Class Graphics2D

java.lang.Object
java.awt.Graphics
java.awt.Graphics2D
public abstract class Graphics2D extends Graphics
This Graphics2D class extends the Graphics class to provide more sophisticated control over geometry, coordinate transformations, color management, and text layout. This is the fundamental class for rendering 2-dimensional shapes, text and images on the Java(tm) platform.

Coordinate Spaces

All coordinates passed to a Graphics2D object are specified in a device-independent coordinate system called User Space, which is used by applications. The Graphics2D object contains an AffineTransform object as part of its rendering state that defines how to convert coordinates from user space to device-dependent coordinates in Device Space.

Coordinates in device space usually refer to individual device pixels and are aligned on the infinitely thin gaps between these pixels. Some Graphics2D objects can be used to capture rendering operations for storage into a graphics metafile for playback on a concrete device of unknown physical resolution at a later time. Since the resolution might not be known when the rendering operations are captured, the Graphics2D Transform is set up to transform user coordinates to a virtual device space that approximates the expected resolution of the target device. Further transformations might need to be applied at playback time if the estimate is incorrect.

Some of the operations performed by the rendering attribute objects occur in the device space, but all Graphics2D methods take user space coordinates.

Every Graphics2D object is associated with a target that defines where rendering takes place. A GraphicsConfiguration object defines the characteristics of the rendering target, such as pixel format and resolution. The same rendering target is used throughout the life of a Graphics2D object.

When creating a Graphics2D object, the GraphicsConfiguration specifies the default transform for the target of the Graphics2D (a Component or Image). This default transform maps the user space coordinate system to screen and printer device coordinates such that the origin maps to the upper left hand corner of the target region of the device with increasing X coordinates extending to the right and increasing Y coordinates extending downward. The scaling of the default transform is set to identity for those devices that are close to 72 dpi, such as screen devices. The scaling of the default transform is set to approximately 72 user space coordinates per square inch for high resolution devices, such as printers. For image buffers, the default transform is the Identity transform.

Rendering Process

The Rendering Process can be broken down into four phases that are controlled by the Graphics2D rendering attributes. The renderer can optimize many of these steps, either by caching the results for future calls, by collapsing multiple virtual steps into a single operation, or by recognizing various attributes as common simple cases that can be eliminated by modifying other parts of the operation.

The steps in the rendering process are:

  1. Determine what to render.
  2. Constrain the rendering operation to the current Clip. The Clip is specified by a Shape in user space and is controlled by the program using the various clip manipulation methods of Graphics and Graphics2D. This user clip is transformed into device space by the current Transform and combined with the device clip, which is defined by the visibility of windows and device extents. The combination of the user clip and device clip defines the composite clip, which determines the final clipping region. The user clip is not modified by the rendering system to reflect the resulting composite clip.
  3. Determine what colors to render.
  4. Apply the colors to the destination drawing surface using the current Composite attribute in the Graphics2D context.

The three types of rendering operations, along with details of each of their particular rendering processes are:
  1. Shape operations
    1. If the operation is a draw(Shape) operation, then the createStrokedShape method on the current Stroke attribute in the Graphics2D context is used to construct a new Shape object that contains the outline of the specified Shape.
    2. The Shape is transformed from user space to device space using the current Transform in the Graphics2D context.
    3. The outline of the Shape is extracted using the getPathIterator method of Shape, which returns a PathIterator object that iterates along the boundary of the Shape.
    4. If the Graphics2D object cannot handle the curved segments that the PathIterator object returns then it can call the alternate getPathIterator method of Shape, which flattens the Shape.
    5. The current Paint in the Graphics2D context is queried for a PaintContext, which specifies the colors to render in device space.
  2. Text operations
    1. The following steps are used to determine the set of glyphs required to render the indicated String:
      1. If the argument is a String, then the current Font in the Graphics2D context is asked to convert the Unicode characters in the String into a set of glyphs for presentation with whatever basic layout and shaping algorithms the font implements.
      2. If the argument is an AttributedCharacterIterator, the iterator is asked to convert itself to a TextLayout using its embedded font attributes. The TextLayout implements more sophisticated glyph layout algorithms that perform Unicode bi-directional layout adjustments automatically for multiple fonts of differing writing directions.
      3. If the argument is a GlyphVector, then the GlyphVector object already contains the appropriate font-specific glyph codes with explicit coordinates for the position of each glyph.
    2. The current Font is queried to obtain outlines for the indicated glyphs. These outlines are treated as shapes in user space relative to the position of each glyph that was determined in step 1.
    3. The character outlines are filled as indicated above under Shape operations.
    4. The current Paint is queried for a PaintContext, which specifies the colors to render in device space.
  3. Image Operations
    1. The region of interest is defined by the bounding box of the source Image. This bounding box is specified in Image Space, which is the Image object's local coordinate system.
    2. If an AffineTransform is passed to drawImage(Image, AffineTransform, ImageObserver), the AffineTransform is used to transform the bounding box from image space to user space. If no AffineTransform is supplied, the bounding box is treated as if it is already in user space.
    3. The bounding box of the source Image is transformed from user space into device space using the current Transform. Note that the result of transforming the bounding box does not necessarily result in a rectangular region in device space.
    4. The Image object determines what colors to render, sampled according to the source to destination coordinate mapping specified by the current Transform and the optional image transform.

Default Rendering Attributes

The default values for the Graphics2D rendering attributes are:
Paint
The color of the Component.
Font
The Font of the Component.
Stroke
A square pen with a linewidth of 1, no dashing, miter segment joins and square end caps.
Transform
The getDefaultTransform for the GraphicsConfiguration of the Component.
Composite
The AlphaComposite.SRC_OVER rule.
Clip
No rendering Clip, the output is clipped to the Component.

Rendering Compatibility Issues

The JDK(tm) 1.1 rendering model is based on a pixelization model that specifies that coordinates are infinitely thin, lying between the pixels. Drawing operations are performed using a one-pixel wide pen that fills the pixel below and to the right of the anchor point on the path. The JDK 1.1 rendering model is consistent with the capabilities of most of the existing class of platform renderers that need to resolve integer coordinates to a discrete pen that must fall completely on a specified number of pixels.

The Java 2D(tm) (Java(tm) 2 platform) API supports antialiasing renderers. A pen with a width of one pixel does not need to fall completely on pixel N as opposed to pixel N+1. The pen can fall partially on both pixels. It is not necessary to choose a bias direction for a wide pen since the blending that occurs along the pen traversal edges makes the sub-pixel position of the pen visible to the user. On the other hand, when antialiasing is turned off by setting the KEY_ANTIALIASING hint key to the VALUE_ANTIALIAS_OFF hint value, the renderer might need to apply a bias to determine which pixel to modify when the pen is straddling a pixel boundary, such as when it is drawn along an integer coordinate in device space. While the capabilities of an antialiasing renderer make it no longer necessary for the rendering model to specify a bias for the pen, it is desirable for the antialiasing and non-antialiasing renderers to perform similarly for the common cases of drawing one-pixel wide horizontal and vertical lines on the screen. To ensure that turning on antialiasing by setting the KEY_ANTIALIASING hint key to VALUE_ANTIALIAS_ON does not cause such lines to suddenly become twice as wide and half as opaque, it is desirable to have the model specify a path for such lines so that they completely cover a particular set of pixels to help increase their crispness.

Java 2D API maintains compatibility with JDK 1.1 rendering behavior, such that legacy operations and existing renderer behavior is unchanged under Java 2D API. Legacy methods that map onto general draw and fill methods are defined, which clearly indicates how Graphics2D extends Graphics based on settings of Stroke and Transform attributes and rendering hints. The definition performs identically under default attribute settings. For example, the default Stroke is a BasicStroke with a width of 1 and no dashing and the default Transform for screen drawing is an Identity transform.

The following two rules provide predictable rendering behavior whether aliasing or antialiasing is being used.

  • Device coordinates are defined to be between device pixels which avoids any inconsistent results between aliased and antialiased rendering. If coordinates were defined to be at a pixel's center, some of the pixels covered by a shape, such as a rectangle, would only be half covered. With aliased rendering, the half covered pixels would either be rendered inside the shape or outside the shape. With anti-aliased rendering, the pixels on the entire edge of the shape would be half covered. On the other hand, since coordinates are defined to be between pixels, a shape like a rectangle would have no half covered pixels, whether or not it is rendered using antialiasing.
  • Lines and paths stroked using the BasicStroke object may be "normalized" to provide consistent rendering of the outlines when positioned at various points on the drawable and whether drawn with aliased or antialiased rendering. This normalization process is controlled by the KEY_STROKE_CONTROL hint. The exact normalization algorithm is not specified, but the goals of this normalization are to ensure that lines are rendered with consistent visual appearance regardless of how they fall on the pixel grid and to promote more solid horizontal and vertical lines in antialiased mode so that they resemble their non-antialiased counterparts more closely. A typical normalization step might promote antialiased line endpoints to pixel centers to reduce the amount of blending or adjust the subpixel positioning of non-antialiased lines so that the floating point line widths round to even or odd pixel counts with equal likelihood. This process can move endpoints by up to half a pixel (usually towards positive infinity along both axes) to promote these consistent results.

The following definitions of general legacy methods perform identically to previously specified behavior under default attribute settings:

  • For fill operations, including fillRect, fillRoundRect, fillOval, fillArc, fillPolygon, and clearRect, fill can now be called with the desired Shape. For example, when filling a rectangle: 
     fill(new Rectangle(x, y, w, h));
    is called.
  • Similarly, for draw operations, including drawLine, drawRect, drawRoundRect, drawOval, drawArc, drawPolyline, and drawPolygon, draw can now be called with the desired Shape. For example, when drawing a rectangle: 
     draw(new Rectangle(x, y, w, h));
    is called.
  • The draw3DRect and fill3DRect methods were implemented in terms of the drawLine and fillRect methods in the Graphics class which would predicate their behavior upon the current Stroke and Paint objects in a Graphics2D context. This class overrides those implementations with versions that use the current Color exclusively, overriding the current Paint and which uses fillRect to describe the exact same behavior as the preexisting methods regardless of the setting of the current Stroke.
The Graphics class defines only the setColor method to control the color to be painted. Since the Java 2D API extends the Color object to implement the new Paint interface, the existing setColor method is now a convenience method for setting the current Paint attribute to a Color object. setColor(c) is equivalent to setPaint(c).

The Graphics class defines two methods for controlling how colors are applied to the destination.

  1. The setPaintMode method is implemented as a convenience method to set the default Composite, equivalent to setComposite(new AlphaComposite.SrcOver).
  2. The setXORMode(Color xorcolor) method is implemented as a convenience method to set a special Composite object that ignores the Alpha components of source colors and sets the destination color to the value: 
     dstpixel = (PixelOf(srccolor) ^ PixelOf(xorcolor) ^ dstpixel);

See Also: