Java 2D Graphics

2.3. All About Alpha

Rendering is an approximation. When you ask to have an ideal shape filled, the rendering engine figures out how the pixels of an output device should be colored to best approximate the shape. For example, suppose the rendering engine is asked to fill a shape with some color. There's a fast way to do it, and then there's a good way to do it.

2.3.1. Aliasing and Antialiasing

The fast method is to color the pixels whose centers fall within the shape. Using this algorithm, pixels are either fully colored or left unchanged. Figure 2.3 shows an example of this technique with a single letter shown on some device with very large pixels. The ideal outline of the shape is also shown. The filled shape exhibits unattractive jaggies, or ragged edges. Images produced using this algorithm are said to be aliased.

Figure 2.3. Aliased rendering

The better method for filling a shape involves a little more work. The basic idea is to calculate the intersection of the shape with each pixel of the output device. Pixels are colored in proportion to the amount they are covered by the shape. This reduces the jaggies that are symptomatic of aliased rendering. Not surprisingly, this technique is called antialiasing. Figure 2.4 shows the same shape as Figure 2.3, but rendered with antialiasing. The pixels on the edge aren't just black or white; they're varying shades of gray.

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