Java 2D

Shapes

A shape in Java 2D is a boundary which defines an inside and an outside. Pixels inside the shape are affected by the drawing operation, those outside are not.

Trying to fill a straight line segment will result in no pixels being affected, as such a shape does not contain any pixels itself. Instead, a thin rectangle must be used so that the shape contains some pixels.

Paints

A paint generates the colors to be used for each pixel of the fill operation. The simplest paint is java.awt.Color, which generates the same color for all pixels. More complicated paints may produce gradients, images, or indeed any combination of colors. Filling a circular shape using the color yellow results in a solid yellow circle, while filling the same circular shape using a paint that generates an image produces a circular cutout of the image.

Composites

During any drawing operation, there is a source (the pixels being produced by the paint) and a destination (the pixels already onscreen). Normally, the source pixels simply overwrite the destination pixels, but the composite allows this behavior to be changed.

The composite, given the source and destination pixels, produces the final result that ultimately ends up onscreen. The most common composite is java.awt.AlphaComposite, which can treat the pixels being drawn as partially transparent, so that the destination pixels show through to some degree.

Filling

To fill a shape, the first step is to identify which pixels fall inside the shape. These pixels will be affected by the fill operation. Pixels that are partially inside and partially outside the shape may be affected to a lesser degree if anti-aliasing is enabled.

The paint is then asked to generate a color for each of the pixels to be painted. In the common case of a solid-color fill, each pixel will be set to the same color.

The composite takes the pixels generated by the paint and combines them with the pixels already onscreen to produce the final result.

Transform

Every Java 2D operation is subject to a transform, so that shapes may be translated, rotated, sheared, and scaled as they are drawn. The active transform is most often the identity transform, which does nothing.

Filling using a transform can be viewed as simply creating a new, transformed shape and then filling that shape.

Stroke

In addition to the fill operation, Java 2D provides a draw operation. While fill draws the interior of a shape, draw draws its outline. The outline can be as simple as a thin line, or as complicated as a dashed line with each dash having rounded edges.

The object responsible for generating the outline is the stroke. Given an input shape, the stroke produces a new shape representing its outline. For instance, an infinitely thin line segment (with no interior) might be stroked into a one-pixel-wide rectangle.

A draw operation can therefore be described as creating a new, stroked object and then filling that object.

Technically speaking, the stroke is only required to accept an input shape and produce a new shape. The stroke implementation provided with Java 2D implements the outline rules described above, but a custom-written stroke could produce any shape it wished.