Graphics2D Drawing Basics

Suppose you draw a diagonal line or the outline of an oval in a Java graphics program and the result looks rough. Instead of a smooth slope or curve, the edge appears as a set of small steps. It is easy to assume that the drawing call is wrong, but the call itself is often correct. The difficulty comes from the drawing surface: it is a grid of pixels, and a smooth geometric edge rarely lines up exactly with that grid.

This article covers two settings that control how a drawing looks. The first is antialiasing, which softens those stepped edges. The second is stroke width, which controls how thick the lines of an outlined shape are. Both settings are applied to the graphics context before the drawing calls they should influence, and both are easy to misplace. Learning to reason about the order of these calls is the key skill of this article.

The term aliasing names the jagged look that appears when shapes are drawn using pixels. Each pixel at a given position is either colored or not colored. A diagonal line that runs across a row of pixels therefore cannot be represented exactly; the renderer must choose which pixels along the edge to fill. Picking whole pixels produces stairs.

The diagram is illustrative: it shows the general idea that each position is either fully part of the shape or not part of it, and that a run of such whole pixels reads as a step. The eye sees those steps as roughness, even though the underlying geometry is perfectly smooth.

Antialiasing is the technique that tries to reduce this roughness. Instead of treating the edge as a hard boundary between colored and uncolored pixels, the rendering process blends the boundary pixels so the transition looks smoother. The practical results are that diagonal lines and the outlines of ovals look much cleaner, and text can also look better.

Antialiasing is enabled through a rendering hint set on the graphics context. A rendering hint is a request that tells the drawing system how to handle a certain aspect of rendering. A hint is made of a key, which names the aspect being controlled, and a value, which says whether that aspect is on or off. For antialiasing, the key is the antialiasing key and the value is the on value.

Because the setting belongs to the context, its position in your program matters. Any drawing call made on g2 after the hint is applied is affected by it, and the placement of the call is the subject of the rest of this article.

Stroke width is the thickness of the lines used to draw shapes. The thickness is set by creating a BasicStroke object and handing it to the graphics context through setStroke. The value passed to the BasicStroke constructor can be an integer or a floating-point number, and it becomes the width that subsequent lines will have.

The timing rule is the most important fact about this command. The setting applies to lines drawn after setStroke runs. Lines drawn before it keep whatever width was in effect when they were drawn. The rule also covers outlined shapes: the thickness affects the outlines of shapes drawn with methods such as drawRect and drawOval, so a width change alters the border of those shapes, not their fill.

The following sequence draws two outlines. The first is drawn before setStroke runs, so it keeps the width already in effect. The second is drawn after the new stroke is set, so only it takes on the new thickness.

The diagram makes the state explicit. The stroke is a piece of context state, not a property of each shape. It remains in effect across later calls until another setStroke replaces it. That is why a width setting can appear to be ignored when it is actually applied to outlines drawn at a different point than the one you were looking at.

When a drawing does not match what you expected, do not start by rewriting shape code. Instead, list the calls that touch the graphics context in the order they execute, and for each outline or edge ask which settings were in effect at the moment it was drawn. The cause of the difference usually shows up at one specific call, and that call is where the investigation should end.