java.awt

Class AlphaComposite

java.lang.Object

java.awt.AlphaComposite

All Implemented Interfaces:

Composite

public final class AlphaComposite
extends Object
implements Composite

This AlphaComposite class implements the basic alpha compositing rules for combining source and destination pixels to achieve blending and transparency effects with graphics and images. The rules implemented by this class are the set of Porter-Duff rules described in T. Porter and T. Duff, "Compositing Digital Images", SIGGRAPH 84, 253-259.

If any input does not have an alpha channel, an alpha value of 1.0, which is completely opaque, is assumed for all pixels. A constant alpha value can also be specified to be multiplied with the alpha value of the source pixels.

The following abbreviations are used in the description of the rules:

• Cs = one of the color components of the source pixel.
• Cd = one of the color components of the destination pixel.
• As = alpha component of the source pixel.
• Ad = alpha component of the destination pixel.
• Fs = fraction of the source pixel that contributes to the output.
• Fd = fraction of the input destination pixel that contributes to the output.

The color and alpha components produced by the compositing operation are calculated as follows:

        Cd = Cs*Fs + Cd*Fd
        Ad = As*Fs + Ad*Fd

where Fs and Fd are specified by each rule. The above equations assume that both source and destination pixels have the color components premultiplied by the alpha component. Similarly, the equations expressed in the definitions of compositing rules below assume premultiplied alpha.

The alpha resulting from the compositing operation is stored in the destination if the destination has an alpha channel. Otherwise, the resulting color is divided by the resulting alpha before being stored in the destination and the alpha is discarded. If the alpha value is 0.0, the color values are set to 0.0.

If either source or destination pixels have color components not premultiplied by the alpha component, appropriate conversions are performed before and after the compositing operation.

Restrictions

Implementations of alpha compositing in this Profile exhibit certain restrictions, specifically:

• An implementation may approximate the result of the SRC_OVER rule in the case that 0.0 < As < 1.0. See:
  • SRC_OVER

For more information, see Profile-specific properties.

Version:

10 Feb 1997

See Also:

Field Summary

Fields

Modifier and Type	Field and Description
static AlphaComposite	Clear AlphaComposite object that implements the opaque CLEAR rule with an alpha of 1.0f.
static int	CLEAR Porter-Duff Clear rule.
static AlphaComposite	Src AlphaComposite object that implements the opaque SRC rule with an alpha of 1.0f.
static int	SRC Porter-Duff Source rule.
static int	SRC_OVER Porter-Duff Source Over Destination rule.
static AlphaComposite	SrcOver AlphaComposite object that implements the opaque SRC_OVER rule with an alpha of 1.0f.

Method Summary

Methods

Modifier and Type	Method and Description
boolean	equals(Object obj) Determines whether the specified object is equal to this AlphaComposite.
float	getAlpha() Returns the alpha value of this AlphaComposite.
static AlphaComposite	getInstance(int rule) Creates an AlphaComposite object with the specified rule.
static AlphaComposite	getInstance(int rule, float alpha) Creates an AlphaComposite object with the specified rule and the constant alpha to multiply with the alpha of the source.
int	getRule() Returns the compositing rule of this AlphaComposite.
int	hashCode() Returns the hashcode for this composite.

Methods inherited from class java.lang.Object

clone, finalize, getClass, notify, notifyAll, toString, wait, wait, wait

Field Detail

CLEAR

public static final int CLEAR

Porter-Duff Clear rule. Both the color and the alpha of the destination are cleared. Neither the source nor the destination is used as input.

Fs = 0 and Fd = 0, thus:

        Cd = 0
        Ad = 0

See Also:

Constant Field Values

SRC

public static final int SRC

Porter-Duff Source rule. The source is copied to the destination. The destination is not used as input.

Fs = 1 and Fd = 0, thus:

        Cd = Cs
        Ad = As

See Also:

Constant Field Values

SRC_OVER

public static final int SRC_OVER

Porter-Duff Source Over Destination rule. The source is composited over the destination.

Fs = 1 and Fd = (1-As), thus:

        Cd = Cs + Cd*(1-As)
        Ad = As + Ad*(1-As)

Note: This operation is subject to restriction in this Profile.

See Also:

Constant Field Values

Clear

public static final AlphaComposite Clear

AlphaComposite object that implements the opaque CLEAR rule with an alpha of 1.0f.

See Also:

CLEAR

Src

public static final AlphaComposite Src

AlphaComposite object that implements the opaque SRC rule with an alpha of 1.0f.

See Also:

SRC

SrcOver

public static final AlphaComposite SrcOver

AlphaComposite object that implements the opaque SRC_OVER rule with an alpha of 1.0f.

See Also:

SRC_OVER

Method Detail

getInstance

public static AlphaComposite getInstance(int rule)

Creates an AlphaComposite object with the specified rule.

Parameters:

rule - the compositing rule

Throws:

IllegalArgumentException - if rule is not one of the following: CLEAR, SRC, or SRC_OVER

getInstance

public static AlphaComposite getInstance(int rule,
                         float alpha)

Creates an AlphaComposite object with the specified rule and the constant alpha to multiply with the alpha of the source. The source is multiplied with the specified alpha before being composited with the destination.

Parameters:

rule - the compositing rule

alpha - the constant alpha to be multiplied with the alpha of the source. alpha must be a floating point number in the inclusive range [0.0, 1.0].

Throws:

IllegalArgumentException - if alpha is less than 0.0 or greater than 1.0, or if rule is not one of the following: CLEAR, SRC, or SRC_OVER

getAlpha

public float getAlpha()

Returns the alpha value of this AlphaComposite. If this AlphaComposite does not have an alpha value, 1.0 is returned.

Returns:

the alpha value of this AlphaComposite.

getRule

public int getRule()

Returns the compositing rule of this AlphaComposite.

Returns:

the compositing rule of this AlphaComposite.

hashCode

public int hashCode()

Returns the hashcode for this composite.

Overrides:

hashCode in class Object

Returns:

a hash code for this composite.

See Also:

Object.equals(java.lang.Object), Hashtable

equals

public boolean equals(Object obj)

Determines whether the specified object is equal to this AlphaComposite.

The result is true if and only if the argument is not null and is an AlphaComposite object that has the same compositing rule and alpha value as this object.

Overrides:

equals in class Object

Parameters:

obj - the Object to test for equality

Returns:

true if obj equals this AlphaComposite; false otherwise.

See Also:

Object.hashCode(), Hashtable