Drawing on the Canvas - HTML5 Canvas - page 53

Figure 2-23. A linear horizontal gradient
To create the horizontal gradient, we must first create a variable ( gr ) to reference the
new gradient. Here's how we set it:
var gr = context.createLinearGradient(0,0,100,0);
The four parameter values in the createLinearGradient method call are the top-left x
and y coordinates to start the gradient, as well as the two bottom-right points to end
the gradient. Our example starts at 0,0 and goes to 100,0 . Notice that the y values are
both 0 when we create a horizontal gradient; the opposite will be true when we create
a vertical gradient.
Once we have defined the size of our gradient, we then add in color stops that take two
parameter values. The first is a relative position origin point along the gradient to start
with color, and the second is the color to use. The relative position must be a value
from 0.0 to 1.0:
gr.addColorStop(0,'rgb(255,0,0)');
gr.addColorStop(.5,'rgb(0,255,0)');
gr.addColorStop(1,'rgb(255,0,0)');
Therefore, in Example 2-14 , we have set a red color at 0 , a green color at .5 (the center),
and another red color at 1 . This will fill our shape with a relatively even red to green to
red gradient.
Next, we need to get the context.fillStyle to be the gradient we just created:
context.fillStyle = gr;
Finally, we create a rectangle on the canvas:
context.fillRect(0, 0, 100, 100);
Notice that we created a rectangle that was the exact size of our gradient. We can change
the size of the output rectangle like this:
context.fillRect(0, 100, 50, 100);
context.fillRect(0, 200, 200, 100);
Example 2-15 adds these two new filled rectangles to Example 2-14 to create Fig-
ure 2-24 . Notice that the gradient fills up the available space, with the final color filling
out the area larger than the defined gradient size.