting rect() to a rectangle that encompasses the region we would like to draw in, and then calling the clip() function. This will set the clip region to be the rectangle we defined with the rect() method call. Now, no matter what we draw onto the current context, it will only display the portion that is in this region. Think of this as a sort of mask that you can use for your drawing operations. Example 2-5 shows how this works, producing the clipped result shown in Figure 2-10.

Example 2-5. The Canvas clipping region

function drawScreen() {

      //draw a big box on the screen
      context.fillStyle = "black";
      context.fillRect(10, 10, 200, 200);

      //clip the canvas to a 50×50 square starting at 0,0
      context.rect(0, 0, 50, 50);

      //red circle
      context.strokeStyle = "red"; //need list of available colors
      context.lineWidth = 5;
      context.arc(100, 100, 100, (Math.PI/180)*0, (Math.PI/180)*360, false);
      //full circle


      //reclip to the entire canvas
      context.rect(0, 0, 500, 500);

      //draw a blue line that is not clipped
      context.strokeStyle = "blue"; //need list of available colors
      context.lineWidth = 5;
      context.arc(100, 100, 50, (Math.PI/180)*0, (Math.PI/180)*360, false);
      //full circle

The Canvas clipping region

Figure 2-10. The Canvas clipping region

Example 2-5 first draws a large 200×200 black rectangle onto the canvas. Next, we set our Canvas clipping region to rect(0,0,50,50). The clip() call then clips the canvas to those specifications. When we draw our full red circle arc, we only see the portion inside this rectangle. Finally, we set the clipping region back to rect(0,0,500,500) and draw a new blue circle. This time, we can see the entire circle on the canvas.


Other Canvas methods can be used with the clipping region. The most obvious is the arc() function:

arc(float x, float y, float radius, float startAngle,
float endAngle, boolean anticlockwise)

This can be used to create a circular clipping region instead of a rectangular one.

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