HTML and CSS Reference
In-Depth Information
var linearGradient = ctx.createLinearGradient(x0, y0, x1, y1);
var radialGradient = ctx.createRadialGradient(x0, y0, r0, x1, y1, r1);
Linear gradients start at x0 , y0 and go until x1 , y1 . They are drawn as an infinitely-wide band that is per-
pendicular to the line created by the passed-in points.
Radial gradients are generated as a cone created between the two circles defined by the passed-in parameters.
Areas outside of both circles are transparent. If you just want to create a single circular gradient, you can set the
first radius to 0 and both points x0 , y0 and x1 , y1 to be the same point.
After you create a gradient, you need to add color stops to it. These define the color at a specific percentage
of the way from the start to the end of the gradient. To add a stop, you need to call addColorStop on the
gradient and pass a number from 0 to 1 that represents the position of the stop and the color:
gradient.addColorStop(position, color);
For example, to create a gradient that goes from black to white at the midpoint and then back to black, you
could write
You can use any valid CSS color definition to pass in as the second parameter to addColorStop .
The last stroke or fill style you can create is a pattern. A pattern is an image repeated over the area of the fill
and is created by calling
var pattern = ctx.createPattern(sourceImage,repeatString);
sourceImage can be an <img> element; this includes objects created with new Image() , a <video>
element, or a <canvas> element. repeatString is one of the following values: "repeat" , "repeat-
x" , "repeat-y" , or "no-repeat" . You should recognize these strings as the repeat values you can set on
a backgroundImage in CSS. They map to the pattern repeated in both directions, repeated horizontally only,
repeated vertically only, and not repeated at all.
Listing 15-4 shows an example of both types of gradients created along with a simple pattern generated from
an offscreen <canvas> element. The result is shown in Figure 15-3 .
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